Page 50 - Summer 2015

HIstory of Psychoacoustics
Figure 3. Hermann von Helm- holtz (1821-1894) and his book On the Sensation of Tones.
perception for many de- cades (Figure 3). With the publication of Fechner’s and Helmholtz’s books, the study of hearing turned from one of observations based on a scientist’s per- ceptions to a more system- atic collection of psychop physical data, often stated in terms of acoustical vari- ables, in somewhat well- controlled experiments.
Many studies of hearing occurred in the 18th cen- tury, and two areas of inter- est, pitch and sound source
perceived pitch. However, Seebeck’s siren was constructed so that the fundamental frequency was not physically pres- ent, only the harmonics. The pitch of this “missing funda- mental” sound appeared to be the same as when the funda- mental was present. This implied that the fundamental of a harmonic sound does not have to be physically present for the pitch to be perceived as that of the fundamental. This was at odds with Ohm’s acoustic law as interpreted by Helm- holtz. Helmholtz cited several reasons why Seebeck’s missing fundamental pitch was an “artifact,” and it was many years before the missing fundamental stimulus was cited as a se- rious challenge to Helmholtz’s spectral/resonance theory of pitch perception. