IMPULSE RESPONSE IMPULSE RESPONSE

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Angelo Farina Angelo Farina

Industrial Engineering Dept.

University of Parma

University of Parma -- ITALYITALY HTTP://

HTTP://www.angelofarina.itwww.angelofarina.it

IMPULSE RESPONSE IMPULSE RESPONSE

MEASUREMENTS MEASUREMENTS

Nordic Sound Symposium XXIII Bolkesjø, Norway

Sept. 27 - Sept. 30, 2007

University of Parma

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30.09.2007Angelo FarinaAngelo Farina UNIPR |UNIPR |All Rights ReservedAll Rights Reserved||ConfidentialConfidential

 Traditional time-domain measurements with pulsive sounds and omnidirectional transducers

 Electroacoustical measurements employing special computer- based hardware, a loudspeaker and an omnidirectional

microphone

Time Line

The The PastPast

The PresentThe Present

 Electroacoustical measurements employing standard sound cards, 2 or more loudspeakers and multiple microphones (2 to 8)

The Future The Future

 Microphone arrays for capturing high-order spatial information

 Artificial sound sources employing a dense array of

loudspeakers, capable of synthesizing the directivity pattern of any real-world source

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Basic

Basic sound sound propagation propagation scheme scheme

Omnidirectional receiver Direct Sound

Reflected Sound

Point Source

Direct Sound

Reverberant tail

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The The Past Past

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Traditional

Traditional measurement measurement methods methods

 Pulsive sourcesPulsive sources: : ballonsballons, , blankblank pistolpistol

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Example

Example of of a a pulsive pulsive impulse impulse response response

Note the

Note the bellbell-shaped-shaped spectrumspectrum

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Test

Test with with binaural binaural microphones microphones

 Cheap Cheap electret mikeselectret mikes in the in the earear ductsducts

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Loudspeaker

Loudspeaker as as sound source sound source

A A loudspeaker loudspeaker is is fed fed with with a a special special test test signal signal x(t), x(t), while

while a a microphonemicrophone recordsrecords the the roomroom responseresponse

A A proper proper deconvolution deconvolution technique technique is is required required for for retrieving

retrieving the the impulse impulse response response h(t) h(t) from from the the recorded

recorded signal signal y(t)y(t)

Portable PC with 4- channels sound board Original Room

SoundField Microphone

B- format 4- channels signal

(WXYZ)

Measurement of B- format Impulse Responses

MLS excitation signal

Portable PC with additional sound card Room

Microphone

Output signal y Measurement of Room

Impulse Response

MLS test signal x Loudspeaker

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Not-linear, time variant

system K[x(t)]

Noise n(t)

input x(t)

+ output y(t) linear system

w(t)h(t) distorted signal

w(t)

Measurement process

 The desidered result is the linear impulse response of the acoustic propagation h(t). It can be recovered by knowing the test signal x(t) and the measured system output y(t).

 It is necessary to exclude the effect of the not-linear part K and of the background noise n(t).

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Electroacoustical

Electroacoustical methods methods

 Different Different types oftypes of test test signalssignals havehave beenbeen developeddeveloped, , providing

providing goodgood immunityimmunity to background to background noisenoise and easy and easy deconvolution

deconvolution of the of the impulse responseimpulse response::

►MLS (Maximum Lenght Sequence, pseudo-random white noise)

►TDS (Time Delay Spectrometry, which basically is simply a linear sine sweep, also known in Japan as “stretched pulse” and in Europe as “chirp”)

►ESS (Exponential Sine Sweep)

 Each ofEach of thesethese test test signalssignals can can bebe employedemployed withwith differentdifferent deconvolution

deconvolution techniques, techniques, resultingresulting in a in a numbernumber of of

“different“different”” measurementmeasurement methodsmethods

 Due Due to to theoretical theoretical and and practical practical considerationsconsiderations, the , the preference

preference is is nowadays nowadays generally generally oriented oriented for for the the usage usage of of ESS ESS with with notnot--circular circular deconvolutiondeconvolution

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The first MLS

The first MLS apparatus apparatus - - MLSSA MLSSA

 MLSSA wasMLSSA was the first the first apparatusapparatus forfor measuringmeasuring impulseimpulse responses withresponseswith MLSMLS

