Measurement of Equivalent Input Distortion AN 20

Transcription

1 Measuremet of Equivalet Iput Distortio AN 2 Applicatio Note to the R&D SYSTEM Traditioal measuremets of harmoic distortio performed o loudspeakers reveal ot oly the symptoms of the oliearities but also the effect of liear loudspeaker parameters, radiatio ito the soud field ad the iteractios with the room. Thus, the iterpretatio ad compariso of results are difficult if the acoustical coditios chage. This problem ca be solved by trasformig the harmoic distortio measured i the soud pressure ito equivalet distortio at the voltage iput. The equivalet distortio is almost idepedet of the radiatio, soud propagatio, room acoustics ad the liear properties of the sesor (Laser, microphoe. The equivalet harmoic distortio are ot oly a miimal set of iformatio but make it possible to predict the traditioal harmoic distortio accordig (IEC stadard at ay poit r i the soud field by performig a simple filterig with a liear trasfer fuctio. CONTENTS: Modelig... 2 Performig the Measuremet... 5 Setup Parameters for the TRF Module... 6 Example... 6 Glossary of Symbols... 9 More Iformatio... 9 updated April 4, 212 Klippel GmbH Medelssohallee Dresde, Germay ifo@klippel.de TEL: FAX:

2 AN 2 Equivalet Harmoic Distortio Modelig Loudspeaker Model u H(f,r 1 H(f,r 2 p(r 1 soud field p(r 2 u D Noliear System H(f,r 3 p(r 3 The loudspeaker may be cosidered as a system havig a sigle iput at the termials ad multiple outputs i the soud field. The small amplitudes the sigal flow betwee iput U 1 ad output P(r at poit r may be represeted by a liear trasfer fuctio H(f,r. I the large sigal domai oliearities iheret i the loudspeaker produce distortio. Most of the oliearities are located i the electrical ad mechaical domai where the sigal flow is still oe-dimesioal. I this case the oliearities ca be lumped ito oe system geeratig distortio U D which are added to the iput sigal U ad the dispersed via the liear systems H(f, r to ay poit i the far field. Measuremet at oe poit i the soud field Harmoic distortio are usually measured oly at few poits i the soud field to make practical performace feasible. After performig a spectral aalysis the reposes of the fudametal compoet P 1 (f ad the 2d-harmoic compoet P 2 (2f ad higher-order harmoics are measured versus excitatio frequecy f. Geerator u(t Noliear System p(t U(f P(f P 1 P 2 P3 f 1 f f 1 2f 1 3f 1 f Applicatio Note R&D SYSTEM page 2

3 Equivalet Harmoic Distortio AN 2 Impulse respose 1 ZOOM Measured Impulse respose H(f= / Stimulus Widowed,75 ZOOM Noliear Impulse respose H(f= / Stimulus Measured Widowed,5 5,25 [V / V] -5 [V / V], -,25 -,5 -, , -1, left:-227. Time [ms] right: left: Time [ms] right: The Trasfer Fuctio Module (TRF uses a siusoidal sweep as excitatio sigal to separate the liear impulse respose (-.2 s from the oliear resposes of 2 d, 3 rd ad higher (appear as peaks betwee secods. Due to the diffuse field i the listeig room both the liear ad the oliear resposes have a relatively log decay time. Thus, higher-order resposes iterfere with each other ad ca ot completely be separated by usig time widowig. Amplitude Respose Fudametal + Harmoic distortio compoets Fudametal 2d Harmoic 3rd Harmoic Applyig a to the widowed liear ad oliear impulse resposes leads to the magitude of the fudametal P 1 (f, 2 d -order compoet P 2 (2f ad the 3 rd -order compoet P 3 (3f. At 1 m distace from the loudspeaker the diffuse field i the room geerates sigificat variatio of the soud pressure values k Relative Harmoic Distortio i the output sigal (IEC stadard 8 7 Harmoic distortio (relative 2d Harmoic 3rd Harmoic Accordig to the IEC 6268 the th-order harmoic distortio compoets geerated by a excitatio frequecy f are referred to the rms amplitude of the total output sigal P(f ad may be expressed i percet d ( p P P t 1%, > 1 with T 1 2 Pt pt ( dt T (1 Applicatio Note R&D SYSTEM page 3

