Electrical, Mechanical and Acoustical Measurements of Loudspeakers and Sound System Equipment. Tutorial to a new IEC Standard Project

Transcription

1 Electrical, Mechanical and Acoustical Measurements of Loudspeakers and Sound System Equipment Tutorial to a new IEC Standard Project 15 by Wolfgang Klippel, IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1 AGENDA Limited Scope of the IEC Standard New requirements and challenges Scope of new standard proposal (Part A and B) Conditions (test signals, equipment, environment,...) Amplitude adjustment (umax,) Maximal output (SPLmax, compression,...) Frequency Response (phase, latency,...) Directional characteristics (near and far field,...) Distortion (THD, IMD, Multi-tone, rub and buzz,...) IEC Standard Project LOUDSPEAKER MEASUREMENTS,

2 SCOPE of the existing IEC Starting point IEC Standard Project LOUDSPEAKER MEASUREMENTS, 3 Characteristics defined by IEC Impedance (rated value, Z(f)-curve, Qts, Vas). Input voltage (rated noise, short + long term maximal) 3. Input power (rated noise, short + long term maximal) 4. Frequency characteristics (rated range, fs, fvent) 5. SPL in stated band, Sensitivity for 1 W 6. SPL response for voltage, H(f), effec. freq. range 7. Output (acoustic) power, efficiency 8. Directivity (pattern, rad. angle, index, coverage) 9. Amplitude nonlinearity (THD, IMD) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 4

3 Active Loudspeaker Systems amplifiers DSP Nonlinear components Tweeter protection Digital audio input Equalizer Limiter X-over Midrange protection Gain Control Woofer protection drivers Control input Integration between signal processing, power amplification and electro-acoustical conversion IEC Standard Project LOUDSPEAKER MEASUREMENTS, 5 New Requirements for Modern Audio Systems Audio systems become active no access to the electrical terminals of the transducer digital signal processing dedicated to the transducer amplifiers with more capabilities Audio systems become portable main axis of radiation, sweet point and position of the listener are not defined battery powered Audio systems become personal (hand-hold devices) listener is in the near field of the source Audio systems become smaller, lighter using green transducer technologies (efficient, nonlinear) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 6

4 Characteristics defined by IEC can be applied, need modification, are not applicable to Active Loudspeaker Systems 1. Impedance (rated value, Z(f)-curve, Qts, Vas). Input voltage (rated noise, short + long term maximal) 3. Input power (rated noise, short + long term maximal) 4. Frequency characteristics (fs, fvent) 5. SPL in stated band, Sensitivity for 1 W 6. SPL response for voltage input, H(f), effec. freq. range, 7. Output (acoustic) power, efficiency 8. Directivity (pattern, rad. angle, index, coverage) 9. Amplitude nonlinearity (THD, IMD) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 7 Need for updated IEC Standard (668-X) OBJECTIVES: Applicable to all kinds of modern audio devices (active, passive) Coping with any input signal (digital, wireless, ) Defining new measurement techniques (e.g. Rub & buzz test) Bridging manufacturing (QC) and system development (R&D) Providing comprehensive information in a shorter measurement time (e.g. directivity) Simplify interpretation (e.g. Root cause analysis) Increasing flexibility to consider particularities of the application (e.g. home, automotive, personal, professional, ) Avoiding redundancy with other standards (IEC, CES, AES, ALMA, ITU) Updating, merging of existing IEC standards (e.g ) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 8

5 Disclaimer This standard provides a physical evaluation of the sound system no fixed values for PASS/FAIL limits and quality grading no characteristics for assessing overall sound quality or preference of the audio system no modeling perceptive and cognitive evaluation of the reproduced sound quality by user Conclusions: This standard describes the general framework of the physical evaluation Further standards are required to consider the particularities of personal equipment, microspeakers, headphones, homeequipment, automotive, professional applications Perceptive evaluation requires a separate standard IEC Standard Project LOUDSPEAKER MEASUREMENTS, 9 How to organize the new standard? Problems: An overwhelming number of meaningful and important measurements and characteristics (not important for all users) Measurement of transducer parameters require access to the electrical terminals and diaphragm Two basic loudspeaker standards are required! Acoustical (output based) measurement (Part A) Applicable to transducers and systems System-oriented modeling Input-output transfer characteristics (distortion) no electrical and mechanical characteristics Important for end-user Electrical and mechanical measurement (Part B) Applicable to transducers and passive systems Access to internal state variables of the transducer Model based (lumped, distributed,...) Essential for transducer and system design, less important for end-user IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1

6 SCOPE OF PART A ACOUSTICAL (OUTPUT BASED) MEASUREMENTS This International Standard applies to passive and active sound systems such as loudspeakers, headphones, TV-sets, multi-media devices, personal portable audio devices, automotive sound systems and professional equipment. The device under test (DUT) may be comprised of electrical components performing analogue and digital signal processing prior to the passive actuators performing a transduction of the electrical input into an acoustical output signal. The measurements presented here determine the transfer behaviour of the DUT between an arbitrary analogue or digital input signal and the acoustical output at any point in the near and far field of the system. This includes operating the DUT in both the small and large signal domains. The influence of the acoustical boundary conditions of the target application (e.g. car interior) can also be considered in the evaluation of the sound system. Note: This standard does not apply to microphones and other sensors. This standard does not require access to the state variables (voltage, current) at the electrical terminals of the transducer. Sensitivity, electric input power and other characteristics based on the electrical impedance will be described in a separate standard document IEC 668-Xb dedicated to electrical and mechanical measurements. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 11 SCOPE OF PART A ACOUSTICAL (OUTPUT BASED) MEASUREMENTS Properties of the black box depend on control parameters and stimulus Evaluation is based on evaluation of acoustical output digital audio stream Black box No access to internal states Near Field Far Field drivers control parameters (e.g. attenuation) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1

7 Digital audio input Equalizer Control input Gain Control Limiter X-over Tweeter protection Midrange protection Woofer protection amplifiers drivers Not covered in Part A amplifiers Digital audio input DSP Equalizer Gain Control Limiter X-over Tweeter protection Midrange protection Woofer protection x x x drivers no access to terminals of the transducers Control input No electric input power, no electric impedance No efficiency, no sensitivity No direct measurement of coil temperature No lumped parameters (linear lumped T/S, nonlinear, thermal) No distributed transducer parameters (no optical access to the diaphragm) No accelerated life testing and long-term measurement with on-line monitoring to evaluate aging, fatigue, climate dependency IEC Standard Project LOUDSPEAKER MEASUREMENTS, 13 Conventional Characteristics applicable to active loudspeaker systems DSP linear All relative characteristics, which are independent of amplitude, such as directivity index coverage angle radiation angle effective frequency range Conditions: based on sound pressure output only ratio of acoustical output signals is considered measurement in small signal domain (limiter, protection, nonlinearities not active) independent of one-dimensional signal processing (equalizer) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 14

8 Equalizer Tweeter protection Midrange protection Woofer protection Relevant information of the Transfer Function H(f) (small signal domain) amplifiers Input signal Digital audio input DSP Gain Control Limiter X-over drivers 1m x SPL Control input ms 1, 7,5 5,,5 Group delay Impulse accuracy Phase response Group delay const. time delay (latency) KLIPPEL H(f) db - [V] (rms) Rel. Amplitude response Sound Pressure Level (rel.) Maximum SPL deviation eff. frequency range (small signal ) KLIPPEL, Hz k k 5k 1k Frequency [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 15 New Challenge 1: Evaluation at Small and High Amplitudes Range of Operation Amplitude Overload Large signal performance Thermal and Nonlinear Model Maximal Output Distortion Compression Stability Small signal performance Linear Model Amplitude of the stimulus should be clearly defined to ensure test repeatability and to protect the DUT Using SPLmax as rated by the manufacturer for calibrating the stimulus Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 16

9 New Challenge : Measurement of regular and irregular distortion Stimulus Measured Signal Input Signal Output Signal Desired Small Signal Performance Accepted Large Signal Performance Linear Model Nonlinear Model Unpredictable Dynamics Regular linear distortion Harmonics, intermodulations impulsive distortion Noise Undesired Loudspeaker Defects (e.g. rub & buzz, loose particles, air leak noise, ) Which is the optimal test signal? Amplitude of the test signal? How to perform signal analysis? Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 17 New Challenge 3: Comprehensive Evaluation in 3D Space Audio System represented as a Signal Flow Chart Multiple Outputs Single Input sound radiation Sound propagation Room Interaction Room Interaction Room Interaction p(r1) Audio signal Amplifier Crossover EQ u(t) Electromechanical Transducer i(t) one-dimensional x(t) Mechanoacoustical Transducer (Cone) Sound radiation Sound radiation two-dimensional Sound propagation Sound propagation Room Interaction Room Interaction Room Interaction Room Interaction Room Interaction Room Interaction Conventional measurements SPL response in the far field are not sufficient for portable devices, studio monitores, automotive require anechoic room of sufficient size are time consuming p(r) sound field p(r3) three-dimensional Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 18

