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Figure 5. Stevens and Newman on the roof (to avoid sound reflections) of James Hall at Harvard University conducting their sound source local- ization experiment (Stevens and New- man, 1936). [Photograph from Ameri- can Journal of Psychology 48:2 (April 1936): pp. 297-306. Copyright 1936 by the Board of Trustees of the University of Illinois. Used with permission from the University of Illinois Press.]
differences were rarely considered as a cue for sound source local- ization until Rayleigh (1907) reevaluated the cues that might be used for sound source localization. Rayleigh argued that the inter- aural level difference (ILD) was a possible cue at high frequencies where the ILDs would be large due to the head shadow, and an interaural time (phase) difference could be a cue at low frequencies. This “duplex theory of sound source localiza-
Today psychoacoustics is usually broadly described as auditory per- ception or just hearing, although the latter term also includes biological aspects of hearing.
ment-supported monopoly for the US telephone system. Bell Laboratories was first established in New York City but moved to a new, specially designed space in Murray Hill, NJ, in 1941. Harvey Fletcher,4 a modest son of a Utah farmer (Figure 6; Table 1), worked at Western Electric and then Bell Laboratoriesfrom1917to1949andwasatonetimedirector of acoustical research and then physical research. In addi- tion to his pioneering roles in the ASA, Fletcher is also well known for his laborious efforts in the Nobel Prize-winning experiment measuring the electron charge carried by a sin- gle atom (awarded to Robert Millikan in 1923). As director
tion” was further validated by others including Stevens and Newman (1936) in their experiment on the roof of James Hall at Harvard (Figure 5). See Jens Blauert’s (1997; Table 1) often-cited book Spatial Hearing for a detailed review of spatial hearing.
Although it was clear for centuries that sound had magni- tude (loudness) in addition to pitch, it was not easy to accu- rately manipulate and control sound magnitude. One could change the length of a tube or string, alter the characteristics of a tuning fork, and vary the density of spikes on a wheel or holes of a siren to change the frequency (pitch). But there was no easy way to vary sound magnitude. So studies on audibility, perceived sound level differences, loudness, and masking were not studied, not because of a lack of interest in these topics but because of a lack of means to accurately vary the sound level. With electroacoustic devices, studies involv- ing sound level proliferated in 1920s and 1930s, especially at Bell Laboratories.
Bell Laboratories (1920-1960)
Bell Laboratories was truly a unique institution both in business and in science (Gertner, 2013). Bell Laboratories (formally founded in 1925) was the “research branch” of the original version of the American Telephone & Telegraph (AT&T) company that was consolidated with Western Elec- tric (the manufacturing arm of AT&T), forming a govern-
Figure 6. Harvey Fletcher (1884- 1981), former Director of Physi- cal Acoustics at Bell Laboratories and first President of the Acousti- cal Society of America.
of acoustical research (see Allen, 1996), Fletcher over- saw a litany of psychoacous- tic research achievements unmatched in the history of the field,5 which included measurements of auditory thresholds (leading to the modern-day audiogram, the gold standard for evaluat- ing hearing loss), intensity discrimination, frequency discrimination, tone-on- tone masking, tone-in-noise masking, the critical band, the phon scale of loudness, and the articulation index. psychoacoustic contribu-
Two of the more important
tions of the Bell Laboratories years are the critical-band and equal-loudness contours.
Fletcher originally conceived of critical bands in terms of both loudness and masking (Allen, 1996). The critical band is a frequency region that is “critical” for masking and/or loudness summation (the masking definition is used most
4 See http://www.nasonline.org/publications/biographical-memoirs/ memoir-pdfs/fletcher-harvey.pdf.
5 In addition to Jont Allen’s article (1996), see Study of Speech and Hearing at Bell Telephone Laboratories, ASA-CD, ASA-DS-02, ASA- Online Book Store. Produced by Christine Rankovic and Jont Allen.
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