Apparent source width
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Apparent source width (ASW) is the audible impression of a spatially extended sound source. Physically, this psychoacoustic impressions results from sound radiation characteristics and room acoustical properties. Wide sources are desired by listeners of music. Apparent source width affects the perceived sound of unplugged concerts of art music, opera, classical music, historically informed performance and contemporary classical music, as well as concerts that use live event support, like live sound mixing, sound reinforcement systems or a public address system, like popular music, rock music, electronic music and musical theatre. Research concerning the ASW comes from the field of room acoustics, architectural acoustics and auralization as well as musical acoustics, psychoacoustics and systematic musicology.
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. 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 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 source localization, intimacy and the apparent source width. 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 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. 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.
Even in absence of room acoustical reflections the pure direct sound of musical instruments already effects the perceived source extent. Unlike hypothetical monopole source musical instruments radiate their sound not evenly in all directions. Rather the overall volume and the frequency spectrum differ in each direction, referred to as sound radiation characteristics or radiation patterns. 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.
Subjective room acoustics
The apparent source width and other subjective sound properties in many concert halls have been rated by experts, like conductors and 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.
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 responses, like the binaural quality index, the lateral energy fraction and the early sound strength. 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 appatent 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.
In audio mastering and sound recording and reproduction a major task of the recording studio`s audio engineers and record producers is to make musical instruments sound huge. The increase of apparent source width is as important as spectral balancing and dynamic range compression.
This is can be achieved with established recording techniques, like A-B technique, Blumlein technique, M-S technique, ORTF stereo technique, X-Y technique, or by experimenting with different types of microphones and microphone locations, like dynamic microphones, ribbon microphones, contact microphones, boundary microphones and 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. These techniques have in common that they decorrelate the stereo channels by applying individual audio filters, reverberation and 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 pseudo-stereophonic sound.
Related sound impressions
Several subjective sound impressions are closely related to apparent source width. Reverberance refers to the impression that spatially and temporally distributed sounds blend due to reverberation. Liveness is the impression that the room contributes more than just repetitions of direct sound. A live concert sounds 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.
- Blau, Matthias (2004). "Correlation of apparent source width with objective measures in synthetic sound fields". Acta Acustica United with Acustica. 90 (4): 720. Retrieved 31 May 2018.
- Beranek, Leo Leroy (2004). Concert Halls and Opera Houses: Music, Acoustics, and Architecture (Second ed.). New York: Springer. doi:10.1007/978-0-387-21636-2. ISBN 978-1-4419-3038-5.
- Braun, Christopher B.; Grande, Terry (2008). "Evolution of peripheral mechanisms for the enhancement of sound reception". In Webb, Jacqueline F.; Fay, Richard R.; Popper, Arthur N. Fish Bioacoustics. New York: Springer. p. 105. doi:10.1007/978-0-387-73029-5_4. ISBN 978-0-387-73029-5.
- Ziemer, Tim (2017). "Source Width in Music Production. Methods in Stereo, Ambisonics, and Wave Field Synthesis". In Schneider, Albrecht. Studies in Musical Acoustics and Psychoacoustics. Current Research in Systematic Musicology. 4. Cham: Springer. pp. 299–340. doi:10.1007/978-3-319-47292-8_10. ISBN 978-3-319-47292-8.
- Ziemer, Tim (2015). Implementation of the Radiation Characteristics of Musical Instruments in Wave Field Synthesis Applications (PhD). Hamburg: Univ. Diss. doi:10.13140/RG.2.1.1997.9769. Retrieved 25 May 2018.
- Bader, Rolf (2012). "Radiation characteristics of multiple and single sound hole vihuelas and a classical guitar". The Journal of the Acoustical Society of America. 131 (1): 819–828. Bibcode:2012ASAJ..131..819B. doi:10.1121/1.3651096. PMID 22280704.
- Meyer, Jürgen (2009). Acoustics and the Performance of Music. Manual for Acousticians, Audio Engineers, Musicians, Architects and Musical Instrument Makers (Fifth ed.). Bergkirchen: Springer. doi:10.1007/978-0-387-09517-2. ISBN 978-0-387-09516-5.
- Pätynen, Jukka; Lokki, Tapio (2010). "Directivities of Symphony Orchestra Instruments". Acta Acustica United with Acustica. 96 (1): 138–167. doi:10.3813/aaa.918265.
- Ziemer, Tim; Bader, Rolf (2017). "Psychoacoustic Sound Field Synthesis for Musical Instrument Radiation Characteristics". Journal of the Audio Engineering Society. 65 (6): 482–496. doi:10.17743/jaes.2017.0014. Retrieved 25 May 2018.
- Zotter, Franz (2009). Analysis and Synthesis of Sound-Radiation with Spherical Arrays (PhD). Graz: University of Music and Performing Arts Graz. Retrieved 25 May 2018.
- Levinit, D.J. (2004-03-11). "Instrument (and vocal) recording tips and tricks". In Greenbaum, K.; Barzel, R. Audio Anecdotes. vol. I. Natick: A K Peters. pp. 147–158. ISBN 978-1568811048.
- Kaiser, C. (2013). 1001 Mastering Tipps. Heidelberg: mitp. p. 23,40.
- Cabrera, Andrés (2011). Hearon, James; Yi, Steven, eds. "Pseudo-stereo Techniques". CSound Journal (14). Retrieved 25 May 2018.
- Faller, Christoph (2005). Pseudostereophony Revisited. Audio Engineering Society Convention 118. pp. paper number 6477. Retrieved 25 May 2018.