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==Physics and perception==
Apparent source width is the aurally perceived extent of a sound source. Sometimes, it is defined as the impression that a source sounds larger than its optical size.<ref name="blau">{{cite journal |last1=Blau |first1=Matthias |title=Correlation of apparent source width with objective measures in synthetic sound fields |journal=Acta Acustica United With Acustica |date=2004 |volume=90 |issue=4 |page=720 |url=http://www.ingentaconnect.com/content/dav/aaua/2004/00000090/00000004/art00015# |accessdate=31 May 2018}}</ref>. The impression results from several auditory cues, which are affected by sound radiation characteristics of the source itself and by characteristics of the room.
The [[auditory system]] has mechanisms that separate the processing of late [[reverberation]] from the processing of direct sound and early [[Reflection (physics)|reflections]] referred to as [[precedence effect]]. While the late reverberation contributes to the [[perception]] of listener envelopment and reverberance, the direct sound and the early reflections mostly affect [[Sound localization|source localization]], intimacy and the apparent source width<ref name="beranek">{{cite book|last1=Beranek|first1=Leo Leroy|title=Concert Halls and Opera Houses: Music, Acoustics, and Architecture|date=2004|publisher=Springer|___location=New York|doi=10.1007/978-0-387-21636-2|isbn=978-1-4419-3038-5|edition=Second|url=https://link.springer.com/book/10.1007/978-0-387-21636-2|accessdate=25 May 2018}}</ref>. The balance of early and late arriving sound affects the perceived clarity, warmth and brilliance.
The auditory system does not process all early sounds together to derive a source ___location. In complicated acoustical scenes the auditory system integrates those parts of sound that share temporal, spectral, and spatial properties into one so-called auditory stream. An auditory stream is the counterpart to a visible [[entity|object]] in [[Gestalt psychology]]. Several auditory streams are segregated from one another. The process of integration and segregation is referred to as [[auditory scene analysis]] and is believed to be the original function of the ear.<ref name="braun">{{cite book |last1=Braun |first1=Christopher B. |last2=Grande |first2=Terry |editor1-last=Webb |editor1-first=Jacqueline F. |editor2-last=Fay |editor2-first=Richard R. |editor3-last=Popper |editor3-first=Arthur N. |title=Fish Bioacoustics |date=2008 |doi=10.1007/978-0-387-73029-5_4 |publisher=Springer |___location=New York |isbn=978-0-387-73029-5 |page=105 |url=https://link.springer.com/chapter/10.1007/978-0-387-73029-5_4 |accessdate=31 May 2018 |chapter=Evolution of peripheral mechanisms for the enhancement of sound reception}}</ref>. Each auditory stream can have its own apparent source width. One auditory stream may contain the direct sound and early reflections of a single musical instrument or a [[musical ensemble]].
A high strength of low frequencies and incoherence of the left and the right ear of one auditory stream, especially of its direct sound and early reflections, increase the apparent source width.<ref name=blau/><ref name=ziemer/><ref name=beranek/>.
Even in absence of room acoustical reflections the pure direct sound of musical instruments already effects the perceived source extent.<ref name="ziemer">{{cite book|last1=Ziemer|first1=Tim|editor1-last=Schneider|editor1-first=Albrecht|title=Studies in Musical Acoustics and Psychoacoustics|doi=10.1007/978-3-319-47292-8_10|date=2017|publisher=Springer|___location=Cham|isbn=978-3-319-47292-8|pages=299–340|url=https://link.springer.com/chapter/10.1007/978-3-319-47292-8_10|accessdate=25 May 2018|chapter=Source Width in Music Production. Methods in Stereo, Ambisonics, and Wave Field Synthesis}}</ref>. Unlike hypothetical [[Point source#Sound|monopole source]] musical instruments radiate their sound not evenly in all directions. Rather the overall [[Amplitude|volume]] and the [[frequency spectrum]] differ in each direction, referred to as sound radiation characterisitcs or radiation patterns<ref name=ziemer/><ref name="ziemerdiss">{{cite thesis|type=PhD|doi=10.13140/RG.2.1.1997.9769|last1=Ziemer|first1=Tim|title=Implementation of the Radiation Characteristics of Musical Instruments in Wave Field Synthesis Applications|date=2015|publisher=Univ. Diss.|___location=Hamburg|url=http://ediss.sub.uni-hamburg.de/volltexte/2016/7939/|accessdate=25 May 2018}}</ref><ref name="bader">{{cite journal|last1=Bader|first1=Rolf|title=Radiation characteristics of multiple and single sound hole vihuelas and a classical guitar|journal=The Journal of the Acoustical Society of America|date=2012|volume=131|issue=1|pages=819–828|doi=10.1121/1.3651096|url=https://asa.scitation.org/doi/10.1121/1.3651096|accessdate=25 May 2018}}</ref>. These may create incoherent ear signals and, consequently, the impression of a wide source. The sound radiation characteristics of musical instruments are typically given as [[radiation pattern]] in a two- to three-dimensional [[polar coordinate system]]<ref name="meyer">{{cite book|last1=Meyer|first1=Jürgen|doi=10.1007/978-0-387-09517-2|title=Acoustics and the Performance of Music. Manual for Acousticians, Audio Engineers, Musicians, Architects and Musical