10 CONDITIONS The measurement of the DUT is performed under Normal Conditions which define Mounting and acoustical loading of the DUT Acoustical environment Positioning of DUT with respect to the measuring microphone and the walls Ambient condition (climate, adjustment) Test signal + amplitude Normal position of attenuators, equalizers or any other active control elements Measuring equipment used To ensure reproducibility and sufficient flexibility The manufacturer has to specify the following Rated Conditions rated maximum sound pressure level SPL max or maximum input value u max evaluation point rated frequency range reference plane reference point reference axis orientation vector Depends on application Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 19 Mounting and Acoustical Loading Mounting and acoustic loading of drive units (transducer) Half-space free-field condition (in plan reflecting surface of sufficient size (d >> λ) Standard baffle Standard measuring enclosure (type-a and type-b) Test cabinet generating used for end-of-line testing and relative measurements Specified acoustic load generated by a defined horn, coupler, Plane wave tube Free air without baffle, enclosure, horn,... Mounting and acoustic loading of an electro-acoustic system Free air State by the manufacturer Klippel, Sound Quality of Audio Systems, Part 1 Introduction,

11 Acoustical Environment Acoustical measurements shall be made under one of the following conditions, the choice being indicated with the results. a) Free-field condition large anechoic full room b) Half-space, field-field conditions ground floor measurement in large anechoic half room c) Simulated free-field conditions d) Half-space simulated free-field conditions gating techniques, holographic and field separation techniques Reflections of the radiated sound on other reflecting surfaces (e.g. walls) shall sufficiently suppressed to ensure an accuracy of ±1 % of the sound pressure measurements. a) Diffuse sound field conditions reverberant room, ISO 3741 b) Target application conditions (e.g. car interior) Unwanted acoustical and electrical signals and noise generated by other sources shall be kept at the lowest possible level. Data related to signals which are less than 1 db above the noise level shall be discarded.. Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 1 Positioning of DUT Polar angle θ Reference plane and normal vector n ref Evaluation point r Azimuthal angle z Reference point r ref and orientation vector o ref O y x Reference point and orientation vector are not obvious for personal sound devices Spherical coordinates are useful for compact sources and far field data Klippel, Sound Quality of Audio Systems, Part 1 Introduction,

12 Recommended Position and Orientation r xe x ye y ze z r cos sin e x r sin sin e y r cos e z x Azimuthal angle orientation vector o ref Measurement point r Polar angle θ y Reference point r ref normal vector n ref z It is strongly recommended to to put the reference point r ref in the origin O of the coordinate system, to point the normal vector n ref of the reference plan into the z direction and to turn the audio system in such a way that the orientation vector o ref points into x-direction Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 3 Measuring Distance between DUT and microphone Evaluation distance r e = r e r ref Evaluation point r Reference point r ref Far-field conditions sound pressure decreases according to the 1/r law with an accuracy of ±1 %. distance >> geometrical DUT dimensions Distance >> wavelength of the signal Near-field conditions provides additional information for assessing studio monitors, personal audio devices, measurements of line array loudspeakers and other DUTs of large size using multiple transducers cannot be performed in the far field of the source due to limited size of the anechoic room. Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 4

13 O O U U T T 1 MI C1 LINE1 P U S H P U S H LIN E MI C I Definition of the Acoustical Environment Measurement in Target Application Measurement under Standard Condition considering room, distance, ambient noise and other conditions (Standard) living room transfer of the loudspeaker system Suppressing the room influence Definition of target performance as perceived by final user Physical characteristics (comprehensive, simple to interpret, comparable, reproducible) Auralization/Listening Test Perceptual Evaluation Loudspeaker Development IEC Standard Project LOUDSPEAKER MEASUREMENTS, 5 Definition of Test Signals Sinusoidal chirp Steady-state single tone signal Steady-state two-tone signal Sparse multi-tone complex Broadband noise signal Narrow-band noise signal Impulsive signal Hann-burst signal x c ( t) A( f ( t)) cos ) x ( t) s cos f t Instantaneous frequency f ( t t xt ( t) A1 cosf 1t A cosf t t TP T M A A 1 N xm ( t) Af i cosf it i t TP T N M i1 A( fi ) pseudo-random phase i1 1 NP NM t TP TM f These terms are explained in IEC f t sin(f t) 1 cos for t 6.5/ f x b ( t) 6.5, elsewhere 1 Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 6

14 Amplitude Adjustment of the Input Signal selected input ~ stimulus u Sound pressure output amplifier, equalizer, ect.) transducer Idea Using only one single value meaningful for engineering, marketing, final user Rating of the maximal amplitude by manufacturer based on design, target application, evaluation Rated value can be applied to input and output DUT will not be damaged by the test stimuli defined by manufacturer Rated maximum input value u max Good for DUTs with a single input and constant transfer function between input and output Not meaningful for active systems Rated maximum (output) SPL max Universal approach for passive and active systems Can be applied to any input channel Can cope with gain controllers, equalizers, ect. Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 7 Rated Loudspeaker Characteristics Manufacturer has to state (in accordance with the performance of the audio device and target application) the following mandatory characteristics: the position of the audio device in the coordinate system used reference point r ref, reference plane and reference axis evaluation point r e or distance r e from the w reference point r ref SPL max or the maximum input value u max rated frequency range Further optional characteristics: shaping of stimulus to simulate program material special setting of the control parameters of the audio device Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 8

15 Should the manufacturer rate u max or SPL max? Maximum input value u max is preferred in the development and (end-of-line) testing of transducers and passive systems Maximum output value SPL max is preferred when Using multiple input channels (digital, analog) Using different parameter settings of the audio device (gain, equalizer,...) describing the physical limits and performance of the audio device comparing competitive products Generating useful information for the end user Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 9 What is the purpose of the evaluation point r e? fast and easy determination of maximum input u max SPL response describes radiated free-field at most relevant listening position according target application The following properties can be assessed at a single point because they are part of the one-dimensional signal path: regular nonlinearities caused by motor and suspension significant rub&buzz of the transducer thermal behavior of the transducer protection system Problem: air leakage noise occuring at different sides of the enclosure can not be evaluated by a single point measurements at multiple points required Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 3

16 Amplitude Adjustment of the Input Signal based on SPL max rated by manufacturer V Note: u max depends on selected input channel, setting of control elements (gain, equalizers, ect.) Evaluation point Broad-band stimulus in rated frequency band u ~ u ~ max f start f end Comparator Characteristics stated by the manufacturer Rated frequency band defined by f start and f end Rated maximum sound pressure level SPL max Evaluation point r e (distance, angle, ) Properties of the stimulus used during calibration (multi-tone or pink noise with shaping, ) Objectives of the calibration process Fast determination of the maximum input value u max based on SPL max Using u max for the calibration of other test stimuli Full flexibility for using any input channel of the active system (analogue, digital, ect. ) Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 31 Calibration of Other Test Stimuli based on maximum input u max Maximum input value sine noise ~ ~ Other test stimuli Comparator V selected input channel, setting of control elements (gain, equalizers, ect.) are identical with those used in the measurement of u max Evaluation point Benefits: Amplitude adjustment of other test stimuli Simplifies automatic testing Clear definition of small signal domain u max <.1 u max Avoiding unintended overload of the DUT Klippel, Sound Quality of Audio Systems, Part 1 Introduction, 3

17 How to rate u max and SPL max? The manufacturer has freedom to rate u max and SPL max but should consider the following requirements: final target application (rated frequency range, spectrum of typical program material, evaluation point, ect. ) DUT can reproduce the stimulus at SPL max for any time without damage sufficient sound quality for the particular application acceptable regular nonlinear distortion (harmonic + intermodulation) low compression of the fundamental (heating, mechanical limiting, protection) effective frequency range corresponds with the rated frequency range no rub & buzz or any other defects IEC Standard Project LOUDSPEAKER MEASUREMENTS, 33 How to rate the maximum input u max or maximum output SPL max 1. Defining a test value u test (based on information from customer, marketing or development). 1 h test with the stimulus at test value u test 3. Measurement of characteristics defined in the data sheet 4. Assigning the test value to the rated maximum reference value u max =u test, if the DUT is not damaged and the within the stated specification Repeat the test with a lower test value if the evaluation was not successful IEC Standard Project LOUDSPEAKER MEASUREMENTS, 34