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More

More recently recently - - the CLIO system the CLIO system

 The ItalianThe Italian--mademade CLIO system CLIO system hashas supersededsuperseded MLSSA MLSSA for mostformost electroacoustics

electroacoustics applicationsapplications ((measurementmeasurement ofof loudspeakersloudspeakers, , qualityquality controlcontrol))

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The first TDS

The first TDS apparatus apparatus - - TEF TEF

 Techron Techron TEF 10 wasTEF 10 was the first the first apparatusapparatus for measuringformeasuring impulseimpulse responses responses with TDSwith TDS

 Subsequent versionsSubsequent versions (TEF 20, TEF 25) (TEF 20, TEF 25) alsoalso supportsupport MLSMLS

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The The Present Present

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Edirol FA-101 Firewire sound

card:

10 in / 10 out 24 bit, 192 kHz ASIO and WDM

Today’s Hardware: PC and audio interface

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Hardware:

Hardware: loudspeaker loudspeaker & & microphonemicrophone

Dodechaedron loudspeaker

Soundfield microphone

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Aurora Plugins

Generate MLS Generate MLS Deconvolve DeconvolveMLSMLS Generate Sweep Generate Sweep Deconvolve

DeconvolveSweepSweep Convolution

Convolution Kirkeby

KirkebyInverse FilterInverse Filter Speech

Speech TransmTransm. Index. Index

The first ESS system - AURORA

 Aurora wasAurora was the first measurementthe first measurement system system based on standard sound based on standard sound cardscards and employingand employing the the ExponentialExponential SineSine SweepSweep methodmethod (1998)(1998)

 It alsoIt also worksworks withwith traditionaltraditional TDS and MLS TDS and MLS methodsmethods, so the , so the comparisoncomparison can becan be mademade employingemploying exactlyexactly the the samesame hardwarehardware

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MLS MLS method method

 X(t) isX(t) is a a periodicperiodic binarybinary signal

signal obtainedobtained withwith a a suitable

suitable shiftshift--registerregister, , configured

configured forfor maximum

maximum lenghtlenght of the of the period

period..

N stages

XOR k stages

x’(n)

1 2

L  ^N 

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MLS MLS deconvolution deconvolution

 The The rere--recorded recorded signal signal y(i) y(i) is is crosscross--correlated correlated with with the the excitation

excitation signalsignal thanksthanks toto a fast a fast HadamardHadamard transformtransform. The . The result

result isis the the required required impulse responseimpulse response h(i), h(i), ifif the system the system was linearwas linear and and timetime--invariantinvariant

y 1 M

L h 1

~



 



 Where M is the Hadamard matrix, obtained by permutation of the original MLS sequence m(i)

 

 ⁱ ^j ² ^mod ^L  ¹

m )

j , i (

M ~    

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MLS MLS measurement measurement procedure procedure

playplay

record record

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Example

Example of of a MLS a MLS impulse impulse response response

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Exponential Sine Sweep method

 x(t) is a band-limited sinusoidal sweep signal,

which frequency is varied exponentially with time, starting at f₁and ending at f₂.

 

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

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 



 

1 e

f ln f

T f

sin 2 )

t (

x

¹

2

f ln f T

t

1

2

1

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Test Signal – x(t)

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Measured signal - y(t)

 The The not-not-linearlinear behaviourbehaviour ofof the loudspeakerthe loudspeaker causescauses manymany harmonics toharmonics to appearappear

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Inverse Filter – z(t)

The The deconvolution deconvolution of of the IR isthe IR is obtained obtained convolving convolving the the measured measured signal signal y(t)

y(t) with the inverse with the inverse filterfilter z(t) [z(t) [equalizedequalized, time, time--reversedreversed x(t)]x(t)]

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Deconvolution of Exponential Sine Sweep

The “time reversal mirror” technique is employed: the system’s impulse response is obtained by convolving the measured signal y(t) with the time-reversal of the test signal x(-t). As the log sine sweep does not have a “white” spectrum, proper equalization is required

