Starting point point : : room room impulse impulse response response

Angelo Farina Angelo Farina

Industrial Engineering Dept.

Industrial Engineering Dept.

University of Parma

University of Parma -- ITALYITALY HTTP://

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

IMPULSE RESPONSE IMPULSE RESPONSE MEASUREMENTS BY MEASUREMENTS BY

EXPONENTIAL SINE SWEEPS EXPONENTIAL SINE SWEEPS

Workshop

Casa della Musica

Parma, 18 October 2008

University of Parma

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

ƒ MLS and TDS methods for electroacoustical masurements

The The PastPast

The The PresentPresent

ƒ Electroacoustical measurements employing the Exponential Sine Sweep method (ESS)

The Future The Future

ƒ Capturing the complete spatial information by means of arrays of transducers

ƒ Employment of not-linear impulse responses in the auralization process

Time Line

Starting

Starting point point : : room room impulse impulse response response

Omnidirectional receiver Direct Sound

Reflected Sound

Point Source

Direct Sound

Reverberant tail

The The Past Past

Traditional

Traditional measurement measurement methods methods

ƒƒ PulsivePulsive sources: sources: ballonsballons, , blankblank pistolpistol

Example

Example of of a a pulsive pulsive impulse impulse response response

Loudspeaker

Loudspeaker as as sound source sound source

ƒƒA A loudspeakerloudspeaker isis fedfed withwith a a specialspecial test test signalsignal x(t), x(t), while

while a microphonea microphone recordsrecords the roomthe room responseresponse

ƒƒA A properproper deconvolutiondeconvolution techniquetechnique isis requiredrequired forfor retrieving

retrieving the the impulseimpulse responseresponse h(t) h(t) fromfrom the the recorded

recorded signalsignal 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

Measurement process

ƒ 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).

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

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)

Electroacoustical

Electroacoustical methods methods

ƒƒ DifferentDifferent typestypes ofof test signalstest signals havehave beenbeen developeddeveloped, , providing

providing goodgood immunityimmunity toto background noisebackground noise and easy and easy deconvolution

deconvolution ofof the the impulseimpulse responseresponse::

►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)

ƒƒ EachEach ofof thesethese test signalstest signals can becan be employedemployed withwith differentdifferent deconvolution

deconvolution techniques, techniques, resultingresulting in a numberin a number ofof

“different“different”” measurementmeasurement methodsmethods

ƒƒ Due Due toto theoreticaltheoretical and and practicalpractical considerationsconsiderations, the , the preference

preference isis nowadaysnowadays generallygenerally orientedoriented forfor the the usageusage ofof ESS ESS withwith notnot--circularcircular deconvolutiondeconvolution

The first MLS

The first MLS apparatus apparatus - - MLSSA MLSSA

ƒƒ MLSSA wasMLSSA was the first apparatusthe first apparatus forfor measuringmeasuring impulseimpulse responsesresponseswithwith MLSMLS

More

More recently recently - - the CLIO system the CLIO system

ƒƒ The ItalianThe Italian--mademade CLIO system hasCLIO system has supersededsupersededMLSSA MLSSA forformostmost low-low-costcost electroacoustics

electroacoustics applicationsapplications (measurement(measurement ofofloudspeakers, loudspeakers, qualityquality controlcontrol))

The first TDS

The first TDS apparatus apparatus - - TEF TEF

ƒƒ TechronTechron TEF 10 wasTEF 10 was the first apparatusthe first apparatus forformeasuringmeasuring impulseimpulse responsesresponses withwith TDSTDS

ƒƒ SubsequentSubsequent versionsversions(TEF 20, TEF 25) also(TEF 20, TEF 25) also supportsupport MLSMLS

The The Present Present

Edirol FA-101 Firewire sound

card:

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

Today’s Hardware: PC and audio interface

Hardware:

Hardware: loudspeakerloudspeaker & & microphonemicrophone

Dodechaedron loudspeaker

Soundfield microphone

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

ƒƒ Aurora wasAurora was the first measurementthe first measurement system system basedbased on standard sound cardson standard sound cards and employingand employing the Exponentialthe Exponential SineSineSweepSweep methodmethod

ƒƒ ItIt alsoalso worksworks withwith traditionaltraditional TDS and MLS methodsTDS and MLS methods, so the , so the comparisoncomparison can becan be mademadeemployingemploying exactlyexactlythe samethe same hardwarehardware

Exponential Sine Sweep method

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

which frequency is varied exponentially with time, starting at f₁ and ending at f₂.