4 AN 2 Equivalet Harmoic Distortio or i db ( p ( p d 2log 1% L, > 1. (2 This measure reflects the iteractios betwee loudspeaker ad room at the poit r 1 i the soud field. The sparse desity of room modes geerates fluctuatios which vary with the measuremet positio r 1. Equivalet Iput Distortio U 1, U 2, U 3 u x H(f,r p(r H(f,r -1 u+u D u D Noliear System P 1, P 2, P 3 U 1, U 2, U 3 By performig a filterig of the iput measured soud pressure sigal with the iverse system fuctio H(f -1, the effect of the radiatio ad room iteractios ca be compesated ad the equivalet harmoic distortio U P Pf ( Hf ( Hf (, > 1 (3 ca be calculated. The trasfer fuctio ca be calculated by P1( f, r1 H( f, r 1 U ( f 1 (4 usig the fudametal P 1 (f ad the iput voltage U 1 (f Noliear Impulse respose H(f= / Stimulus Measured Widowed 7,5 ZOOM 5, 2,5, Noliear Impulse respose H(f= / Stimulus Measured Widowed [V / V] [V / V] -2,5-5, -7,5-1, left:-227. Time [ms] right: left:-227. Time [ms] right: Thus, the liear impulse respose becomes close to a ideal Dirac impulse ad the oliear impulse resposes become much shorter because the effect of the room respose is removed. Applicatio Note R&D SYSTEM page 4

5 Equivalet Harmoic Distortio AN 2 Absolute Equivalet Distortio Compoets (voltage Fudametal + Harmoic distortio compoets Fudametal 2d Harmoic 3rd Harmoic k The "iverse filterig" of the measured fudametal ad harmoic resposes with the liear trasfer fuctio H(f,r 1 geerates to the almost costat liear trasfer fuctio U 1 (f ad 2 d -order compoet U 2 (2f ad the 3 rd -order compoet U 3 (3f. These curves describe the oliear distortio at the source ad are almost idepedet o the radiatio, soud propagatio, room iteractios ad the properties of the sesor. Relative Distortio (Percet 6 5 Harmoic distortio (relative 2d Harmoic 3rd Harmoic I a aalogue way as i the IEC stadard the amplitude of the th-order harmoic distortio is referred to the rms value of the total voltage sigal d ( u U( f ( f 1% U ( f U (2 f U (3 f (5 Assessig loudspeaker system The large sigal performace of loudspeakers ca be more easily assessed by comparig the equivalet harmoic distortio i the iput voltage because the ifluece of the acoustical eviromet (room, distace ad the sesor is miimal. It is also possible to predict the harmoic distortio at a preferred listeig positio (drivers seat based o equivalet harmoic distortio (represetig the loudspeaker ad the liear trasfer fuctio H(f describig the room acoustics. Performig the Measuremet Requiremets Setup Preparatio The followig hardware ad software is required Distortio Aalyzer PC Software modules (TRF, db-lab Sesor microphoe (or laser Power Amplifier (set gai to maximum Coect the termials of the driver with SPEAKER 1. Switch the power amplifier betwee OUT1 ad coector AMPLIFIER. Coect the microphoe to iput IN1,or coect a laser head to the coector LASER ad adjust the laser beam to a white dot o the diaphragm. 1. Create a ew database 2. Ope the database withi db-lab 3. Create a ew object DRIVER based o the template Equivalet Iput Dist. AN 2. Applicatio Note R&D SYSTEM page 5

6 AN 2 Measuremet Equivalet Harmoic Distortio 1. Adjust sesor. Whe usig a microphoe prefer a measuremet poit givig sufficiet SNR (earfield measuremet or distace < 1m is preferable!!. 2. Start the measuremet "1. TRF Small Sigal Measuremet". 3. Ope Property Page IM/EXPORT i the measuremet "1. TRF Small Sigal Measuremet" ad select the trasfer fuctio "Fudametal + Total Phase". Press the butto "Export to Clipboard". Select the measuremet "2. TRF Equivalet Distortio". Ope the Property Page PROCESSING ad press butto IMPORT uder Referece". Press the butto "From Clipboard" to trasfer data. 4. Ope the Property Page STIMULUS of the measuremet "2. TRF Equivalet Distortio". Adjust the voltage U of the stimulus i dbu accordig to the permissible load. Start the measuremet. 5. Ope the Result Widow "Eergy-Time Curve" ad adjust the marker of the time widow to separate the liear respose from the oliear impulse resposes (small spikes at later times. Setup Parameters for the TRF Module Template Create a ew Object, usig the object template Equivalet Iput Dist. AN 2 i db- Lab. If this database is ot available you may geerate a object TRF Equivalet Distortio AN 2 based o the geeral TRF module. 1. Geerate a object Equivalet Iput Dist. AN Assig to the object a measuremet "1. TRF Small Sigal Measuremet" ad "2. TRF Equivalet Distortio" based o the default TRF measuremet. 3. Select "1. TRF Small Sigal Measuremet". Ope the PP STIMULUS ad set startig frequecy Fmi= 2 Hz ad Fmax =2kHz. Set resolutio to.73 Hz. Set Voltage to dbu at SPEAKER 1 Termials. Select 16 times averagig. Ope PP INPUT, select IN1 at Chael 1 ad disable Chael 2. Ope PP Processig ad select No Widow. 4. Select "2. TRF Equivalet Distortio ". Ope the PP STIMULUS ad set startig frequecy Fmi= 2 Hz ad Fmax =2kHz. Set resolutio to.73 Hz. Set Voltage to 12 dbu or higher at SPEAKER 1 Termials. Ope PP INPUT, select IN1 at Chael 1 ad disable Chael 2. Ope PP Processig ad use default settig of Rectagular Widow. You may also modify the setup parameters accordig to your eeds. Example Liear Amplitude Respose 11 fudametal soud pressure 1 m distace 6 cm distace 3 cm distace earfield 1 db - [mm] k The soud pressure respose of a loudspeaker was measured at four positios (earfield, 3 cm, 6 cm ad 1 m distace. With risig distace the diffuse field i the room causes sigificat variatios i the respose (> 2 db variatio at 5 Hz. Applicatio Note R&D SYSTEM page 6

