Apparent source width (ASW) refers to the perceived "spatial extent" of a sound source. This

psychoacoustic Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, ...

phenomenon is influenced by both the sound radiation pattern of the source itself and the acoustic properties of the environment in which it is located. A wide ASW is often a desirable characteristic, particularly in genres like

classical music Classical music generally refers to the art music of the Western world, considered to be #Relationship to other music traditions, distinct from Western folk music or popular music traditions. It is sometimes distinguished as Western classical mu ...

opera Opera is a form of History of theatre#European theatre, Western theatre in which music is a fundamental component and dramatic roles are taken by Singing, singers. Such a "work" (the literal translation of the Italian word "opera") is typically ...

, and

historically informed performance Historically informed performance (also referred to as period performance, authentic performance, or HIP) is an approach to the performance of Western classical music, classical music which aims to be faithful to the approach, manner and style of ...

, as it is associated with the immersive sound of acoustic spaces. The study of ASW draws upon research from various fields, including

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 architectur ...

, auralization,

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 buil ...

psychoacoustics Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, ...

, 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 explor ...

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 the 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 ear, sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, incre ...

has mechanisms that separate the processing of late

reverberation In acoustics, reverberation (commonly shortened to reverb) is a persistence of sound after it is produced. It is often created when a sound is reflection (physics), reflected on surfaces, causing multiple reflections that build up and then de ...

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

precedence effect The precedence effect or law of the first wavefront is a binaural psychoacoustical effect concerning sound reflection and the perception of echoes. When two versions of the same sound presented are separated by a sufficiently short time delay ...

. 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 syste ...

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 and a theory of perception that emphasises the processing of entire patterns and configurations, and not merely individual components. It emerged in the early twent ...

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

. 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 the absence of room acoustical reflections, the pure direct sound of musical instruments already affects the perceived source extent. Unlike a theoretical monopole source, musical instruments do not radiate their sound evenly in all directions. Rather, the overall

volume Volume is a measure of regions in 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) ...

and the

frequency spectrum In signal processing, the power spectrum S_(f) of a continuous time signal x(t) describes the distribution of power into frequency components f composing that signal. According to Fourier analysis, any physical signal can be decomposed int ...

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 a

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- or three-dimensional

polar coordinate system In mathematics, the polar coordinate system specifies a given point in a plane by using a distance and an angle as its two coordinates. These are *the point's distance from a reference point called the ''pole'', and *the point's direction from ...

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 m ...

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 subjectivity, subjective perception of sound pressure. More formally, it is defined as the "attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relat ...

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 concert halls that exhibit the desired acoustical properties.

Music production

audio mastering Mastering is a form of audio post production which is the process of preparing and transferring recorded audio from a source containing the final mix to a data storage device called a master recording, the source from which all copies will ...

and

sound recording and reproduction Sound recording and reproduction is the electrical, Mechanical system, 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 ...

, a major task of the

audio engineer An audio engineer (also known as a sound engineer or recording engineer) helps to produce a recording or a live performance, balancing and adjusting sound sources using equalization, dynamics processing and audio effects, mixing, reproduc ...

s and

record producer A record producer or music producer is a music creating project's overall supervisor whose responsibilities can involve a range of creative and technical leadership roles. Typically the job involves hands-on oversight of recording sessions; ensu ...

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 c ...

. This can be achieved with established recording techniques, like A-B technique, Blumlein pair, M-S technique, and the ORTF stereo technique, 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 ...

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 from a tiny

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

but rather from 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 recordings 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"). A progra ...

is the impression that the room contributes more than just repetitions of direct sound. A live concert sounds better in a reverberant hall than in a dead or dry hall. In intimate halls, instruments sound close to the listener and the hall sounds small. Listener envelopment is the impression that the listener is bathed in sound, i.e., that the sound comes from all directions. Spaciousness is a term that summarizes apparent source width and listener envelopment.

References

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