⎥ ⎥

⎥ ⎥

⎦

⎤

⎢ ⎢

⎢ ⎢

⎣

⎡

⎟⎟

⎟ ⎟

⎠

⎞

⎜⎜

⎜ ⎜

⎝

⎛

−

⋅

⎟⎟ ⎠

⎜⎜ ⎞

⎝

⎛

⋅

⋅ π

= ⋅ ^{⎟⎟ ⎠}

⎜⎜ ⎞

⎝

⋅ ⎛

1 e

f ln f

T f

sin 2 )

t (

x

2

f ln f T

t

1

2

1

Test Signal – x(t)

Measured signal - y(t)

ƒƒ The The not-not-linearlinear behaviourbehaviourofof the loudspeakerthe loudspeaker causescauses manymany harmonicsharmonics totoappearappear

Inverse Filter – z(t)

The The deconvolutiondeconvolution ofof the IR isthe IR is obtainedobtained convolvingconvolving the the measuredmeasured signalsignal y(t)

y(t) withwith the inverse filterthe inverse filter z(t) [equalizedz(t) [equalized, , time-time-reversedreversed x(t)]x(t)]

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)

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°

Maximum Length Sequence vs. Exp. Sine Sweep

Distortion measurements Distortion measurements

ƒƒA headphone was driven A headphone was driven with a 1 V RMS signal, with a 1 V RMS signal,

causing severe causing severe

distortion in the small distortion in the small

loudspeaker.

loudspeaker.

ƒƒThe measurement was The measurement was made placing the

made placing the

headphone on a dummy headphone on a dummy

head.

head.

ƒƒMeasurements: ESS and Measurements: ESS and traditional sine at 1 kHz traditional sine at 1 kHz

Distortion measurements Distortion measurements

ƒƒ Comparison between:Comparison between:

ƒƒ traditional distortion measurement with fixed-traditional distortion measurement with fixed-frequency sine (the black frequency sine (the black histogram)

histogram)

ƒƒ the new exponential sweep (the 4 narrow, coloured lines)the new exponential sweep (the 4 narrow, coloured lines)

Spatial

Spatial analysisanalysis byby directivedirective impulseimpulse responsesresponses

ƒƒ The initialThe initial approachapproach waswas totouseuse directivedirective microphonesmicrophonesforfor gatheringgathering some some information

information aboutabout the spatialthe spatial propertiesproperties ofofthe sound the sound fieldfield“as“as perceivedperceived byby the listenerthe listener””

ƒƒ TwoTwo apparentlyapparentlydifferentdifferent approachesapproachesemerged: emerged: binauralbinaural dummydummy headsheads and and pressure

pressure--velocityvelocity microphones:microphones:

Binaural Binaural microphone

microphone (left(left)) and and

Pressure

Pressure-velocity-velocity microphone

microphone (right)(right)

IACC

IACC “ “ objective objective ” ” spatial spatial parameter parameter

ƒƒ ItIt waswas attemptedattemptedtoto “quantify“quantify”” the “the “spatialityspatiality”” ofofa rooma room bybymeansmeans ofof

“objective“objective”” parameters, parameters, basedbased on 2-on 2-channels channels impulseimpulse responsesresponses measuredmeasured withwith directivedirective microphonesmicrophones

ƒƒ The mostThe most famousfamous “spatial“spatial”” parameterparameter isis IACC (Inter AuralIACC (Inter Aural Cross Cross Correlation

Correlation), ), basedbased on binauralon binaural IR measurementsIR measurements

LeftLeft

Right Right

80 ms 80 ms

( )

( ) ( )

( ) ( )

pp_LL(τ(τ))

pp_RR((ττ))

[ ]( )^{t t} [ ^{1ms... 1ms}]

