Critical bands

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The term critical band, introduced by Fletcher in the 1940s, referred to the frequency bandwidth of the then-loosely-defined auditory filter. Since Georg von Békésy’s studies (1960), the term also refers literally to the specific area on the basilar membrane (an elongated thin sheet of fibers located in the inner ear, inside the cochlea) that goes into vibration in resonance to an incoming sine wave. Its length depends on the elastic properties of the membrane and on active feedback mechanisms operating within the hearing organ. Converging psychophysical and psycho physiological experiments indicate that the average length of the critical band is ~1mm. Psycho-physiologically, beating and auditory roughness sensations can be linked to the inability of the auditory frequency-analysis mechanism to resolve inputs whose frequency difference is smaller than the critical bandwidth and to the resulting instability or periodic “tickling” (Campbell and Greated 1987:61) of the mechanical system (basilar membrane) that resonates in response to such inputs. Critical bands are also closely related to auditory masking phenomena (i.e. reduced audibility of a sound signal when in the presense of a second signal of higher intensity and within the same critical band). Masking phenomena have wide implications, ranging from a complex relationaship between loudness (perceptual frame of reference) and intensity (physical frame of reference) to sound compression algorithms.

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As an example, the perceived loudness of a very narrow-band noise-source with a constant sound pressure level initially remains constant as the noise-bandwidth is gradually increased. Beyond a certain noise-bandwidth, called the critical bandwidth, the loudness begins to increase significantly.

[edit] Sources:

Backus, J. (1977) (2nd ed). The Acoustical Foundations of Music. New York: W.W. Norton and Company. (Click here for notes and comments on Chapter 5 by S. Hamm, University of Florida)
Békésy, G. von. (1960) [1989]. Experiments in Hearing. New York: Acoustical Society of America Press.
Campbell, M. and Greated, C. (1987). The Musician’s Guide to Acoustics. New York: Schirmer Books.
Vassilakis, P.N. (2005). Auditory roughness as means of musical expression. Selected Reports in Ethnomusicology, 12: 119-144.
Vassilakis, P.N. and Fitz, K. (2007). SRA: A Web-based Research Tool for Spectral and Roughness Analysis of Sound Signals. Supported by a Northwest Academic Computing Consortium grant to J. Middleton, Eastern Washington University.