Instrument Makers|date=2009|publisher=Springer|___location=Bergkirchen|isbn=978-0-387-09516-5|edition=Fifth |url=https://link.springer.com/book/10.1007/978-0-387-09517-2|accessdate=25 May 2018}}</ref><ref name="patynen">{{cite journal|last1=Pätynen|first1=Jukka|last2=Lokki|first2=Tapio|title=Directivities of Symphony Orchestra Instruments|journal=Acta Acustica United With Acustica|date=2010|volume=96|issue=1|pages=138–167|doi=10.3813/aaa.918265|url=https://doi.org/10.3813/AAA.918265|accessdate=25 May 2018}}</ref><ref name="ziemerj">{{cite journal|last1=Ziemer|first1=Tim|last2=Bader|first2=Rolf|title=Psychoacoustic Sound Field Synthesis for Musical Instrument Radiation Characteristics|date=2017|volume=65|issue=6|pages=482–496|doi=10.17743/jaes.2017.0014|url=http://www.aes.org/e-lib/browse.cfm?elib=18781|accessdate=25 May 2018}}</ref><ref name="zotter">{{cite thesis|type=PhD||institution=University of Music and Performing Arts Graz|last1=Zotter|first1=Franz|title=Analysis and Synthesis of Sound-Radiation with Spherical Arrays|date=2009|___location=Graz|url=https://iem.kug.ac.at/en/projects/workspace/projekte-bis-2008/dsp/analysis-and-synthesis-of-sound-radiation-with-spherical-arrays.html|accessdate=25 May 2018}}</ref>.
==Subjective room acoustics==
The apparent source width and other subjective sound properties in many concert halls have been rated by experts, like [[conductor]]s and [[Music criticism|music critics]]. Together, apparent source width and listener envelopment are the most important contributors to the spaciousness impression of a concert hall which is the most important contributor to the quality ratings of concert halls<ref name=beranek />.
In the field of subjective [[room acoustics]] the sound radiation characteristics are ignored and the apparent source width is explained by means of objective measures of room [[impulse response]]s, like the binaural quality index, the lateral energy fraction and the early sound strength.<ref name=beranek /><ref name=ziemer /><ref name=blau />. These tend to correlate with the subjective expert ratings. Accordingly, early, incoherent, lateral reflections together with a high [[loudness]] of low frequencies in the early reflections of the room reverberation increase the appatentapparent source width and thus the overall spaciousness and quality of a concert hall. This knowledge is used in [[architectural acoustics]] to design a concert hall that exhibits the desired acoustical properties.
==Music production==
This is can be achieved with established recording techniques, like [[Microphone practice#A-B technique: time-of-arrival stereophony|A-B technique]], [[Blumlein pair|Blumlein technique]], [[Microphone practice#M/S technique: Mid/Side stereophony|M-S technique]], [[ORTF stereo technique]], [[Microphone practice#X-Y technique: intensity stereophony|X-Y technique]], or by experimenting with different types of microphones and microphone locations, like [[Microphone#Dynamic|dynamic microphones]], [[Microphone#Ribbon|ribbon microphones]], [[Microphone#contact microphone|contact microphones]], [[boundary microphone]]s and [[Microphone#Speakers as microphones|loudspeakers as microphones]].
Signals that sound too narrow — like too coherent stereo recordings monophonic recordings or synthetic sounds — can be widened by so-called pseudostereophony.<ref name="csound">{{cite journal|last1=Cabrera|first1=Andrés|editor1-last=Hearon|editor1-first=James|editor2-last=Yi|editor2-first=Steven|title=Pseudo-stereo Techniques|journal=CSound Journal|date=2011|issue=14|url=http://csoundjournal.com/issue14/PseudoStereo.html|accessdate=25 May 2018}}</ref><ref name="faller">{{cite conference|last1=Faller|first1=Christoph|title=Pseudostereophony Revisited|conference=Audio Engineering Society Convention 118|pages=paper number 6477|date=2005|url=http://www.aes.org/e-lib/browse.cfm?elib=13193|accessdate=25 May 2018}}</ref><ref name=ziemer/>. These techniques have in common that they [[decorrelation|decorrelate]] the stereo channels by applying individual [[audio filter]]s, [[Reverberation#Creating reverberation effects|reverberation]] and [[Delay (audio effect)|delay effects]] to each. This way the two channels' signals are similar enough to be heard as one integrated stream, i.e., one auditory sound object. At the same time the signals are so diverse that they do not seem to originate in a tiny [[point source]] but rather in a broad source.
Such techniques were also used in [[Duophonic]] sound to re-release monophonic recording with pseudostereophonic sound.
==Related sound impressions==
Several subjective sound impressions are closely related to apparent source width. Reverberance refers to the impression that spatially and temporally distributes sounds blend du to reverberation. [[Liveness]] is the impression that the room contributes more than just repititions of direct sound. A live concert sound better in a reverberant than in a dead or dry hall. In intimate halls instruments sound near and the hall sounds small. Listener envelopment is the impression that the listener takes a bath in sound, i.e., that sound comes from all over the place. Spaciousness is a term that summarizes apparent source width and listener envelopment.<ref name=beranek/>.
==References==
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