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

+

−

∈ ρ

= τ

⋅ + τ

⋅ τ

⋅ τ

τ

⋅ + τ

⋅ τ

= ρ

∫

∫

∫

Lateral

Lateral Fraction Fraction (LF) (LF) spatial spatial parameter parameter

ƒƒ AnotherAnother “spatial“spatial”” parameterparameterisis the Lateralthe Lateral FractionFraction LFLF

ƒƒ ThisThis isis defineddefined fromfrom a 2-a 2-channels channels impulseimpulse response, the first response, the first channelchannelisis a a standard

standard omniomni microphonemicrophone, the second, the second channelchannel isisa a “figure“figure--ofof--eighteight”” microphone

microphone::

Figure Figure

of of 88 OmniOmni

( )

∫

∫

τ

⋅ τ

τ

⋅ τ

= 80ms

ms 0

o2 ms 80

ms 5

82

d h

d h

LF

hh_oo(τ(τ))

hh₈₈((ττ))

Are Are binaural binaural measurents measurents reproducible reproducible ? ?

ƒƒ ExperimentExperiment performedperformedin anechoicin anechoic roomroom --samesame loudspeakerloudspeaker, same, same source source and receiverand receiver positions, 5 positions, 5 binauralbinaural dummydummy headsheads

Are IACC

Are IACC measurents measurents reproducible reproducible ? ?

ƒƒ Diffuse fieldDiffuse field -- hugehuge differencedifference amongamong the 4 dummythe 4 dummy 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

Are LF

Are LF measurents measurents reproducible reproducible ? ?

ƒƒ ExperimentExperiment performedperformedin the Auditorium ofin the Auditorium of Parma -Parma - samesame loudspeaker, loudspeaker, samesame source and receiversource and receiver positions, 4 positions, 4 pressure-pressure-velocityvelocity microphonesmicrophones

Are LF

Are LF measurents measurents reproducible reproducible ? ?

ƒƒ 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

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

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

ƒƒ The SoundfieldThe Soundfield microphonemicrophone allowsallowsforfor simultaneoussimultaneous measurementsmeasurements ofofthe the omnidirectional

omnidirectional pressurepressure and ofand of the threethe three cartesiancartesian componentscomponentsofofparticleparticle velocity

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

Directivity

Directivity of of transducers transducers

Soundfield

Soundfield ST-ST-250 250 microphonemicrophone

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

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4000 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

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8000 Hz

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

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16000 Hz

A A - - format format microphone microphone arrays arrays

ƒƒ TodayTodayseveralseveral alternativesalternativestotoSoundfieldSoundfield microphonesmicrophonesdo existsdo exists. . AllAll ofofthemthem are providingare providing “raw“raw”” signalssignalsfromfrom the 4 capsulesthe 4 capsules, and the , and the conversionconversion fromfrom these

these signalssignals (A(A--formatformat) ) toto the standard Ambisonicthe standard Ambisonic signalssignals (B(B--formatformat) ) isis performed

performed digitallydigitally byby meansmeansofof software runningsoftware running on the computeron the computer

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

ƒƒ The originalThe original idea ofidea of Michael GerzonMichael Gerzon waswas finallyfinallyput in practiceput in practice in 2003, in 2003, thanks

thanks toto the Israelithe Israeli-based-based company WAVEScompany WAVES

ƒƒ More thanMore than 50 theatres50 theatres allall aroundaround the world werethe world were measured, measured, capturingcapturing 3D 3D IRsIRs (4(4-channels -channels BB-format-format withwith a Soundfielda Soundfield microphone)microphone)

ƒƒ The measurmentsThe measurments diddid alsoalso include binauralinclude binaural impulseimpulseresponses, and a responses, and a circular

circular--arrayarray ofof microphonemicrophone positionspositions

ƒƒ More detailsMore details on WWW.ACOUSTICS.NETon WWW.ACOUSTICS.NET

ƒ Pre-ringing at high and low frequency before the arrival of the direct sound pulse

ƒ Pre/post equalization of the test signal performed in a way which avoids time-smearing of the

impulse response

ƒ Sensitivity to abrupt pulsive noises during the measurement

ƒ Skewing of the measured impulse response when the playback and recording digital clocks are

mismatched

ƒ Cancellation of high frequencies in the late part of the tail when performing synchronous averaging