18 OTHER MAXIMUM LEVELS Short-term maximum sound pressure level SPL short Objectives: Maximum SPL limited by mechanical load without thermal heating without causing damage of the DUT Method: Conventional method defined in IEC Broad-band stimulus 1s on/1 min off with 6 repetitions will not damage DUT Long-term maximum sound pressure level SPL long Objectives: Maximum SPL limited by applying mechanical and thermal load without causing damage of the DUT Method: Conventional method defined in IEC Broad-band stimulus 1min on/ min off with 1 repetitions will not damage DUT IEC Standard Project LOUDSPEAKER MEASUREMENTS, 35 Sound Pressure in Stated Frequency Band pink noise ~ slopes of at least 4 db/octave Bandpass filter V Amplitude adjustment Characteristics to be specified: The sound-pressure p(r) produced by a DUT at a stated measurement point r excited with a band-limited pink-noise signal with a stated rms value αu max. The sound pressure level SPL(r) p SPL log ~ ~ p ref Application: - Sound pressure in 1/3 octave or octave band standard reference soundpressure ( Pa). IEC Standard Project LOUDSPEAKER MEASUREMENTS, 36

19 Mean Sound Pressure in Stated Frequency Band 1/3rd oct. filter pink noise slopes of at least 4 db/octave Bandpass filter V 1/3rd oct. filter 1/3rd oct. filter Energetic Averaging Amplitude adjustment Characteristics to be specified: The square root of the arithmetic mean of the squares of the sound-pressure p i from all the 1/3 octave frequency bands in the stated frequency band 1 i n. ~ p mean sound pressure: n 1/ 1 ~ m p i n i1 mean sound pressure level ~ p SPL m log ~ p IEC Standard Project LOUDSPEAKER MEASUREMENTS, 37 m ref standard reference soundpressure ( Pa). Frequency response of the fundamental sound pressure component 1. Complex Transfer Function H(f) between input U(f) and sound pressure output P(f) - gives magnitude response (in db, Pascal/volt, ect.) - gives phase response (mean time delay, group delay) - Measurement by using any broad-band stimulus (shaping can be applied). Frequency response of the sound pressure level SPL(f) describes the output in generated by a narrowband stimulus of defined (constant) amplitude a) Direct measurement by using a single tone or narrow-band noise b) Calculation from the complex transfer function measured by using broad-band stimulus Measurements at low amplitudes (scaling factor α.1) : - linear time-invariant modeling can be applied (nonlinearities and heating are negligible) - measurement results are independent of the stimulus properties - Transfer Function H(f) corresponds to frequency response SPL(f) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 38

20 Transfer Function Measurement Amplitude adjustment Chirp Multi tone pink noise ~ V Fourier Transform Frequency domain Smoothing Fourier Transform Characteristics to be specified: The transfer function H(f,r) between the input signal u(t) and the sound pressure output p(t, r) at the measurement point r excited by the a broadband stimulus with rms value αu max. Large signal domain (scaling factor α 1) reveals linear, nonlinear and thermal properties of the DUT. Small signal domain (scaling factor α.1) reveals the linear transfer response only. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 39 Smoothing of the Transfer Function Spectral averaging of the complex transfer function Magnitude Spectral Averaging Impulse Response Remove Time Delay Phase Unwrapping Spectral Averaging Energy Time curve MAXIMUM BANDWIDTH B Constant time delay is estimated from the impulse response and removed from the phase response before unwrapping and spectral averaging is applied. Unwrapping is a necessary but ambiguous, noise-sensitive and errorprone process for frequency-discrete phase data (at least in acoustics). The frequency resolution should be high enough that the phase difference between two discrete frequencies will not exceed ±9 degree. Add Time Delay bandwidth B, which is typically between 1 octave and 1/4th octave, determines the spectral resolution of the magnitude and phase response IEC Standard Project LOUDSPEAKER MEASUREMENTS, 4

21 Direct Measurement of the frequency response SPL(f) Single tone, Narrow band noise ~ Constant gain V Narrow Bandpass (center) frequency Characteristics to be specified: The sound-pressure level SPL(f,r) as a function of frequency, measured under normal conditions at the measurement point r using a narrow band signal at the center frequency f. The input signal u(t) has a constant rms value αu max for all frequencies f varied in the rated frequency range. The properties of the stimulus u(t), the measurement time T s and either the rms-value or the scaling factor α shall be stated. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 41 Frequency response SPL(f) calculated from the transfer function H(f,r) Chirp Multi tone pink noise ~ (α.1) V Fourier Transform rms value of the input signal Smoothing Fourier Transform bandwidth B Method of measurement: 1. Measuring the transfer function H(f,r) by using a broad-band stimulus at low amplitudes (scaling factor α.1) within the measurement time T s in accordance with clause The magnitude response is smoothed by applying a spectral averaging with a specified bandwidth B 3. The SPL(f) in db is calculated by H( f, r) u ~ max SPL( f, r) log p ref IEC Standard Project LOUDSPEAKER MEASUREMENTS, 4

22 Short-term amplitude compression of the fundamental component Chirp Multi tone pink noise ~ Amplitude variation V No heating!! Small Signal Domain Large Signal Domain Compression Cshort(f) Characteristics to be specified: Short term amplitude compression C short (f) is the level difference between transfer function measured in the small and large signal domain Cshort ( f ) logh lin( f, r, umax ) logh ( f, r, umax ) by using a broad-band stimulus with a short measurement time T s = 1s. The short amplitude compression C short (f) reveals the nonlinear mechanisms of the transducer, the effect of the protection system and the limiting effects from other electronics (e.g. amplifier). IEC Standard Project LOUDSPEAKER MEASUREMENTS, 43 Short-Term Compression reveals mechanical nonlinearities only (no voice coil heating ) system excited by a chirp (T=1 s) generating SPLmax at the evalution point db - [V] (rms) linear prediction short-term fundamental (1 s) KLIPPEL k k Frequency [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 44

23 Long-term amplitude compression of the fundamental component Chirp Multi tone pink noise ~ Amplitude variation V pre-excitation time T pre =1 min heats up the DUT Small Signal Domain Large Signal Domain Compression Clong(f) Characteristics to be specified: Long-term amplitude compression C long (f) is the level difference between transfer function measured in the small and large signal domain Clong ( f ) logh lin( f, r, umax ) logh ( f, r, umax ) by using a broad-band stimulus over a pre-excitation time (T pre =1 min + short measurement time T s = 1s ) The long-term amplitude compression C long (f) reveals the thermal and nonlinear mechanisms of the transducer and the effect of the active protection system. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 45 Long-Term Compression reveals effects of mechanical nonlinearities and voice coil heating system excited by a chirp (T=1 min) generating SPLmax at the evalution point db - [V] (rms) KLIPPEL linear prediction long-term fundamental (1 min) 5 5 k k Frequency [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 46

24 Equalizer Tweeter protection Midrange protection Woofer protection Effective Frequency Range 1 db SPL mean 95 Fundamental 9 rated frequency range 85 effective frequency range f l f u k k 5k 1k k Frequency [Hz] Method: a) The frequency response SPL(f,r) shall be measured in the rated frequency range according at resolution corresponding to narrow band filter with a stated bandwidth (typically B = 1/9). b) The mean sound pressure level SPL mean is calculated in the stated frequency range. c) The limits of the effective frequency range shall be determined where the smoothed frequency response is not more than 1dB below the mean sound pressure level SPL mean. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 47 Latency in the electrical system Latency Sound propagation amplifiers u(t) Digital audio input DSP Gain Control Limiter X-over drivers r ref r r ref p(r) Control input transfer function H(f,r) Phase response Group delay ms 1, 7,5 5, mean group delay mean (r) KLIPPEL The latency of the DUT is the difference between the mean group delay of the DUT and the time required for the sound wave to propagate from the reference point r ref to the measurement point r defined by,5, Latency Hz lat mean r rref c With speed of sound c IEC Standard Project LOUDSPEAKER MEASUREMENTS, 48

