binaural approach in acoustic scene simulations in audio forensics

8/12/2019 Binaural Approach in Acoustic Scene Simulations in Audio Forensics

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Francesca Ortolani - Binaural approach in acoustic scene simulations in audio forensics

Radio Shack School of Audio Technologies 2014

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Binaural approach in acoustic scene simulations in audio forensics

Francesca OrtolaniIronbridge Electronics

[email protected]

Concept

Gunshots, steps, door noises, vehicles, barking dogs each sound could be a critical proof on thecrime scene. Along with acoustic measurements (such as reverberation time, wall isolation,

outdoor noise), binaural miking in acoustic simulations is indeed a powerful instrument fortesting a witness capabilities in hearing sound evidences and check for their direction of arrival onthe crime scene. This article is a short introduction to binaural audio and reverberation issues.

1. Introduction and background

!inaural recording is a "# audio techni$ue ai ed at the %la&back through head%hones or a %air ofs%eakers. 't should not be istaken for stereo%honic sound( )hich rather refers to a

bidi ensional soundsca%e. !inaural iking ai s at rendering the e*act %osition of the sourcesrelati+e to the listener in a "# en+iron ent( that is( sound is all around the listener )earinghead%hones or being in front of the s%eakers. !inaural audio through s%eakers is uch oredifficult to i %le ent due to cross-talk bet)een the channels.

There e*ist " %rinci%al techni$ues in order to get a binaural rendering, The use of a dumm head, an artificial hu an-like head e$ui%%ed )ith icro%hones in the

ears( %laced at eardru s the du & head substitutes for the hu an head and itscharacteristic shado)( de%ending on the direction of arri+al of the i %inging front.

The si ulation of a hu an head and the co %utation of a RT/ !ead "elated Transfer#unction .

*%eri ents )ith real hu an heads and real %eo%le )earing icro%hone ca%sules in theears.

The first tests in binaural audio go back to 1331. A %air of coal icro%hones )ere %laced in front ofthe stage of the %era in 5aris and s%aced like hu an ears. The acoustic signals )ere transducedand sent to destination b& tele%hone.Se+eral &ears later( in 16"1-"2( the researchers at !ell Tele%hone 7aboratories in 8e) 9erse& :SA

founded so e of the ost i %ortant bases of electroacoustics. ar+e& /letcher( ;/ather ofstereo%honic sound


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cur+es started in+estigating about the nature of sound( s%eech and listening and %atented thefirst edical acoustic hearing aid de+ice.

The accurate descri%tion of each %art of an acoustic s&ste ( in a %articular )a& the s%eakingsource( the icro%hone( the electric trans ission line( the recei+er head%hones( louds%eaker orlistener soon beca e essential.

The first binaural du & head )as called ; scar


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The '7# e*hibits instead greater +ariabilit& changing the listener and it de%ends $uite criticall& onfre$uenc&. ThatHs )h& the '7# turns out to be ore useful to source localisation.

Referring to /igure 1( the for ula for calculating the 'T# can be Iood)orth for ula e*tended forfre$uencies belo) 1.= k ? E"F,

( ) ( )sin

, cosa

ITD c

+= 1.1

)here a is the radius of the head su%%osing it is s%herical ( c is the s%eed of sound about "44 Jsin air ( K is the a?i uth and * is ele+ation.

Iood)orthHs original for ulation did not consider that the 'T# di inishes outdistancing thesource on the listenerHs hori?ontal %lane. The factor cos reckons )ith that.

At fre$uencies abo+e 1.= k ? Iood)orthHs for ula gets less accurate.

onsider another sensor& cue, the '5# ( nteraural +hase ifference) . Ihen the '5# is greater ore$ual to 130L( the source %osition is a biguous. This a biguit& is induced b& the %eriodic natureof %hase.

Since the )a+elength for higher fre$uencies is shorter( it a& ha%%en that the distance bet)eenthe ears is greater than a )a+elength for ade$uatel& high fre$uencies M1.=k ? .This gi+es rise to spatial aliasing ( as entioned abo+e( and the %ercei+ed 'T# %ro+es to bea biguous( as the ear is res%onsi+e onl& to the %hase and not to ti e differences.

Figure 1 Duple binaural model


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So( for such fre$uencies the '7# su%%orting infor ation is necessar&. 'n other )ords( a highfre$uenc& signal M1.= k ? %roduces an 'T# greater than the signal %eriod( )hile a lo) fre$uenc&

signal is such that( because of the 'T# ranging in a %eriod of the signal itself( the %hase difference%ercei+ed at the ear allo)s the listener to e+aluate the 'T# una biguousl&.

The hu an listener takes ad+antage of the 'T# and '7# cues in order to decode the direction ofarri+al of the sound. 'n addition( the %resence of the %inna in the ear %roduces further ore acoloration of the sound( de%ending on the direction of arri+al of the sound )a+e./igure 2 sho)s the hu an ear anato &.'n general( localisation %erfor ance is e*cellent in the front stage of the hori?ontal %lane( good onthe rear and uch )orse on the +ertical %lane.

