Apparent source width (ASW) is the audible impression of a spatially extended sound source. This

psychoacoustic Psychoacoustics is the branch of psychophysics involving the scientific study of sound perception and audiology—how humans perceive various sounds. More specifically, it is the branch of science studying the psychological responses associated wi ...

impression results from sound radiation characteristics and properties of an

acoustic space Room acoustics is a subfield of acoustics dealing with the behaviour of sound in enclosed or partially-enclosed spaces. The architectural details of a room influences the behaviour of sound waves within it, with the effects varying by frequency ...

. Wide sources are desired by listeners of music because these are associated with sound of

acoustic music Acoustic music is music that solely or primarily uses instruments that produce sound through acoustic means, as opposed to electric or electronic means. While all music was once acoustic, the retronym "acoustic music" appeared after the ad ...

opera Opera is a form of theatre in which music is a fundamental component and dramatic roles are taken by singers. Such a "work" (the literal translation of the Italian word "opera") is typically a collaboration between a composer and a libre ...

, classical music, historically informed performance. Research concerning ASW comes from the field of

room acoustics Room acoustics is a subfield of acoustics dealing with the behaviour of sound in enclosed or partially-enclosed spaces. The architectural details of a room influences the behaviour of sound waves within it, with the effects varying by frequency ...

architectural acoustics Architectural acoustics (also known as building acoustics) is the science and engineering of achieving a good sound within a building and is a branch of acoustical engineering. The first application of modern scientific methods to architectura ...

and

auralization Auralization is a procedure designed to model and simulate the experience of acoustic phenomena rendered as a soundfield in a virtualized space. This is useful in configuring the soundscape of architectural structures, concert venues, and public s ...

as well as

musical acoustics Musical acoustics or music acoustics is a multidisciplinary field that combines knowledge from physics, psychophysics, organology (classification of the instruments), physiology, music theory, ethnomusicology, signal processing and instrument buildi ...

psychoacoustics Psychoacoustics is the branch of psychophysics involving the scientific study of sound perception and audiology—how humans perceive various sounds. More specifically, it is the branch of science studying the psychological responses associated ...

and

systematic musicology Systematic musicology is an umbrella term, used mainly in Central Europe, for several subdisciplines and paradigms of musicology. "Systematic musicology has traditionally been conceived of as an interdisciplinary science, whose aim it is to explo ...

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 visible 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. Since the term ''apparent source width'' has been used a lot in the field of subjective room acoustics to characterize how the room affects the perception of source size, the term ''perceived source extent'' has been introduced to highlight that the perception results from both the sound source and the room. The

auditory system The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, increasin ...

has mechanisms that separate the processing of late

reverberation Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abs ...

from the processing of direct sound and early reflections referred to as

precedence effect The precedence effect or law of the first wavefront is a binaural psychoacoustical effect. When a sound is followed by another sound separated by a sufficiently short time delay (below the listener's echo threshold), listeners perceive a single a ...

. While the late reverberation contributes to the

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system, ...

of 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 Gestalt-psychology, gestaltism, or configurationism is a school of psychology that emerged in the early twentieth century in Austria and Germany as a theory of perception that was a rejection of basic principles of Wilhelm Wundt's and Edward ...

. Several auditory streams are segregated from one another. The process of integration and segregation is referred to as

auditory scene analysis In perception and psychophysics, auditory scene analysis (ASA) is a proposed model for the basis of auditory perception. This is understood as the process by which the human auditory system organizes sound into perceptually meaningful elements. T ...

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 musical ensemble, also known as a music group or musical group, is a group of people who perform instrumental and/or vocal music, with the ensemble typically known by a distinct name. Some music ensembles consist solely of instrumentalists ...