25 Directional transfer function Azimuthal angle x Measurement point r Polar angle θ z P( f, r,, ) H ( f, r,, ) U( f ) Chirp Multi tone pink noise ~ V y Characteristics to be specified: Direct sound radiated by a DUT into 3D space any point near field or far field free field or simulated free field conditions Polar coordinate system is recommended The directional transfer function H(f, r, φ, θ) between the input signal u(t) and the sound pressure p(t, r) of the direct sound at the measurement point r shall be specified. The measurement point r is described by the spherical coordinates distance r= r-r ref azimuthal angle φ and angle θ in the stated acoustical environment. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 49 Measurement of direct sound field in 3D space region of validity region of validity Ss Ss Near field Far field Far-Field Measurement - Valid measurements at a distance r>r far - Extrapolation based on 1/r law - Directional information is independent of distance r - Sufficient information for home, pro and other applications - Large anechoic room required Near-Field Measurement - measured at a distance r<r far - Holografic wave expansion required - Extrapolation to any point outside S s - Important for personal audio, car, monitors - Applicable in small, non-anechoic rooms IEC Standard Project LOUDSPEAKER MEASUREMENTS, 5

26 Outpu t P U S H Input Measurement of the Far-field Response Conventional Techniques: Measurement of the sound field spherical surface at constant distance r in the far field of the source with sufficient angular resolution anechoic room Problems: Anechoic condition (free field, half-space condition) required Room reflections occuring in nonanechoic conditions can only be suppressed at higher frequencies by windowing of the impulse response High amount of redundant measurement data produced measurement distance r should be much larger than dimensions of the radatior d Accurate measurmeent of the phase response difficult at at large distance r Amplifier A M P Loudspeaker Turntable Analyzer r Multiplexer IEC Standard Project LOUDSPEAKER MEASUREMENTS, 51 Extrapolation of Far Field data Near field r far Not applicable Far field r 1 r Extrapolation r Measurement distance Requirements: Direct sound radiated by the DUT free field condition far field (r > r far and r 1 > r far ) same direction ( = 1, θ = θ 1 ) r H( f, r,, ) H( f, r1, 1, 1 ) e r 1 jk( r r1 ) Characteristics to be specified: A directional transfer function H(f, r,, θ ) between the input signal u(t) and the sound pressure p(t,r ) of the direct sound in the far field at distance r > r far and angles (, θ ) is extrapolated from a transfer function H(f, r 1, 1, θ 1 ) measured in the far field at the distance r 1 > r far at the same angles ( = 1, θ = θ 1 ) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 5

27 Das verknüpfte Bild kann nicht angezeigt werden. Möglicherweise wurde die Datei verschoben, umbenannt oder gelöscht. Stellen Sie sicher, dass die Verknüpfung auf die korrekte Datei und den korrekten Speicherort zeigt. Evaluation of Personal Audio Equipment near field The receiving point r in target application is close to the source (e.g. personal audio equipment, car, multimedia) d far field conditions: distance r >> dimension d distance r >> wave length λ ratio r/d >> d/λ far field data are less meaningful IEC Standard Project LOUDSPEAKER MEASUREMENTS, 53 Particularities of the Near-Field Sound Pressure Field at 1 khz baffle Surround cone Dust cap 1. High Sound Pressure Amplitudes high signal to noise ratio room reflections are neglibigle no windowing of the impulse response high spectral resolution at low frequencies Direct sound larger than room reflections Good conditions for simulated free field condition. Complexity of the Sound Field not a plane wave velocity gives additional information evanescent waves (exponential decay) relationship between sound pressure of a surface and full 3D field information holographic techniques IEC Standard Project LOUDSPEAKER MEASUREMENTS, 54

28 Short History on Near-Field Measurement Single-point measurement close to the source Multiple-point measurement on a defined axis Scanning the sound field on a surface around the source On-axis.... Don Keele 1974 Ronald Aarts (8) Weinreich (198), Evert Start () Melon, Langrenne, Garcia (9) Bi (1) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 55 Holographic wave expansion transfer function General solutions of the wave equation used as basic functions in the expansion H( f, r) C( f ) B( f, r) region of validity Coefficients of the expansion Ss S1 Conditions to be specified The coefficients C(f), the order N(f) depending on frequency f, the validity radius a and the general basic functions B(f,r) of the wave expansion describe the directional transfer function H( f, r) C( f ) B( f, r) between the input signal u(t) and the sound pressure output p(t,r) at measurement point r at a distance r= r r ref from the reference point r ref which is larger than the validity radius a IEC Standard Project LOUDSPEAKER MEASUREMENTS, 56

29 Example of a Holografic Measurement spherical waves used as basic functions 1. Measurement Scanning the sound pressure in the near field of the source at a single or multiple surfaces. Holografic Data Processing Expansion into spherical waves using Lengendre and Hankel functions Determination of the free parameters of the expansion (order N(f) and coefficients C(f)) 3. Extrapolation Calculation of the transfer function H(r,f) betweeen input u and sound pressure p(r) at an arbitrary point r in the 3D space outside the scanning surface Calculation of derived characteristics (directivity, beam pattern,sound power) monopole dipoles a IEC Standard Project LOUDSPEAKER MEASUREMENTS, 57 Expansion into Spherical Waves region of validity surface r p( r,,, ) + sound source external boundaries (walls) general solution of the wave equation in spherical coordinates p( r,,, ) p ( r,,, ) p ( r,,, ) p( r,,, ) out outgoing wave Coefficients outgoing wave N n out cn, m n mn ( ) h N n in cn, m( n mn Coefficients incoming wave Hankel function of the second kind () n ) h (1) n in incoming wave ( kr) Y (, ) e m n m n Spherical Harmonics jt ( kr) Y (, ) e Hankel function of the first kind jt Spherical Harmonics external sound source (ambient noise) useful choice of the coordinate system results in three factors: depending on frequency ω depending on distance r depending on angular direction IEC Standard Project LOUDSPEAKER MEASUREMENTS, 58

30 Angular Dependency (θ, φ) Spherical Harmonics, Legendre Polynomials p ( r,,, ) out N n n mn () m cn, m( ) hn ( kr) Yn (, ) Spherical Harmonics m n 1 ( n m)! m jm Y n (, ) Pn (cos ) e 4 ( n m)! Legendre Function Real part Imaginary part monopol dipols quadropols Bellmann 1 IEC Standard Project LOUDSPEAKER MEASUREMENTS, 59 Radial Dependency Bessel, Neumann and Hankel functions Hankel function of first kind h ( kr) j ( kr) jy ( kr) (1) n n n Hankel function of second kind h ( kr) j ( kr) jy ( kr) () n n n Bessel function j n (kr) Neumann function y n (kr) singularity! small amplitude approximation (near field) log kr / ka 1 large amplitude approximation (far field) h () n (n 1)!! kr) j jyn( kr) n ( kr) ( 1 log 1 h n (1) (kr)/hn (1) (ka) a log 1 r n 1 / r Region () n h ( kr) j n 1 jkr e kr IEC Standard Project LOUDSPEAKER MEASUREMENTS, 6

31 Extrapolation of Near Field data a Near field Far field r 1 r far r r Measurement distance r Extrapolation N > N > 5 N > 1 1 Hz 1 khz 1 khz order of the expansion frequency Benefits of the near field measurement Comprehensive assessment of direct sound in 3D space (near + far field) High signal to noise ratio Suppression of room reflections (simulated far field conditions) Minimal influence air properties (air convection, temperature field) Low redundancy in the generated data set Spatial resolution can be controlled by order N(f) of the expansion Spatial interpolation is based on acoustical model IEC Standard Project LOUDSPEAKER MEASUREMENTS, 61 Far-Field Characteristics Surface S Beam Pattern b(, ) log H, db db 9 Far-Field Sound Pressure p( r,, ) Sound Pressure On-Axis p ax ( r) p( r,, ) SPL On-Axis p( r,,) SPLax r db p ( ) log o With p = Pa Directional Factor pr,, H ( r,, ) p r Directivity pax( r) S D p ( r) H (, ) ds S ax Directivity Index DI 1log1( D) db 1 c Sound Power S p ( r,, ) ds p ax ( r ) H ( r,, ) ds c S S p S ( r ) c Sound Power Level L db P 1log 1 With P =1-1 W DI SPL ax ( r.4m ) L Radiation into half space (using baffle) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 6

32 Sound Power derived from the coefficients of spherical wave expansion Total Sound Power radiated into the far field * 1 * pv dsr pp c Sr S r N n 1 C n, m( ck n mn ds r ) see Williams, Fourier Acoustics Sound Power Level L db P 1log 1 With P =1-1 W Apparent power of nth-order spherical wave n 1 1 () n( ) pn dsr rhn ( kr) Cn, m( ) c c Sr mn mismatch between position r m of a point source and development point r of the expansion order n of the spherical waves monopol r m +r Near field Far field contribution of the coefficients to the radiated sound power N n n r r IEC Standard Project LOUDSPEAKER MEASUREMENTS, 63 Directivity derived from the coefficients of spherical wave expansion Pax D P a p ( r) p ( r) ax s as the ratio of the virtual sound power P ax generated by the on-axis response p ax on the sphere in the far field and the total power P a with 1 Pa 4r c p ( r) s where cpa ps ( r) 4r N n 1 ck n mn C n, m ( ) c 4r Using sound pressure in far field kr>>1 from above we get D( ) 8 N n n mn C n, m ( ) j N n n1 n mn spherical harmonics on-axis Y ( /, ) m n C n, m ( ) Directivity index in db DI( ) 1log1 DdB sound power IEC Standard Project LOUDSPEAKER MEASUREMENTS, 64