7 Equivalet Harmoic Distortio AN 2 Harmoic Distortio Compoets 9 8 2d harmoic absolute 1 m distace 6 cm distace 3 cm distace earfield rd harmoics absolute 1 m distace 6 cm distace 3 cm distace earfield k k The absolute soud pressure respose of the 2 d ad 3 rd -order harmoics vary sigificatly withi the soud field. Relative Harmoic distortio i soud pressure d harmoic distortio (IEC 3 cm distace 1 m distace earfield 6 cm distace 8 7 3rd harmoics (IEC stadard 3 cm distace earfield 6 cm distace 1 m distace Calculatig the 2 d ad 3 rd -order harmoic distortio i percet accordig to the stadard IEC 6268 the variatio of the fudametal are cosidered i the assessmet of the distortio. However, there are substatial variatios due to the iterfereces with the room modes. Applicatio Note R&D SYSTEM page 7

8 AN 2 Equivalet harmoic distortio i voltage -1 Equivalet Harmoic Distortio 2d harmoics i voltage 1 m distace 6 cm distace 3 cm distace earfield k 3rd harmoic distortio i voltage -5 earfield 3 cm 6 cm 1 m distace k The 2 d ad 3 rd -order harmoics measured at the poit r i are filtered with the iverse liear trasfer fuctio H(f,r i -1 to geerate the equivalet harmoic distortio i the soud pressure iput. If the oliearities are located i the electrical, mechaical ad acoustical part before the soud disperses the equivalet harmoic distortio are idepedet of radiatio, soud propagatio ad room iterfereces. Equivalet harmoic distortio i voltage iput (i percet 5, 4,5 4, 3,5 3, 2d order harmoics i voltage 1 m distace 3 cm distace 6 cm distace earfield 2,5 2, 1,5 1,,5, rd harmoic i voltage 6 cm distace 3 cm distace 1 m distace earfield The equivalet harmoic distortio may be referred to the total voltage sigal ad expressed i percet. Applicatio Note R&D SYSTEM page 8

9 Glossary of Symbols Equivalet Harmoic Distortio AN 2 H(f,r liear trasfer fuctio betwee voltage ad soud pressure at poit r u voltage at termials p(r soud pressure at poit r P 1 (f fudametal compoet i soud pressure versus excitatio frequecy f P 2 (2f 2 d -order harmoic compoet i soud pressure versus excitatio frequecy f P 3 (3f 3 rd -order harmoic compoet i soud pressure versus excitatio frequecy f U 1 (f voltage at termials versus excitatio frequecy f U 2 (2f equivalet 2 d -order harmoic compoet i voltage versus excitatio frequecy f U 3 (3f equivalet 3 rd -order harmoic compoet i voltage versus excitatio frequecy f G 2 (f 1,f 2 2 d -order system fuctio geeratig 2 d -order harmoics G 3 (f 1,f 2,f 3 3 rd -order system fuctio geeratig 3 rd -order harmoics d (p (f th-order harmoic distortio i the soud pressure output sigal i percet (accordig to IEC 6268 d (u (f equivalet th-order harmoic distortio i the iput voltage i percet More Iformatio Literature Related Specificatio Software W. Klippel, Measuremet of Equivalet Iput Distortio Preseted at the 115th Covetio of the Audio Eg. Society, 23 October 1 13, New York, USA, Preprit TRF, S7 User Maual for the R&D SYSTEM. Klippel GmbH Medelssohallee Dresde, Germay ifo@klippel.de updated April 4, 212 TEL: FAX: Applicatio Note R&D SYSTEM page 9