User:Sarah polanc/sandbox

Apparent Source Width

Apparent Source Width (ASW) is the audible impression that a sound source extends its visual width. 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 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 direc 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 characterisitcs 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.

In the field of subjective room acoustics the round 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. The apparent source width and other subjective sound properties in many concert halls have been rated by experts like conductors and music critics. Together, aApparent 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.

Music Production
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 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 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.