33 Nonlinear Distortion Measurements 1. Harmonic Distortion (single-tone stimulus) Total harmonic distortion Nth-order harmonic distortion component Maximum SPL for defined THD limit Equivalent harmonic input distortion. Intermodulation Distortion (two-tone stimulus) nd and 3rd-order intermodulation component Amplitude modulation distortion 3. Multi-tone Distortion ( multi-tone stimulus) 4. Impulsive distortion (chirp stimulus) Impulsive distortion level Maximum impulsive distortion ratio Mean impulsive distortion level Crest factor of impulsive distortion IEC Standard Project LOUDSPEAKER MEASUREMENTS, 65 Harmonic Distortion Measurement Single tone, Chirp ~ Constant gain V Spectral Analysis THD n th HD excitation frequency Conditions: Excitation with single tone or sinusoidal shirp Amplitude defined as the rms input value u or by the attenuation factor α corresponding the input value u ref or Smax ax Short measurement time is used (1s) or stated The sound pressure is measured at the evaluation point under normal measurement condition IEC Standard Project LOUDSPEAKER MEASUREMENTS, 66

34 Open Question: Is the SPL Level of Harmonic Distortion Components required? SPL of spectral components nd -order harmonic distortion in percent db - [V] (rms) fundamental nd order harmonic displayed versus excitation frequency 1 % 1 % 1 %.1 % nd Harmonic displayed versus excitation frequency k k 5k Frequency [Hz] k k Frequency [Hz] Not defined in IEC and in new draft BUT useful ~ p f ( f ) HD ( f ) ~ 1% p ( f ) As defined in IEC and in new draft IEC Standard Project LOUDSPEAKER MEASUREMENTS, 67 Equivalent Harmonic Input Distortion H(f,r 1 ) p(r 1 ) Sinusoidal sweep Equivalent Input Distortion U(f) D Nonlinear System H(f,r ) p(r ) sound field Sound pressure measurement rd harmonic distortion in voltage Signal at IN1 nearfield 3 cm 6 cm 1 m distance KLIPPEL Independent of room rd harmonics absolute Signal at IN1 1 m distance 6 cm distance 3 cm distance nearfield KLIPPEL Dependent on position db - [V] k Frequency [Hz] Transformation to the input by inverse filtering db - [V] k Frequency [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 68

35 Equivalent Harmonic Input Distortion Measurement Constant Transfer Function Single tone, Chirp ~ V Linear Filter Spectral Analysis ETHD n th EHD H(f,r) H(f,r) -1 Conditions: Excitation with single tone or sinusoidal shirp Amplitude defined as the rms input value u or by the attenuation factor α corresponding the input value u ref or Smax ax Short measurement time is used (1s) or stated The sound pressure is measured at the evaluation point under normal measurement condition Distortion are transformed to the input by inverse filtering IEC Standard Project LOUDSPEAKER MEASUREMENTS, 69 Localization of Speaker Nonlinearity Distributed nonlinearities (e.g. Higher-order modes after cone break up) EHD measured at different points in the sound field 3rd-order EHD measured at different point.1 m.5 m 1 m d,1 N,1 h(t,r1) p(r1) 1 1 u h(t,r) p(r) sound field d1 N1 d, d,i N, N,i h(t,ri) p(ri) [Percent] 1 Lumped nonlinearities (e.g. motor and suspension) 1-1 Nonlinearities located in one-dimensional signal path Frequency [Hz] Nonlinear distributed parameters IEC Standard Project LOUDSPEAKER MEASUREMENTS, 7

36 Frequency Domain f [Hz] Frequency Domain f [Hz] Intermodulation Distortion spectrum of two-tone Response 1 Stimulus dbu (Uo = 1V) input output Response 1 spectrum of reproduced stimulus dbu (Uo = 1V) Nonlinear System Amplitude nd 3 rd sound pressure spectrum nd nd 3 rd 3 rd Intermodulation Distortion harmonics n th n th difference tones summed tones n th f f 1 1 bass component nf 1 f n 1) ( f 1 f f 1 f f1 f voice component f n 1) ( f 1 frequency IEC Standard Project LOUDSPEAKER MEASUREMENTS, 71 Two-tone Intermodulation Distortion Measurement ~ + ~ f 1 f Constant gain V Spectral Analysis IMD IMD3 rms values excitation frequency nd-order intermodulation in percent ~ p ( f ) ~ f1 p ( f f1) IMD ( f1, f ) ~ 1% p ( f ) 3rd-order intermodulation in percent ~ p ( f ~ f1) p ( f f1) IMD3( f1, f ) ~ 1% p ( f ) in db IMD ( f1, f f1, f ) lg 1% L IMD ( ) in db L3 IMD IMD3( f1, f ( f1, f ) lg 1% ) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 7

37 Contribution from nd and 3 rd order modulation Modulation distortion (U1=1 V) 5-5 Ld Ld3 Ldm (cumul) KLIPPEL Total modulation -1 db nd order 3 rd order -3 4*1 6*1 8*1 1 3 *1 3 4*1 3 6*1 3 8*1 3 Frequency f1 [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 73 Classification of IM-Distortion Type of Nonlinearity determines Modulation Principle Bl(x), Le(x) Doppler, Sound propagation Amplitude Modulation FrequencyModulation IEC Standard Project LOUDSPEAKER MEASUREMENTS, 74

38 Amplitude Modulation two-tone stimulus f 1 < f s, f > f s Bl(x) 5, [N/A] 4,5 4, 3,5 3,,5, 1,5 1,,5 Symmetrical Force factor Bl(x) KLIPPEL [mm] x Pfar [ N / m^ ] 5,,5, -,5 Sound pressure Pfar(t) in far field vs time Pfar(t) peak Bottom Rest position Mean -5, Cycle,5,1,15,,5,3 Time [s] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 75 Phase (Frequency) Modulation caused by Doppler Effect Sound pressure Pfar(t) in far field vs time 1, without Doppler with Doppler 7,5 Sound pressure Pfar(t) in far field vs time 1, without Doppler 5, with Doppler Pfar [ N / m^ ] 7,5 5,,5, Pfar [ N / m^ ],5, -,5-5, -,5-7,5-5, -7,5-1, Phase variation,8,9,1,11,1,13,14 Time [s],16,17,18,19,11,111,11,113 Time [s] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 76

39 Two-tone Amplitude Modulation Distortion Measurement ~ + ~ f 1 f Constant gain V Spectral Analysis E(t) Amplitude + Phase Information envelope excitation frequency AMD ENVELOPE E(t) 1 T 1 E E( t) T dt 1 in percent in db Amplitude Modulation AMD( f, f ) L AMD 1 T 1 T1 E( t) E dt E AMD( f1, f f1, f ) lg 1 ( *1% ) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 77 Contribution from Amplitude Modulation Modulation distortion (U1=1 V) 5 AM distortion (Lamd) Ldm (cumul) KLIPPEL -5-1 Total modulation Doppler db AM modulation 4*1 6*1 8*1 1 3 *1 3 4*1 3 6*1 3 8*1 3 Frequency f1 [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 78

40 Modeling of Loudspeaker Defects Voice coil vibration Air noise backplate hitting backplate Buzzing Coil rubbing Air leakage Loose particles Deterministic Process Semi- Deterministic Process Random Process reproducible Deterministic modulation of a random process Not predictable IEC Standard Project LOUDSPEAKER MEASUREMENTS, 79 Loudspeaker Defect: Buzz problem Most defects behave as a nonlinear oscillator active above a critical amplitude new mode of vibration powered and synchronized by stimulus constant output power Externally excited mass parasitic resonator spring Loose joint (Nonlinearity) vibration distortion signal one period time IEC Standard Project LOUDSPEAKER MEASUREMENTS, 8