". #$%F ead-$elated %ransfer Function'

'n linear s&ste s anal&sis the transfer function is defined as the co %le* ratio bet)een the out%utsignal s%ectru and the in%ut signal s%ectru . Then( the transfer function !( f ) of a linear ti e-in+ariant s&ste is defined as,

( ) ( )

( )Y f

H f X f

= 1.2

)here ( f ) is the out%ut signal fre$uenc& s%ectru and -( f ) is the in%ut signal s%ectru .

A transfer function or( e$ui+alentl& in ti e do ain( the impulse response characteri?es anddescribes the channel bet)een the source and the recei+er.The RT/ is the head related transfer function( a function of 4 +ariables (f, r, K( N ( )here f is thefre$uenc&( r is the distance of the source fro the listener( K and N are res%ecti+el& the a?i uthand the ele+ation.So( )e ha+e a left RT/ and a right RT/ and these describe the channel bet)een the ear and thesource relati+el& to the left and right ear.

A RT/ atches onl& one configuration of source and recei+er and the left and right RT/s doe bed the 'T# and '7# infor ation.

'n order to e*tract the 'T# and '7# fro the RT/s E4F )e co %ute the odulus of the interauraltransfer function ( )hich is defined as,

( ) ( )

( )

= , ,

, ,

R

r

INT L

r

H H

H 1."

)here / 01f is the angular fre$uenc& and the su%erscri%ts " (& denote res%ecti+el& the right andleft channel. The odulus e*%ressed in d! of ( ) INT H is,


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=

( ) ( ) ( ) ( ) = = 10 , , , ,20log R L

INT INT r r dB dB dB H H H H 1.4

and it is an e*%ression of the '7# )ith +ar&ing fre$uenc&.The grou% dela& is defined as,

( ) ( ) ( )( )

=

R LINT d d

d d 1.=

)here ( ) ( ) ( ) ( ) ( ) ( ) = = = , , , ,, ,R L

INT R r L r H H H re%resents the 'T# )ith

+ar&ing fre$uenc&.

The RT/ ethod does not re%lace the #u%le* odel( but su%%orts the latter in order to obtain abetter esti ation of the source %osition in the listening en+iron ent. Research brought to oreco %licated hu an odels and the !RT/ (2od $"elated Transfer #unction) )ere in+ented.These ne) transfer functions describe hu an bod& +ibrations and their trans ission to theauditor& s&ste thus contributing to the o+erall sound %erce%tion.

(. $e)erberation effects on source localisation

As a first effect re+erberation reduces the efficac& of the infor ation used b& the listener. 'n%resence of reflections( in fact( the auditor& s&ste a& decode the source %osition erroneousl&due to high intensit& echoes interfering the direct sound and su%eri %osing the sel+es to it.

Tests in anechoic cha ber and re+erberant cha ber brought so e significant results through the&ears.So e of the are listed belo),

a )ide band stationar& noise is localised ore scarcel& in a re+erberant field E=F a )ide band noise is localised easier in a re+erberant field than single s%arse tones( that is(

localisation i %ro+es )ith the s%ectral densit& of the source high attack transient sounds are better localised than others inde%endentl& on

re+erberation deca& ti e .

Thanks to the precedence effect the listener is able to )eight )a+efronts in different )a&s.The first front on arri+al direct sound is gi+en ore )eight.Stee% attack transients trigger the %recedence effect and fa+our localisation( )hile stationar&

sources are decoded )ith a greater difficult&.


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Studies about se%aration of concurrent sources E>F re+ealed that the ca%abilit& of grou%ing soundsaccording to their har onicit& de%ends on the e*tent of their funda ental fre$uenc& f 0 range.

'n free s%ace the listener identifies )ith ease those sounds ha+ing a %ro%er %itch difference.The situation changes in a re+erberant en+iron ent, direct sound and echoes )ith the sa efunda ental fre$uencies relate to each other on the basis of har onic ratios. A flutter in f 0 causesinhar onicit& bet)een direct sound and reflections. /or this reason( in a re+erberant field( the

echanis of grou%ing sounds according to har onicit& loses its efficac&.

'n a confined s%ace distance %erce%tion de%ends on 2 factors EOF, the ratio bet)een direct soundenerg& and reflected energ& and the ti e dela& bet)een direct sound and reflected sound.Roo reflections also contribute to the general s%atial i %ression charactering the listeningen+iron ent and the sound source.

So e %ara eters are introduced here, ASI (Apparent 3ource 4idth) ( )hich is the %ercei+edlength of a source( and 7 P (&istener 5nvelopment) ( the %erce%tion of a surrounding sound.ASI is deter ined %rinci%all& b& the intensit& le+el of the earl& lateral reflections )ithin the first30 s fro the arri+al of the direct sound at the listenerHs %osition.7 P is the subDecti+e i ersi+e sound sensation %roduced b& re+erb and de%ends on the nature oflate reflections( in %articular on their distribution o+er ti e( their le+el and direction of arri+al.

'n E3F !radle& e*%lains ho) ASI and 7 P are caused b& the %recedence effect( during )hich thedirect sound and the earl& reflections are te %oraril& erged together.

