Division of Physical Metrology Center for Fluid Flow & Acoustics Korea Research Institute of Science & Standards Hyu-Sang Kwon & Wan-Ho Cho

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Ami Heath
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Fluid Flow & Acoustics Korea Research Institute

1 Report on the Bilateral Comparison between KRISS & NIM Division of Physical Metrology Center for Fluid Flow & Acoustics Korea Research Institute of Science & Standards Hyu-Sang Kwon & Wan-Ho Cho

Introduction Artifact & Method No Artifact Supply institute Type Calibration range Measurement method 1 B&K 418 2341431 KRISS LS2p 1 ~ 31.5 khz, 4 khz IEC 6194-3 2 B&K 418 266992 NIM LS2p 1 ~ 31.

2 Introduction Artifact & Method No Artifact Supply institute Type Calibration range Measurement method 1 B&K KRISS LS2p 1 ~ 31.5 khz, 4 khz IEC B&K NIM LS2p 1 ~ 31.5 khz, 4 khz IEC Lason Davis A974 KRISS Class 1 ~ 2 khz Sequential comparison Time table of comparison No 1 2 Calibration laboratory KRISS Artifact Starting date Finishing date B&K Lason Davis A974 6-August August 212 NIM B&K KRISS B&K NIM B&K Lason Davis A974 1-September October October KRISS B&K Lason Davis A974 NIM B&K October November November 212 Mid. December 212 2

3 Comparison planning Dr. Wan-Ho Cho Visiting Sep. 6~8 212 KRISS Bilateral comparison NIM Visiting Nov. 11~13 Mr. Niu Feng Discussion on going for 213 Other APMP Members APMP comparison or study on free-field mic. calibration CC comparison on free-field mic. calibration DFM CENAM 3

NEXUS (Modified) Switch Box Preamp. (B&K 2673) CH. 2 AUX B&K PULSE 356-C d=1 mm Microphone (B&K 418) PC LAN CH.

4 Measurement in KRISS Measurement System Renewal for FF Calibration Introducing the automation system DFM-B&K free-field reciprocity calibration system Signal processing procedure will be compared with KRISS method Position controller PC B&K NEXUS (Modified) Switch Box Preamp. (B&K 2673) CH. 2 AUX B&K PULSE 356-C d=1 mm Microphone (B&K 418) PC LAN CH. 1 Generator Transmitter (B&K ZE796) IEEE B&K 5998 Pressure Probe (Testo ) RH Probe (Testo ) TESTO 4 Monitoring device 4

amplifier (B&K 276) Sound level meter Sine sweep generator (B&K 149)

5 Measurement in KRISS System configuration for sound level meter Monitor TESTO 4 Monitoring device Voltmeter (Keithley 2) Power amplifier (B&K 276) Sound level meter Sine sweep generator (B&K 149) Reference loudspeaker Microphone (B&K 4165) Preamp. (B&K 2636) Voltmeter (Keithley 2) 5

6 Measurement result Free-field sensitivity of LS2 microphone Sensitivity (dbref. 1 V/Pa) B&K B&K Free-field correction (KRISS) Measured by KRISS Measured by NIM 1 1 Frequency (Hz) Sensitivity (db ref. 1 V/Pa) Free-field correction (KRISS) Measured by KRISS Measured by NIM 1 1 Frequency (Hz) 6

Highlights on Free-field Reciprocity Calibration Development of Post Processing Method (Published on JASA 213) Applying low-pass filtering for stabilizing impulse response Low frequency limit of

7 Highlights on Free-field Reciprocity Calibration Development of Post Processing Method (Published on JASA 213) Applying low-pass filtering for stabilizing impulse response Low frequency limit of calibration can be extended below 2 khz without fitting LPF Steady state response Filtered FRF ifft Impulse response (Filtered) Direct path IR (Filtered) Time gating FFT Direct path FRF (Filtered) Inverse LPF Direct path response Tukey Rectangular Chebyshev Minimizing ripple Sensitivity (db) Simulation result with equivalent circuit model Frequency (Hz) Theoratical value Rectangular Tukey Chebyshev Calibration result of LS2 mic. Sensitivity (db) Rectangular -3 Tukey -31 Chebshev Pressure sensitivity -32 with free-field correction A Frequency (Hz) Sensitivity (db) -28 Rectangular -29 Tukey -3 Chebshev -31 Pressure sensitivity -32 with free-field correction Frequency (Hz) Sensitivity (db) Rectangular Tukey -3 Chebshev Pressure sensitivity -33 with free-field -34 correction M1 M2 M3 Frequency (Hz) 7

8 Reciprocity calibration of microphone Based on the electro-acoustic reciprocity principle Mic. 1 Mic. 2 U 2 i 1 d 12 The ratio of the volume velocity in the cavity to the input current when used as a sound source is equal to the ratio of the open-circuit voltage across the electrical terminals to the sound pressure acting on the diaphragm when used as a receiver Z Electrical impedance vs. acoustical impedance e, tr e p u Z R R T R = = = a, tr it ut it pr e M T M R Z = Z M M e,12 a, Z = Z M M e,23 a, Z = Z M M e,31 a, M 2 1 Z e = e i Z a = Ze,12 Ze,31 Za,23 = Z Z Z e,23 a,12 a,31 p u 8

Measurements of transfer impedances Electrical transfer impedance Difficult to measure the electric current directly Use of well defined passive device Cancellation of freq.