Problems with ESS measurements

Pre-ringing at high and low frequency

ƒƒ Pre-Pre-ringingringing at high frequencyat high frequency due todue to improperimproperfadefade-out-out

ThisThispicturepicture showsshows the preringingthe preringing obtainedobtained deconvolvingdeconvolvingdirectlydirectly the test the test signal

signal, , withoutwithout passingpassingthroughthrough the system under testthe system under test

Pre-ringing at high and low frequency

ƒƒ PerfectPerfect Dirac’Dirac’s delta s delta afterafter removingremovingthe fadethe fade--outout

ThisThis picturepictureshowsshows the resultthe result obtainedobtained deconvolvingdeconvolvingdirectlydirectly the test signalthe test signal, , without

without passingpassingthroughthrough the system under test, and employingthe system under test, and employing a sinea sine sweep

sweep goinggoing up toup to the Nyquistthe Nyquist frequencyfrequency

Pre-ringing at high and low frequency

ƒƒ Pre-Pre-ringingringing at low frequencyat low frequency due due totoa bad sound card featuringa bad sound card featuring frequency-frequency- dependent

dependent latencylatency

ThisThis artifactartifact can becan be correctedcorrected ifif the frequencythe frequency-dependent-dependent latencylatencyremainsremains the samethe same, , byby creatingcreatinga suitablea suitable inverse filterinverse filter withwiththe the KirkebyKirkebymethodmethod

Kirkeby

Kirkeby inverse inverse filter filter

ƒƒ The KirkebyThe Kirkeby inverse filterinverse filter isis computedcomputedinvertinginverting the measuredthe measured IRIR

( ) [ ( )]

[^H( )^f ] ^H( ) ( )^{f f}

Conj

f H f Conj

C = ⋅ +ε

1) The IR to be inverted is FFT transformed to frequency domain:

H(f) = FFT [h(f)]

2) The computation of the inverse filter is done in frequency domain:

Where ε(f) is a small, frequency-dependent regularization parameter

3) Finally, an IFFT brings back the inverse filter to time domain:

c(t) = IFFT [C(f)]

ε_est

ε_int

f_low f_high

Δf Δf

Inverse Inverse filterfilter Frequency

Frequency--dependentdependent regularizationregularizationparameterparameter

Pre-ringing at high and low frequency

ƒƒ ConvolvingConvolvingthe timethe time-smeared-smeared IR withIR with the Kirkebythe Kirkeby compactingcompacting filter, a filter, a veryvery sharp

sharp IR isIR is obtainedobtained

The sameThe same methodmethod can alsocan also bebeappliedapplied forfor correctingcorrectingthe responsethe response ofof the the loudspeaker

loudspeaker/microphone/microphone system, ifsystem, if anananechoicanechoic preliminarypreliminary test istest is donedone

ƒ Pre-ringing at high and low frequency before the arrival of the direct sound pulse

ƒ Pre/post equalization of the test signal performed in a way which avoids time-smearing of the

impulse response

ƒ Sensitivity to abrupt pulsive noises during the measurement

ƒ Skewing of the measured impulse response when the playback and recording digital clocks are

mismatched

ƒ Cancellation of high frequencies in the late part of the tail when performing synchronous averaging

Problems with ESS measurements

Equalization

Equalization of of the the whole whole system system

ƒƒ An anechoicAn anechoic measurementmeasurement isis first performedfirst performed

Equalization

Equalization of of the the whole whole system system

ƒƒ A suitableA suitable inverse filterinverse filter isis generatedgenerated withwith the Kirkebythe Kirkeby methodmethodbyby invertinginverting the anechoicthe anechoic measurementmeasurement