. 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 affects the perceived source extent. Unlike hypothetical monopole source musical instruments radiate their sound not evenly in all directions. Rather the overall

volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). Th ...

and the

frequency spectrum The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...

differ in each direction. This is referred to as sound radiation characteristics or radiation patterns. These may create incoherent signals at the ears and, consequently, the impression of a wide source. The sound radiation characteristics of musical instruments are typically given as

radiation pattern In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: � ...

in a two- to three-dimensional

polar coordinate system In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...

Subjective room acoustics

The apparent source width and other subjective sound properties in many concert halls have been rated by experts, including conductors and

music critic ''The Oxford Companion to Music'' defines music criticism as "the intellectual activity of formulating judgments on the value and degree of excellence of individual works of music, or whole groups or genres". In this sense, it is a branch of mu ...

s. 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

the sound radiation characteristics are ignored and the apparent source width is explained by means of objective measures of room

impulse response In signal processing and control theory, the impulse response, or impulse response function (IRF), of a dynamic system is its output when presented with a brief input signal, called an impulse (). More generally, an impulse response is the reac ...

s, 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 In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relation of phys ...

of low frequencies in the early reflections of the room reverberation increase the apparent source width and thus the overall spaciousness and quality of a concert hall. This knowledge is used in

to design a concert hall that exhibits the desired acoustical properties.

Music production

audio mastering Mastering, a form of audio post production, is the process of preparing and transferring recorded audio from a source containing the final mix to a data storage device (the master), the source from which all copies will be produced (via meth ...

and

sound recording and reproduction Sound recording and reproduction is the electrical, mechanical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recordin ...

a major task of the audio engineers and

record producer A record producer is a recording project's creative and technical leader, commanding studio time and coaching artists, and in popular genres typically creates the song's very sound and structure. Virgil Moorefield"Introduction" ''The Producer as ...

s is to make musical instruments sound huge. The increase of apparent source width is as important as spectral balancing and

dynamic range compression Dynamic range compression (DRC) or simply compression is an audio signal processing operation that reduces the volume of loud sounds or amplifies quiet sounds, thus reducing or ''compressing'' an audio signal's dynamic range. Compression is ...

. This is can be achieved with established recording techniques, like

A-B technique There are a number of well-developed microphone techniques used for recording musical, film, or voice sources or picking up sounds as part of sound reinforcement systems. The choice of technique depends on a number of factors, including: * The w ...

Blumlein pair Blumlein pair is a stereo recording technique invented by Alan Blumlein for the creation of recordings that, upon replaying through headphones or loudspeakers, recreate the spatial characteristics of the recorded signal. The pair consists of an ...

, M-S technique,

ORTF stereo technique The ORTF stereo technique, also known as ''side-other-side'', is a microphone technique used to record stereo sound. It was devised around 1960 at the Office de Radiodiffusion Télévision Française (ORTF) at Radio France. ORTF combines both t ...

, or by experimenting with different types of microphones and microphone locations. Signals that sound too narrow — like too coherent stereo recordings, monophonic recordings or synthetic sounds — can be widened by so-called pseudostereophony. These techniques decorrelate the stereo channels by applying individual

audio filter An audio filter is a frequency dependent circuit, working in the audio frequency range, 0 Hz to 20 kHz. Audio filters can amplify (boost), pass or attenuate (cut) some frequency ranges. Many types of filters exist for different audio a ...

or delay effects to each. The resulting channels' signals are similar enough to be heard as one integrated auditory sound object but are so diverse that they do not seem to originate in a tiny

point source A point source is a single identifiable ''localised'' source of something. A point source has negligible extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources can ...

but rather in a broad source. Such techniques were also used in

Duophonic Duophonic sound was a trade name for a type of audio signal processing used by Capitol Records on certain releases and re-releases of mono recordings issued during the 1960s and 1970s. In this process monaural recordings were reprocessed into a ...

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 Properties of an execution of a computer program —particularly for concurrent and distributed systems— have long been formulated by giving ''safety properties'' ("bad things don't happen") and ''liveness properties'' ("good things do happen"). ...

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.

References

{{Reflist Psychoacoustics