41 How to get Symptoms of Irregular Loudspeaker Defects Generation of Stimulus Measurement of State Variables Analysis Symptoms High displacement x and/or velocity v is required Stimulus with sufficient low frequency content Defects produce only acoustical symptoms Sensitive microphone required Defects produce high frequency components Low-pass filtered stimulus and high-pass filtered microphone signal Defects are similar to ambient noise Microphone is located close to the source (near-field measurement) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 81 Measurement of Impulsive Distortion V chirp microphone high-pass ~ filter electro-acoustical system instantaneous frequency f variable cut-off frequency fc > 1f d h (t) squarer integrator peak detector Impulsive Distortion ID peak value CID crest factor MID rms value 13 Sound Pressure [db] peak value rms value Frequency Response Peak value is a sensitive measure for most irregular defects such as rub and buzz, loose particles Frequency [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 8

42 Open Question: Do we need relative characteristics for impulsive distortion? Alternatives: A: The ratio between impulsive distortion and the total sound pressure (in percent) Interpretation and use for QC is more difficult Problems with noise floor B: The ratio between impulsive distortion and mean value of the fundamental response in the rated frequency band Curve shape is identical with absolute level ID of the impulsive Applicable to QC IEC Standard Project LOUDSPEAKER MEASUREMENTS, 83 Multi-Tone Distortion (MTD) Stimulus Spectrum p(f) of microphone signal Output signal Signal lines Noise + Distortions Noise floor Signal level MTND 75 KLIPPEL 5 f Sparse multi-tone complex [db] 5 Distortion -5-5 MTD don t show the generation process in detail k k 5k 1k k Frequency [Hz] Fingerprint (good for quality control) distortion at fundamental frequencies harmonic components difference-tone components intermodulation summed tone components IEC Standard Project LOUDSPEAKER MEASUREMENTS, 84

43 Time [ms] KLIPPEL Curve k k 5k 1k Frequency [Hz] KLIPPEL KLIPPEL k k 5k 1k k Frequency [Hz] Measurement of Multitone Distortion V Fundamental Response ~ Spectral Analysis MDS(f) excitation Frequencies fi i=1,...n at frequencies f fi Sparse multi-tone complex f f f Spectrum of distortion Problem: Result depend on excitation lines selected Standard for multi-tone stimulus is required!! IEC Standard Project LOUDSPEAKER MEASUREMENTS, 85 Phase of the Excitation Tones is important!! Time signal (logarithmic sweep) Stimulus (t) vs time Amplitude spectrum Phase spectrum [V] Stimulus (t) KLIPPEL db - [V] (rms) f Time [ms] At one time there is only one frequency component!!! Harmonics only [V] Time Signal (Multi-tone complex) [db] db = 1 V Amplitude spectrum Phase spectrum At any time there are multiple frequency components interacting!!! random Intermodulation + Harmonics f IEC Standard Project LOUDSPEAKER MEASUREMENTS, 86

44 Defined Properties of Multi-tone Stimulus Objective: - ensure comparability of the results measured by different instruments - easy to generate (by software implementation) - Modification of the stimulus should be possible (bandwidth, resolution R) Amplitude spectrum N i1 f cosf t x( t) U i i i Frequencies of the sparse line spectrum logarithmically spaced 1 resolution / i R T f with i 1 N fi int start,..., T duration Starting frequency Max. Number of frequencies Pseudo-random phase ai * m i 1 mod m m a=4871, m= 31-1 and 1 =1 IEC Standard Project LOUDSPEAKER MEASUREMENTS, 87-1 Multi-tone Distortion Measurement compared with traditional THD, IMD MTND THD IMD IMD KLIPPEL - -3 MTD 1 % IMD: f1 = 5 V + 3V [db] -4 1 % MTD: 15V -5 THD: 15 V -6.1 % -7 4 Hz 6 Hz THD k k 5k 1k k Frequency [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 88

45 Summary What is new in Part A? updating measurement techniques using new stimuli (chirp, multi-tone complex, burst) ( Comprehensive ) physical evaluation of the acoustical output A single value (Umax or SPLmax) rated by the manufacturer to calibrate the rms value of the stimulus Assessing large signal performance (considering heating, nonlinearities) complete assessment of the 3D sound field radiated by the loudspeaker in an anechoic environment (near and far field) physcial measurement of impulsive distortion in the time domain to assess rub & buzz and other loudspeaker defects IEC Standard Project LOUDSPEAKER MEASUREMENTS, 89 SCOPE OF PART B ELECTICAL AND MECHANICAL MEASUREMENTS u terminals i Electro- Mechanical Conversion F coil Voice coil x coil Mechano- Acoustical Conversion p( r c ) radiator s surface x( r c ) Air Load This International Standard applies to electro-acoustical transducers and passive and active sound systems such as loudspeakers, headphones, TV-sets, multi-media devices, personal portable audio devices, automotive sound systems and professional equipment. The device under test (DUT) allows access to electrical signals at the terminals or to the mechanical signals of the transducer. The measurements use physical models describing the transduction process, modal vibration and sound radiation while considering nonlinear and time-variant properties of the DUT. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 9

46 IEC 668 PART B Electrical and Mechanical Measurement LIST OF CONTENT: Measurement of the electrical signals at the terminals (u, i) Electrical characteristics (input impedance, power, ) Efficiency, sensitivity, Lumped parameters (TS, other linear, nonlinear) Coil and magnet temperature, thermal parameters Mechanical characteristics and distributed parameters (cone) Long-term testing Time varying parameters (aging, fatigue, ) Climate impact IEC Standard Project LOUDSPEAKER MEASUREMENTS, 91 Not covered in Part B sound radiation Sound propagation Room Interaction Room Interaction Room Interaction p(r1) Audio signal Amplifier Crossover EQ u(t) x(t) Electromechanical Transducer Mechanoacoustical Transducer (Cone) Sound radiation Sound propagation Room Interaction Room Interaction Room Interaction p(r) sound field i(t) Sound radiation Sound propagation Room Interaction Room Interaction Room Interaction p(r3) Radiation and propagation of sound into 3D space Black box modeling of DSP, crossover, amplification Linear and nonlinear distortion in the output signal IEC Standard Project LOUDSPEAKER MEASUREMENTS, 9

47 Exploiting A Priory Information from Physics and Psychoacoustics Grey Model parameters Black Box state variables Input variables Structure Output variables Model 1. Structure, Relationship, Operators (e.g. equivalent circuit). Parameters (e.g. moving mass M ms,...) 3. State variables (e.g. displacement x,...) IEC Standard Project LOUDSPEAKER MEASUREMENTS, State Variables Grey Model parameters Black Box state variables Input variables Structure Output variables Displacement x(t) describe the instantaneous state vary with time depend on input variable (stimulus) large amount of information Sound pressure p(t) Temperature T(t) Current i(t) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 94

48 i R E (T V ) u L E (x) L (x) R (x) b(x)v b(x) v b(x)i C MS (x) F m (x,i). Structure of the Model Grey Model parameters Black Box state variables Input variables Structure Output variables 1. Linear vs nonlinear. Deterministic vs. stochastic 3. Static vs. dynamic 4. Lumped vs. distributed parameters shows the relationship between the state variables gives general description of the physical mechanisms depends on the scope (micro or macroscopic view) Distributed model M MS R MS Lumped Parameter Model IEC Standard Project LOUDSPEAKER MEASUREMENTS, Model Parameters Grey Model parameters Black Box state variables Input variables Structure Output variables Parameters of the model (Exogenous variables) should be independent of stimulus and measurement conditions are constantvalues or functions of one or more variables describe the properties of the particular unit Material parameters Geometry Transfer functions Thiele-Small Parameter Nonlinear Parameter IEC Standard Project LOUDSPEAKER MEASUREMENTS, 96

49 Electrical signals at the terminals (u, i) Measurement voltage and current Four wire sensing Peak and rms values Maximum Input Voltage Characteristics Rated noise voltage Short term maximum noise voltage Long term maximum noise voltage Rated sinusoidal voltage Related to maximum input power and rated impedance IEC Standard Project LOUDSPEAKER MEASUREMENTS, 97 Rated Noise Voltage Characteristic to be specified: The voltage of a noise signal, simulating normal program, which the loudspeaker can handle without any thermal or mechanical damage shall be specified by the manufacturer. Method of measurement See current standard IEC section 17.1 Applying pink noise shaped and clipped by a network under controlled climatic condition for 1 h test at a rated voltage 4 storing under normal climatic condition Testing electrical, mechanical and acoustical characteristics TEST fulfilled if there are no defects and no significant changes IEC Standard Project LOUDSPEAKER MEASUREMENTS, 98