Equalization

Equalization of of the the whole whole system system

ƒƒ The inverse filterThe inverse filter can becan be eithereither pre-pre-convolvedconvolved withwith the test signalthe test signal or postor post-- convolved

convolved withwith the resultthe result ofof the measurementthe measurement

ƒƒ Pre-Pre-convolutionconvolution usuallyusually reducesreduces the SPL the SPL beingbeing generatedgenerated bybythe the loudspeaker

loudspeaker, , resultingresultingin worstin worst S/N ratioS/N ratio

ƒƒ On the otherOn the other hand, hand, postpost--convolutionconvolution can makecan make the background noisethe background noise toto become

become “coloured“coloured”, and ”, and hencehence more more perciptibleperciptible

ƒƒ The resultingThe resulting anechoicanechoicIR becomesIR becomes almostalmost perfectlyperfectly a Diraca Dirac’’s Delta s Delta function

function::

ƒ Pre-ringing at high and low frequency before the arrival of the direct sound pulse

ƒ Pre/post equalization of the test signal performed in a way which avoids time-smearing of the

impulse response

ƒ Sensitivity to abrupt pulsive noises during the measurement

ƒ Skewing of the measured impulse response when the playback and recording digital clocks are

mismatched

ƒ Cancellation of high frequencies in the late part of the tail when performing synchronous averaging

Problems with ESS measurements

Sensitivity to abrupt pulsive noises

ƒƒ OftenOften a pulsivea pulsive noisenoise occursoccurs duringduring a sinea sine sweepsweep measurement

measurement

Sensitivity to abrupt pulsive noises

ƒƒ AfterAfter deconvolution, the deconvolution, the pulsivepulsive sound causessound causes untolerableuntolerable artifacts

artifacts in the impulsein the impulse responseresponse

The artifactThe artifact appearsappears asas a downa down--slopingsloping sweepsweep on the impulseon the impulse response. response. At the 2 kHz

At the 2 kHz octaveoctave band the band the decaydecay isis distorteddistorted, and the reverb, and the reverb. . timetime isis artificiallyartificially increasedincreased fromfrom 2.13 to2.13 to 2.48 s2.48 s

Sensitivity to abrupt pulsive noises

ƒƒ SeveralSeveraldenoisingdenoisingtechniquestechniquescan becan be employed:employed:

► Brutely silencing the transient noise

► Employing the specific “click-pop eliminator” plugin of Adobe Audition

► Applying a narrow-passband filter around the frequency which was being generated in the moment in which the pulsive noise occurred

ƒƒ The thirdThe thirdapproachapproachprovidesprovidesthe betterthe betterresults:results:

ƒ Pre-ringing at high and low frequency before the arrival of the direct sound pulse

ƒ Pre/post equalization of the test signal performed in a way which avoids time-smearing of the

impulse response

ƒ Sensitivity to abrupt pulsive noises during the measurement

ƒ Skewing of the measured impulse response when the playback and recording digital clocks are

mismatched

ƒ Cancellation of high frequencies in the late part of the tail when performing synchronous averaging

Problems with ESS measurements

Clock mismatch

ƒƒ WhenWhen the measurementthe measurement isisperformedperformed employingemploying devicesdeviceswhichwhich exhibitexhibit signifcant

signifcant clock mismatchclock mismatch betweenbetween playback and recordingplayback and recording, the , the resultingresulting impulse

impulse responseresponseisis “skewed“skewed”” (stretched(stretched in timein time):):

The picturesThe pictures show the show the resultsresults ofof anan electricalelectrical measuremntmeasuremnt performedperformed connecting

connecting directlydirectly a CD-a CD-player player withwith a DAT a DAT recorderrecorder

Clock

Clock mismatch mismatch

ƒƒ ItIt isispossiblepossible totore-re-pack the pack the impulseimpulse responseresponseemployingemploying the alreadythe already-- described

described approachapproach basedbased on the usageon the usage ofof a Kirkebya Kirkeby inverse filterinverse filter::

However

However, , thisthis isis possiblepossible onlyonly ifif a “a “referencereference”” electricalelectrical (or anechoic(or anechoic) ) measurement

measurement hashas beenbeen performedperformed. But. But, in , in manymany casescases, one, one onlyonly getsgets the rethe re--recordedrecorded signalssignals, and no reference, and no reference measurementmeasurement isis available, so available, so

the Kirkebythe Kirkeby inverse filterinverse filter cannotcannot bebe computed.computed.