9 Measurements of transfer impedances Electrical transfer impedance Difficult to measure the electric current directly Use of well defined passive device Cancellation of freq. using capacitance Z e i R R e, tr = = = T T e R e, tr j 2π fc e j 2π fc transfer voltage ratio well-defined capacitor Measurement of voltage ratio Pressure cal. vs. free-field cal. Use of wave propagation inside of a coupler use of a point source model free-field acoustical transfer impedance the radiated sound field by a point source density of medium effects of scattering Z a, tr = Acoustical transfer impedance ρf j 2 d e jkd distance between microphones Z = (, θ ) a M f M p S f Za + Za, r radiation impedance Compensation of preamplifier gain e e e G e G e R = = = e e e G e G e 1, acoust 2, elect R, oc 1 ref 2 R, oc 2, acoust 1, elect T, oc 2 ref 1 T, oc LS1 LS2 M 1 d12d31 R12R31 jk( d12 + d31 d23 e ) = πρ f C d23 R correction factor C f = 1log 1 M M f p 2 9

Measurements of Voltage ratio (B&K 5998).539 V 2.319 V 3.16 V 3.

10 Measurements of Voltage ratio (B&K 5998).539 V V 3.16 V 3.16 V V V 1.56 V 1.56 V r U r U j i U = U j i exp j( ϕ ϕ ) j i

11 Freq. response of voltage ratio Used microphones ¼ B&K 4133 Distance between microphones.1 m Inverse FFT Directly propagated wave Reflected wave Sine sweep test Cross-talk

12 Time selective windowing tech. Inverse FFT Windowing Measured FRF of voltage ratio Impulse response FFT Compensated FRF of voltage ratio Time gated impulse response

13 Signal analysis P O V I V O P I V I H 1 H 2 H P,direct H 3 H 4 V O H P,1 H P,2 Amp. or signal cond. (Electric) Electric- Acoustic Acoustic Acoustic -Electric Amp. or signal cond. (Electric) Freq. X Time *

14 Compensating filter P O V I P I V O Separable in time domain.. V I H direct V O H reflect 1 H reflect 2

15 Compensating filter P O V I P I V O Compact signals in time domain for better separation.. V I H comp H direct V O H reflect 1 H reflect 2

16 Improved signal processing Smoothing filtering (Hann window) Reduction of leakage Inverse FFT

17 Freq. windowing functions No (boxcar) Tukey Chebyshev 17

Simulation (2 khz) Z 1 ds = + jωlds jωc + ds R ds Parameter Value C ds (Diaphragm system compliance) 2.83 1-13 m 5 /N L ds (Diaphragm system mass) 896 kg/m 4 R ds (Diaphragm system resistance) 56.

18 Simulation (2 khz) Z 1 ds = + jωlds jωc + ds R ds Parameter Value C ds (Diaphragm system compliance) m 5 /N L ds (Diaphragm system mass) 896 kg/m 4 R ds (Diaphragm system resistance) Ns/m x 111 SNR=2dB Mic. sensitivity Mic. sensitivity x Voltage ratio (cross-talk) x True value No window Tukey window Chebyshev window Mic. sensitivity 2 x Voltage ratio, Compensated 5 x x 11 No (boxcar) Tukey Estimation error (db) x x x 11 Chebyshev Impulse response

19 Developed S/W (MATLAB) 19

20 Measured results Sensitivity (db) Sensitivity (db) Freq. (Hz) Freq. (Hz) Sensitivity (db) Sensitivity (db) Sensitivity (db) Sensitivity (db) Freq. (Hz) Freq. (Hz) w/o filter (Hz) Freq. (Hz) Chebyshev filter 2

25 1 9 1/u.2.15.1.5 1 khz 2 khz 4 khz 8 khz 1 khz Acoustic Center(mm) 8 7 6 5 4 3 2 1

21 Acoustic center measurement Output voltage of probe microphone u 1/d t d t = d + d a : True distance i 1 d: Physical distance d a : Acoustic center 1/u = md + md a = md + b /u khz 2 khz 4 khz 8 khz 1 khz Acoustic Center(mm) IEC 측정값 Distance(mm) Frequency(kHz)

22 Thank you!

Report on Research Activities of KRISS CCAUV

Report on Research Activities of KRISS CCAUV Division of Physical Metrology Center for Fluid Flow & Korea Research Institute of Standards and Science KRISS - CCAUV Dr. H. S. Kwon hyusang@kriss.re.kr (Air,