50 Electrical Input Impedance Definition Ratio between complex voltage spectrum and current spectrum (transfer function of a linear system) Condition Sufficient spectral excitation small signal domain (distortion THD < 1 %) General Characteristis: Magnitude and phase response Transducer Characteristics Rated Impedance (based on minimum electrical impedance) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 99 Input Electrical Power Definition of Input Electrical Power Real input power Power dissipated in DC resistance R e Power dissipated at nominal impedance Maximum Input Power Characteristics Rated noise power (power handling capacity) Short term maximum noise power Long term maximum noise power Rated sinusoidal power 1 Preal ( t) u( t) i( t) d T P ( t) i ( t) R ( t) P Re nom T rms rms u ( t) Z ( t) nom Related to maximum input voltage and rated impedance e IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1

51 Electrical Lumped Parameters Small Signal Parameters (based on linear modeling) DC resistance R e (Tv) Equivalent circuit of the mechanical resonator Lossy inductance parameters (Wright, Leach, LR,...) Large Signal Parameters (based nonlinear modeling) Inductance L e (x) versus voice coil displacement Inductance L e (i) versus input current Nonlinear variation N L (x,i) of impedance representing lossy inductance L( x, i) Le ( x, i) L ( x, i) R( x, i) N L( x, i) L( x, i ) Le ( x, i ) L ( x, i ) R( x, i ) LR-Model IEC Standard Project LOUDSPEAKER MEASUREMENTS, 11 Interpretation of the Electrical Input Impedance Z e (j) Electrical Impedance at the Terminals Z e Re(TV) ( j ) ZL(j ) voltage U ( j ) I ( j ) current [Ohm] Magnitude of electric impedance Z(f) Measured Fitted KLIPPEL Z L (j) i 1 R e u Cmes Lces Res k Frequency [Hz] f s,q es,q ms f s Resonance Frequency 1 1 C M ms ms 1 1 C L mes ces Electrical Quality Factor Qes C ms R Bl e f s f C s mes R e Mechanical Quality Factor Q ms 1 f scmesr C R f ms ms s es IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1

52 Mechanical Measurements u terminals i Motor F coil Voice coil x coil Radiator p( r c ) radiator s surface x( r c ) Air Load Displacement x(t,r c ) at arbitrary point r c on the radiatior surface Non-destructive, non-contact measurement without additional load optical sensor Dynamic measurement required (full audio band) Scanning technique provides sufficient spatial resolution Forces are difficult to measure ( x, v, a) Measurement of displacement x provides dc-component generated dynamically by transducer nonlinearities IEC Standard Project LOUDSPEAKER MEASUREMENTS, 13 Voice Coil Displacement u terminals i Motor F coil Voice coil x coil Radiator F( r c ) radiator s surface x( r c ) Air Load Mean coil displacement x coil averaged over coil x coil L x( t, rc ) dr N 1 ( t) x( t, rc, L N ) n n1 Measurements at 4 points x x x Voice coil x Radiator s surface IEC Standard Project LOUDSPEAKER MEASUREMENTS, 14

53 Identification of Mechanical Parameters We need more information about the mechanical system Known Perturbation of Mechanical System (traditional technique) Direct Measurement of a Mechanical Signal Requires second measurement with additional mass or enclosure Based on impedance measurement No mechanical sensor required Time consuming Problems with mass attachment, box leakage Requires mechanical (acoustical) sensor (e.g. Laser) Only one measurement (fast) Driver in free air or in enclosure Reliable and reproducible data Can be applied to tweeters IEC Standard Project LOUDSPEAKER MEASUREMENTS, 15 Mechanical Lumped Parameters Small Signal Parameters (based on linear modeling) Transducer operated in free air (#=s), box (#=c) or vacuum (#=d) Moving mass M m# Force factor Bl(x) Stiffness K m# and compliance C m# of the suspension Creep parameters (Knudsen, Ritter) Large Signal Parameters (based on nonlinear) Force factor Bl(x) Stiffness K m# (x) and compliance C m# (x) versus voice coil displacement Mechanical resistance R m# (v) versus voice coil velocity v IEC Standard Project LOUDSPEAKER MEASUREMENTS, 16

54 Mechanics Separated from Air Load by performing a measurement in vacuum electro-dynamical transducer operated in air radiation resistance + turbulences pure mechanical elements measured in vacuum moving air mass air Cavities IEC Standard Project LOUDSPEAKER MEASUREMENTS, 17 air leaks Example: Micro-speaker (3) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 18

55 Acoustical Lumped Parameters Small Signal Parameters (based on linear modeling) Nominal effective radiation area S d (f=fs) Mechano-acoustical transduction factor Sd(f) versus frequency Acoustical compliance C AB of the enclosed air Acoustical mass of the air in the port Equivalent air volume V AS of loudspeaker compliance Large Signal Parameters (based on nonlinear modeling) Mechano-acoustical transduction factor S d (f,x) versus frequency f and displacement x corresponding to effective radiation area S d =S d (f=f s,x=) Acoustical compliance C AB (p) of the enclosed air depending on sound pressure IEC Standard Project LOUDSPEAKER MEASUREMENTS, 19 Effective Radiation Area S D Electrical domain Mechanical domain Acoustical domain Re ZL(f) qa=sdv i Bli psd qb ql U Blv Bl Mmd Cmd Rmd -1 p Sd Cab V=dx/dt Ral Map S D is an important parameter of the lumped parameter model describes coupling between mechanical and acoustical domain determines the acoustical output (sensitivity, efficiency) affects the precision of the lumped parameter measurement if the test box perturbation technique is used (M ms, Bl, K ms, C ms ) Precise Measurement of S D is important! IEC Standard Project LOUDSPEAKER MEASUREMENTS, 11

56 Effective Radiation Area Sd 1. Geometrical Definition based on surround geometry Easy to use Applicable to woofers (surround area is much smaller than cone area). Acoustical Definition Based on voice coil displacement and acoustical output Required for headphones, microspeakers IEC Standard Project LOUDSPEAKER MEASUREMENTS, 111 A Good Approximation? Calculation of Effective Radiation Area S D based on measured diameter Assumption: displacement decreases linearly over the surround displacement in constant in the inner part d i d SD d 4 i di d 4 d 4 do d i do di d i 3 d R. Small: less than 1% error if.8 d < d i ) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 11

57 Limits of the Approximation In headphone, micro-speakers, tweeters, compression drivers : Voice coil No constant displacement in the piston area No linear decay of displacement in the surround area IEC Standard Project LOUDSPEAKER MEASUREMENTS, 113 Effective Radiation Area S D Definition Radiator s surface replaced by Rigid piston v, r ) ( c D Sc S ( ) v(, r ) ds v coil c ( ) c () v coil S D () using mean voice coil velocity r coil q() v coil ( ) v(, r coil, ) d q() S ( ) Reading the absolute value at fundamental D S D resonance The effective radiation area S D is an important lumped parameter describing the surface of a rigid piston moving with the mean value of the voice coil velocity v coil and generating the same volume velocity q as the radiator s surface. The integration of the scanned velocity can cope with rocking modes and other asymmetrical vibration profiles. IEC Standard Project LOUDSPEAKER MEASUREMENTS, 114

58 Total Sound Pressure Level Frequency [Hz] Predicting the Acoustical Output at higher frequencies based on effective radiation area S D (f) Effective radiation Surface (Sd) Sd KLIPPEL Sd [cm^] using effective radiation area S D (f) as a function of frequency f f [Hz] c RAR S ( f ) d radiated sound power, (SPL in a duct) Total Sound Pressure Level SPL [db] useful for transducers having high complexity of the mechanical vibration low complexity of the acoustical system (ka < 1) e.g. (in-ear) headphones, microspeaker application IEC Standard Project LOUDSPEAKER MEASUREMENTS, 115 Relative Small Signal Parameters Transducer operated in air (#=s), enclosure (#=c) or vacuum (#=d) (nd order system) Resonance frequency f # Total quality factor Q t# Electrical quality factor Q e# Mechanical quality factor Q m# Additional Resonantor (4th order system) Resonance frequency f p of the additional resonator (port, passive radiator) Quality factor Q p of an additional mechanical or acoustical resonator IEC Standard Project LOUDSPEAKER MEASUREMENTS, 116