ASI finds Dustification in the circu stance that the identification of the direction of arri+al isdistorted b& the enhance ent of the erged e+ent %roduced b& earl& lateral reflections.

The re+erberation tail instead is not erged )ith direct sound and a%%ears as a sort of diffusedhalo around the direct sound thus gi+ing the listener a sense of i ersion $uantifiable )ith 7 P.

*. $e)erberation effects on sound perception

Ihen the sound is e itted in a re+erberant en+iron ent( reflections are su%eri %osed to directsound( causing the ;s%reading< of it in ti e and filling u% )ith energ& those s%aces in ti e that)ould ha+e re ained e %t& in an anechoic a bient( that is( a reflectionless cha ber.Iith i ediate effect( the listener encounters a reduction in signal intelligibilit&./igure 2 a b co %ares the s%ectrogra 1 of the +ocal recording ;Thank &ou for tra+elling on thecentral line< b& a fe ale s%eaker in the case a of a oderate re+erberant a bient and the case

b )ith a 1-second deca&-ti e re+erb.

/igure 2 b highlights re+erberation filling u% those darker ?ones )ith lo)er energ&( )hich areuch shar%er in /igure 2 a .

The attack %hone es are less susce%tible to the effect of re+erberation than the final ;releasec n nn:

d 3 a= 1.O

)here : is a directi+it& %ara eter 1: = for an o nidirectional source ( n3 is the n-th surface

area ha+ing the absor%tion coefficient na .

'f the absor%tion coefficients are unkno)n( an alternati+e for ula for cd is the follo)ing,

>0100

c

: ; d

"T p= 1.3

)here ; is the roo +olu e "m and

>0"T is the esti ated reverberation time 00( RT"0( RT20( RT10 , t&%icall& defined as the ti e inter+al sound energ& takes todeca& >0( "0( 20( 10 d!( after the e*citation in the roo has ceased.

Figure * %he reflection order is the number of impacts the sound carries out before reaching the listener recei)er 01"


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nne e B/ %he human ear

The %inna( the out ost %art of the ear( is a cartilaginous structure )ith the ai of con+e&ing thesound( through the auditor& channel( to)ards the eardru e brane. The sound is trans ittedto the ossicles alleus( an+il and stirru% ( )hich for a s&ste of le+ers beha+ing like ana %lifier. So( the force i %inging on the o+al )indo) is ade tri%le co %ared to that i %ingingon the alleus and the acoustic %ressure is 2 orders of agnitude greater relati+e to the %ressureon the eardru E4F. The cochlea is the center of the auditor& %rocess. 't has the sha%e of a snailand itHs about "= long )ith a dia eter of 2 at its e*ternal entrance and graduall& shrinking.'nside the cochlea there is the basilar e brane co+ered in ciliated cells. Ihen the e brane+ibrates( the cilias bend and this o+e ent causes the ner+e ter inations of the relati+e cells totrans it i %ulses to the brain( )hose nu ber de%ends on the fre$uenc& of the sound )a+e. 'n

other )ords( the cochlea beha+es like a filter bank E4F( o%erating a s%ectru anal&sis andtransduces a fre$uenc& infor ation into an electric signal to)ards the brain( according to the%osition of the in+ol+ed ;resonator


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3cene Anal sis? +rinciples, Algorithms, and Applications ( Iile&-' 5ress( 200>

2. /. 7. Iight an e #. 9. istler( #actors affecting the relative salience of sound locali>ationcues ( in 2inaural and 3patial !earing in "eal and ;irtual 5nvironments ( 1-2"( 7a)rence

rlbau Associates( ah)ah( 89( 166O

". !rian art& e Pictor 7a??arini( 2inaural !"T# based spatialisation? new approaches and implementation ( 12th 'nt. onference on #igital Audio ffects #A/*-06 ( o o( 'tal&(Se%te ber 1-4( 2006

4. A. :ncini( Audio igitale ( ",30-31( cBra)- ill( 200>

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O. B.9. !ro)n e . 5alo Uki( "everberation ( 'n 'omputational Auditor 3cene Anal sis?

+rinciples, Algorithms, and Applications ( Iile&-' 5ress( 200>

3. 9. S. !radle&( +redictors of speech intelligibilit in rooms ( 9ournal of the Acoustical Societ& ofA erica( 21 > ,=OO-=30( 1646

6. R. . !olt e A. #. ac#onald( Theor of speech masking b reverberation ( 9ournal of theAcoustical Societ& of A erica( 21 > ,=OO-=30(1646

10. !. 7ibbe& e 5. . Rogers( The effects of overlap$masking on binaural reverberant wordintelligibilit ( 9ournal of the Acoustical Societ& of A erica( 11=,"141-"1=1( 2004

11. A erican 8ational Standards 'nstitute( American national ps choacoustical terminolog ,S".20( 16O"( A erican Standards Association

12. 9. 5. A. 7ochner e 9. /. !urger( The influence of reflections on auditorium acoustics (9ournal of Sound and Pibration( 1 4,42>-4=4( 16>4

1". /. A. +erest( The %aster !andbook of Acoustics ( cBra)- ill 2006

binaural approach in acoustic scene simulations in audio forensics

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