Test Signal x(t) Inverse Filter z(t)

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Result of the deconvolution

The last impulse response is the linear one, the preceding are the harmonics distortion products of various orders

2° 1°

5° 3°

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IR IR Selection Selection

 After the After the sequencesequence ofof impulseimpulse responses hasresponseshas beenbeen obtainedobtained, , itit isis possiblepossible toto select

select and and extractextract just onejust one of themofthem (the 1(the 1°°--order order -- Linear in Linear in thisthis exampleexample):):

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Example

Example of of an an ESS ESS impulse impulse response response

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Maximum Length Sequence vs. Exp. Sine Sweep

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Post processing of impulse responses

 A specialA special pluginplugin hashas beenbeen developeddeveloped forfor the the computationcomputation ofof STI accordingSTI according to to IEC-IEC-EN 60268EN 60268--16:200316:2003

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Post processing of impulse responses

 A specialA special pluginplugin hashas beenbeen developeddeveloped forfor performingperforming analysisanalysis of acousticalofacoustical parameters

parameters accordingaccording to ISOtoISO-3382-3382

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The new AQT plugin for Audition

 The newThe new module ismodule is stillstill under under developmentdevelopment and and willwill allowallow forfor veryvery fast fast computation

computation ofof the AQT (Dynamicthe AQT (Dynamic FrequencyFrequency ResponseResponse) curve from) curve from withinwithin Adobe

Adobe AuditionAudition

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Spatial

Spatial analysis analysis by by directive directive microphonesmicrophones

 The initialThe initial approachapproach waswas toto useuse directivedirective microphonesmicrophones forfor gatheringgathering some some information

information aboutabout the the spatialspatial propertiesproperties ofof the sound the sound field “field“asas perceivedperceived byby the listenerthe listener””

 Two apparentlyTwo apparently differentdifferent approachesapproaches emergedemerged: : binauralbinaural dummydummy headsheads and and pressure

pressure-velocity-velocity microphonesmicrophones::

Binaural Binaural microphone

microphone (left(left))

and and

variable

variable--directivitydirectivity microphone

microphone (right)(right)

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IACC

IACC “ “ objective objective ” ” spatial spatial parameter parameter

 It wasIt was attemptedattempted toto “quantify“quantify”” the the ““spatialityspatiality”” ofof a a roomroom byby meansmeans ofof

“objective“objective”” parametersparameters, , based on 2based on 2--channels channels impulseimpulse responsesresponses measuredmeasured with directivewith directive microphonesmicrophones

 The The most famousmost famous ““spatialspatial”” parameterparameter isis IACC (Inter IACC (Inter AuralAural Cross Cross Correlation

Correlation), ), basedbased on on binauralbinaural IR IR measurementsmeasurements

LeftLeft

Right Right

80 ms 80 ms

pp_L_L(())

pp_R_R(())

 

   

  ^t ^t  ¹^ms^... ¹^ms

Max IACC

d t p

d p

d t p

p

t _E

ms 80

0 2R ms

80

0 2L

ms 80

0

R L



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



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

















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LF LF “ “ objective objective ” ” spatial spatial parameter parameter

 Other “Other “spatialspatial”” parametersparameters are the are the LateralLateral Energy Energy ratioratio LFLF

 This isThis is defineddefined fromfrom a 2a 2--channels channels impulseimpulse responseresponse, the first , the first channelchannel isis a a standard

standard omniomni microphone, the microphone, the secondsecond channelchannel isis a a ““figurefigure--ofof--eighteight” ” microphone

microphone::

Figure Figure

of of 88 OmniOmni

 



 













 80ms

ms 0

o2 ms 80

ms 5

82

d h

LF

hh_o_o(())

hh₈₈(())

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Are IACC

Are IACC measurents measurents reproducible reproducible ? ?

 Experiment Experiment performed in performedin anechoicanechoic roomroom -- samesame loudspeakerloudspeaker, same, same source source and and receiver positionsreceiverpositions, 5 , 5 binauralbinaural dummydummy headsheads

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Are IACC

Are IACC measurents measurents reproducible reproducible ? ?