59 Force factor Bl vs. displacement X Bl(X) 5, 4,5 4, 3,5 3,,5, 1,5 1,,5, -1, -7,5-5, -,5,,5 5, 7,5 1, Displacement X [mm] K ms(x) N/mm K MS ( -x peak ) K MS( x peak) 1 -x peak x peak -5, -,5,,5 5, coil in x mm coil out Overview on Single-Valued Parameters derived from loudspeaker nonlinearities L(x,i) Re (Tv) Le(x,i) Fm (x,i) Mms Rms Cms(x) Sdv qp v i R(x,i) Bl(x)v Bl(x) Bl(x)i u Sd Cab pbox Ral Map Bl [N/A] Parameters at x= Nonlinear Parameters at x=x peak 1 % distortion in IMD or THD Electrical Parameters R, e L e Mechanical Parameters Bl, M, R, C, K ms ms Relative Parameters f, Q, Q, Q, f, Q s ms ts ms ms es b b Stiffness asymmetry A K Voice Coil Offset x offset from reference DUT Symmetry point in the Bl(x) curve Compliance limited displacement x C Force factor limited displacement x B IEC Standard Project LOUDSPEAKER MEASUREMENTS, 117 Characteristics derived from Nonlinear Curve Shape NONLINEAR FORCE FACTOR Force factor limited displacement X Bl generating 1 % distortion Symmetry point X sym (X ac ) depending on AC amplitude Xac Offset x off of the voice coil from a defined reference rest position NONLINEAR STIFFNESS K ms (x) Compliance limited displacement X c generating 1 % distortion Suspension asymmetry A k NONLINEAR INDUCTANCE L e (x) Inductance limited displacement XL generating 1 % IEC Standard Project LOUDSPEAKER MEASUREMENTS, 118

60 Force Factor Limited Displacement x Bl defined according IEC standard , Bl N/A 4, Bl min=8 % Bl(x=) Bl(x Bl) Steps: 1. Operate transducer in large signal domain 3,, 1,, x Bl -5, -,5,,5 5, << Coil in X mm coil out >>. Read displacement X Bl where force factor Bl(x ac ) decreases to 8 % of the value Bl(x=) at rest position IEC Standard Project LOUDSPEAKER MEASUREMENTS, 119 Compliance Limited Displacement x C defined according IEC standard 6458 C ms(x) mm/n C MS(x=).75C MS(x=) x C -5, -,5,,5 5, coil in x mm coil out Steps: 1. Operate transducer in large signal domain. Read displacement X C where compliance value C ms (x ac ) decreases to 75 % of the value C ms (x=) at rest position IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1

61 KLIPPEL << Coil in X [mm] coil out >> KLIPPEL << Coil in X [mm] coil out >> << Coil in X [mm] coil out >> Peak Displacement limited by Nonlinearities Compliance Bl(X) N/A KLIPPEL 3,5 Force Factor Cms(X) Cms (-X) mm/n KLIPPEL,9 5 Inductance Magnitude of electric impedance Z(f) x= mm x = - 4 mm x = + 4 mm KLIPPEL Doppler 3,,8,5, 1,5 1,,5 C lim = 75 %,7,6,5,4,3,,1 Bl lim = 8 % [Ohm] Z lim = 1 %, << Coil in coil out >> X [mm], << Coil in X [mm] coil >> Frequency [Hz] X c X limited by Cms(x) 1 % THD X Bl X limited by Bl(x) 1 % IMD minimum X L X limited by Le(x) 1 % IMD X D X limited by Doppler 1 % IMD X max,1% Generating not more than 1 % THD or 1 % IMD IEC Standard Project LOUDSPEAKER MEASUREMENTS, 11 How to check the Voice Coil Rest Position Force factor Bl (X) magnet pole plate magnet pole plate Induction B voice coil Bl [N/A] voice coil rest position Induction B voice coil pole piece x= displacement pole piece x=x b displacement Offset from Symmetry Point Offset from Reference Curve Force factor Bl (X) Force factor Bl(X) Symmetry Reference Bl [N/A] Symmetry point Bl [N/A] Offset Can not cope with B field asymmetry No reference curve required Important for product development Transducer diagnostics Reference curve is required (Golden Reference DUT) Coil height and gap depth are constant Important for QC end-of-line testing can cope with asymmetrical curve shape IEC Standard Project LOUDSPEAKER MEASUREMENTS, 1

62 Symmetry point X sym defined according IEC standard , Bl N/A x ac Bl max x ac x ac 4, 3,, x 1 x sym x Definition: Symmetry point x sym is the centre point between two points having the same Bl value at a distance x ac 1, Bl ( xsym xac ) Bl ( xsym xac ), -5, -,5,,5 5, << Coil in X mm coil out >> IEC Standard Project LOUDSPEAKER MEASUREMENTS, 13 Stiffness Asymmetry A K defined according IEC standard 6458 K ms(x) N/mm K MS( -x peak) K MS( x peak) -x peak x peak -5, -,5,,5 5, coil in x mm coil out Steps: 1. Operate transducer in large signal domain. Read stiffness values X ms (X peak ) and X ms (- X peak ) at maximal peak displacement 3. Calculate stiffness asymmetry according A ( x K peak K ) K MS MS ( x ( x peak peak ) K ) K MS MS ( x ( x peak peak ) 1%, ) IEC Standard Project LOUDSPEAKER MEASUREMENTS, 14

63 Electro-acoustical Efficiency 1. Efficiency in a specified frequency band (e.g. pass band) Ratio between measured electrical nominal input power P nom and measured acoustical output power P nom Pa Pnom Based on lumped parameter modeling ( Bl) R M e ms Sd c for f >f s and ka<1, radiation on one side considered. Mean Efficiency in a specified frequency band Average of efficiency measured in third-octave bands IEC Standard Project LOUDSPEAKER MEASUREMENTS, 15 Sensitivity Calculated from the frequency response and effective frequency range, as the sound pressure level produced at 1 m on the reference axis by an applied voltage of,83 V. Narrow-band sensitivity: the test signal is 1/3-octave filtered noise centered at 1 khz, or at the geometric mean of the limit frequencies of the effective frequency range if different from 1 khz. The frequency shall be stated. Broad-band sensitivity: the test signal is -octave filtered noise centered at 1 khz, or at the geometric mean of the limit frequencies of the effective frequency range if different from 1 khz. The frequency shall be stated. Reference: AES recommended practice Methods of measuring and specifying the performance of loudspeakers for professional applications Part 1: Drive units, AES -1-R IEC Standard Project LOUDSPEAKER MEASUREMENTS, 16

64 Mechanical Distributed Characteristics u terminals i Motor F coil Voice coil x coil Radiator F( r c ) radiator s surface x( r c ) Air Load X ( f, rc ) H x( f, rc ) U ( f ) Set of transfer functions between input voltage u(t) and displacement X(t,r c ) at arbitrary point r c on the radiatior surface IEC Standard Project LOUDSPEAKER MEASUREMENTS, 17 Mechanical and Acoustical Characteristics derived from Displacement Transfer function Scanning Vibrometer X ( f, rc ) H x( f, rc ) U ( f ) Diagnostics On Cone vibration Predicted Sound Pressure Level SPL(f,r) Accumulated Acceleration level AAL(f,r) Relative Rocking Level RRL n of the nth rocking mode Lowest radiation efficiency η rad Acoustical cancellation distance L can Cone break-up frequency f break-up IEC Standard Project LOUDSPEAKER MEASUREMENTS, 18

65 178,1 Hz Accumulated Acceleration a( r c ) a( r c ) Integral of the absolute value of weighted cone acceleration p aa ( r ) W a( r ) a Sc cone s surface S c c ds c paa( ra ) AAL a p ( r ) log o db 9 db Accumulated Acceleration Level Rigid body modes with reference sound pressure p and a useful scaling W to comparable ALL with SPL output W r r a c ra r c r a f [Hz] IEC Standard Project LOUDSPEAKER MEASUREMENTS, 19 Accumulated Acceleration Level (AAL) 9 db AAL Rigid body modes Rigid body mode Acceleration level SPL Total sound Pressure level f [Hz] describes total mechanical vibration is comparable with SPL is never smaller than SPL predicts potential acoustical output neglects acoustical cancellation is identical with SPL for a rigid body mode IEC Standard Project LOUDSPEAKER MEASUREMENTS, 13

66 Characteristics for Diagnostics Single-valued parameter derived from AAL (5) How critical is the rocking mode? db Total AAL RRL Circular Component (AAL) Quadrature Component (AAL) f rock 1 1 f [Hz] Woofer A with paper cone 1. Search for first maximum in quadrature component in AALon-axis!. Determine the relative rocking level RRL defined by RRL(f rock )=AAL quad -AAL in 3. rocking mode is negligible if RRL< -5dB IEC Standard Project LOUDSPEAKER MEASUREMENTS, 131 Thermal Measurements Increase of mean voice coil temperature ΔT v based on monitored DC voice coil resistance R e (t) Increase of magnet temperature ΔT m IEC Standard Project LOUDSPEAKER MEASUREMENTS, 13