Development of computer program ENAUDIBL for computation of the sensation levels of multiple, complex, intrusive sounds in the presence of residual environmental masking noise Page: 4 of 17
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THE FUNDAMENTAL SIGNIFICANCE OF USING CRITICAL-BAND
AND ERB FREQUENCY DISTRIBUTIONS
In 1940, Dr. Harvey Fletcher of Bell Telephone Laboratories, in his classic paper, reported on the
existence of a set of "critical bands" of frequency by which human hearing could be modeled;' i.e.,
human perception of sound can be described as though sensed by approximately 36 separate
frequency filters, all operating in parallel, and with different individual bandwidths. This revelation
was actually preceded by other research publications," and, since 1933, extensive additional
research work by psychophysicists, experimental psychologists, and related investigators has
refined the concept, especially as it pertains to defining the sensation level ("audibility") of each
component sound in the presence of others.
Sound is detected by the ears through separately measurable, physiological, "pseudo parallel-
filter" channels that are not either uniformly logarithmic or linear in distribution of frequency limits
or bandwidths. This natural frequency-filtering characteristic of hearing is determined directly by
the physical structure of the approximately 35-mm-long basilar membrane in the inner ear
(cochlea), which contains nature's "analog-to-digital-converter," the Organ of Corti.'9 The analog
sound-pressure waves are sensed by resonances of approximately 3,500 inner hair cells
(approximately 100 per mm), which, in turn, mechanically excite approximately 20,000 outer hair
cells, which produce neural impulses transmitted to the brain.'9 This anatomy, determined by
Nobel prize winner, Georg von Bekesy, is documented by Greenwood'2 to correlate the
equivalent rectangular (amplitude-response shape) bandwidths (ERBs) with equal intervals of
length along the basilar membrane. Since the natural frequency-filtering characteristics of hearing
can be modeled mathematically by 36 ERB intervals, the curious coincidence occurs that each
ERB represents approximately 1 mm of length on the basilar membrane.'2
Scharf states, "The critical-band concept is fundamental to the understanding of human
hearing.... Nowhere in auditory theory or in acoustic psychophysiological practice is there
anything more ubiquitous than the critical band. [authors' italics] It turns up in the measurement
of pitch, in the study of loudness, in the analysis of masking and fatiguing signals, in the
perception of phase, and even in the determination of the pleasantness of music. And likely, in one
way or another, it will be a part of the final understanding of how and why we perceive anything
that reaches our ears." 13
Cohen and Fielder state, "An acoustic signal is detected if the energy within a particular critical
band exceeds a certain level, whether the signal is sine-wave tone, a band of noise, or a
combination of the two. The signal is perceived only if the energy within at least one critical band
exceeds threshold, independent of the levels in other critical bands. This independence between
bands at threshold greatly simplifies the analysis of the audibility of sound..." 20
Cohen and Fielder further describe the basic audibility-modeling methodology: "The signal
composed of sine-wave tones, noise, or any combination of them, is divided into critical
bandwidths and the level in each band is examined to see whether a particular threshold is
exceeded. If that threshold is exceeded, then the listener perceives (the) noise, independent of any
other critical band. This independence of bands greatly simplifies the analytical process since it
is only the highest band relative to the hearing threshold that is significant..." 20 [authors' italics].
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Liebich, R. E.; Chang, Y.-S. & Chun, K. C. Development of computer program ENAUDIBL for computation of the sensation levels of multiple, complex, intrusive sounds in the presence of residual environmental masking noise, article, March 31, 2000; Argonne, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc706726/m1/4/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.