 Diffuse fieldDiffuse field -- huge differencehuge difference amongamong the 4 the 4 dummydummy headsheads

IACCe - random incidence

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

31.5 63 125 250 500 1000 2000 4000 8000 16000

Frequency (Hz)

IACCe

B&K4100 Cortex Head Neumann

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Are LF

Are LF measurents measurents reproducible reproducible ? ?

 Experiment Experiment performed in the Auditorium performedin the Auditorium ofof Parma Parma -- samesame loudspeakerloudspeaker, , same source and same source and receiverreceiver positionspositions, 4 , 4 pressure-pressure-velocityvelocity microphonesmicrophones

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Are LF

Are LF measurents measurents reproducible reproducible ? ?

 At 25 m distanceAt 25 m distance, the , the scatterscatter isis reallyreally bigbig

Comparison LF - measure 2 - 25m distance

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

31.5 63 125 250 500 1000 2000 4000 8000 16000

Frequency (Hz)

LF

Schoeps Neumann Soundfield B&K

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3D 3D Impulse Impulse Response Response ( ( Gerzon Gerzon , 1975) , 1975)

Portable PC with 4- channels sound board Original Room

Sound Source

SoundField Microphone

B-format 4-channels signal (WXYZ) Measurement of B-format

Impulse Responses

MLS or sweep excitation signal

Convolution of dry signals with the B-format Impulse

Responses Sound Source

Mono Mic.

B-format Imp. Resp.

of the original room

B-format 4-channels signal (WXYZ) Convolver

Ambisonics decoder

Speaker array in the reproduction room

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3D extension3D extension of the of the pressure-pressure-velocityvelocity measurementsmeasurements

 The SoundfieldThe Soundfield microphonemicrophone allowsallows forfor simultaneoussimultaneous measurementsmeasurements ofof the the omnidirectional

omnidirectional pressure and pressure and ofof the threethe three cartesian componentscartesian components ofof particleparticle velocity

velocity (figure(figure--ofof--8 8 patternspatterns))

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The The Waves Waves project (2003) project (2003)

 The originalThe original idea ofidea of Michael Michael GerzonGerzon waswas finallyfinally put in put in practicepractice in 2003, in 2003, thanks

thanks toto the the IsraeliIsraeli-based-based company WAVEScompany WAVES

 More thanMore than 50 theatres50 theatres allall aroundaround the world the world werewere measured, measured, capturing 3D capturing 3D IRs (4IRs (4-channels -channels BB-format-format withwith a a SoundfieldSoundfield microphonemicrophone))

 The The measurments didmeasurmentsdid alsoalso include include binauralbinaural impulseimpulse responsesresponses, and a , and a circular

circular--arrayarray of microphoneofmicrophone positionspositions

 More detailsMore details on on WWW.ACOUSTICS.NETWWW.ACOUSTICS.NET

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The Future

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The Future The Future

Microphone arraysMicrophone arrays capable capable of of synthesizing synthesizing aribitrary

aribitrary directivity directivity patternspatterns

Advanced Advanced spatial spatial analysis analysis of of the sound fieldthe sound field employing

employing sphericalspherical harmonicsharmonics ((AmbisonicsAmbisonics -- 11°° order

order or or higher)higher)

Loudspeaker Loudspeaker arrays arrays capable ofcapable of synthesizingsynthesizing arbitrary

arbitrary directivity directivity patternspatterns

Generalized Generalized solution solution in in which which both both the the directivities

directivities of of the source and the source and of the of the receiverreceiver are are represented

represented as as a sphericala spherical harmonicsharmonics expansionexpansion

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Directivity

Directivity of of transducers transducers

Soundfield

Soundfield STST--250 250 microphonemicrophone

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

60

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

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250 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

60

90

120

150 180

210 240 270

300 330

500 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

60

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150 180

210 240 270

300 330

1000 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

60

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150 180

210 240 270

300 330

2000 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

60

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210 240 270

300 330

4000 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

30

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210 240 270

300 330

8000 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0

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300 330

16000 Hz

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How How to to get get better better spatial spatial resolution resolution ? ?

 The answerThe answer isis simplesimple: analyze: analyze the spatialthe spatial distributiondistribution ofof bothboth source and source and receiverreceiver byby means

means ofof higherhigher--orderorder sphericalspherical harmonicsharmonics expansionexpansion

 Spherical harmonicsSphericalharmonics analysisanalysis isis the the equivalentequivalent, in space, in space domain, domain, ofof the Fourier the Fourier analysis

analysis in in timetime domaindomain



 As a complexAs a complex timetime--domaindomain waveformwaveform can can bebe thoughthough asas the sum the sum ofof a a numbernumber ofof sinusoidal

sinusoidal and and cosinusoidalcosinusoidal functionsfunctions, so a , so a complexcomplex spatialspatial distributiondistribution aroundaround a a given

given notionalnotional pointpoint can can bebe expressedexpressed asas the sum the sum ofof a a numbernumber ofof sphericalspherical harmonicharmonic functions

functions

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

0 0.5 1 1.5 2 2.5 3

dc Cos(5) Sin(5) Cos(10) Sin(10)

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

0 0.5 1 1.5 2 2.5 3

Sum

-20 0

0 5 10 1520

2530 35

40 45

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70 75

80 85 90 95 100 105 110 115 120 125 130 135 140 145 155150 165160 175170 185 180 195190 205200 210 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285 290

295 300

305 310

315 320

325330335340345350 355 0

0.35

0 5 10 1520

2530 35

40 45

50 55

60 65

70 75

80 85 90 95 100 105 110 115 120 125 130 135 140 150145 160155 170165 180 175 190185 200195 210205 215 220 225 230 235 240 245 250 255 260 265 270 275 280 285

290 295

300 305

310 315

320325330335340345350 355

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Higher

Higher--order order spherical spherical harmonics harmonics expansionexpansion

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3 3 ° ° - - order order microphone microphone ( ( Trinnov Trinnov - - France) France)

 Arnoud Arnoud Laborie developedLaborie developed a 24a 24--capsule compact capsule compact microphone microphone array

array -- byby meansmeans ofof advancedadvanced digitaldigital filteringfiltering, , sphericalspherical ahrmonicahrmonic signals

signals up up toto 33°° order are order are obtainedobtained (16 (16 channelschannels))

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4 4 ° ° - - order order microphone microphone (France Telecom) (France Telecom)

 Jerome Daniel and Jerome Daniel and Sebastien Moreau Sebastien Moreau builtbuilt samples ofsamples of 3232--capsules capsules spherical

spherical arraysarrays -- thesethese allow forallow for extractionsextractions ofof microphonemicrophone signalssignals up up to 4to 4°° orderorder (25 (25 channelschannels))

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4 4 ° ° - - order order microphone microphone (Italy) (Italy)

 Angelo Farina’Angelo Farina’s s sphericalspherical mike mike(32 capsules(32 capsules))

IMPULSE RESPONSE IMPULSE RESPONSE

IMPULSE RESPONSE IMPULSE RESPONSE

MEASUREMENTS MEASUREMENTS

Time Line

Basic

Basic sound sound propagation propagation scheme scheme

The The Past Past

Traditional

Traditional measurement measurement methods methods

Example

Example of of a a pulsive pulsive impulse impulse response response

Test

Test with with binaural binaural microphones microphones

Loudspeaker

Loudspeaker as as sound source sound source

Electroacoustical

Electroacoustical methods methods

The first MLS

The first MLS apparatus apparatus - - MLSSA MLSSA

More

More recently recently - - the CLIO system the CLIO system

The first TDS

The first TDS apparatus apparatus - - TEF TEF

The The Present Present

Today’s Hardware: PC and audio interface

The first ESS system - AURORA

MLS MLS method method

1 2

L  N 

MLS MLS deconvolution deconvolution

y 1 M

L h 1



 



 

 i j 2 mod L  1

m )

j , i (

M ~    

MLS MLS measurement measurement procedure procedure

Example

Example of of a MLS a MLS impulse impulse response response

 

 





 

 

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



 







 









 

 











   

 



 

1 e

f ln f

T f

sin 2 )

t (

x

f ln f T

t

L  ^N 

 ⁱ ^j ² ^mod ^L  ¹

  ^ ^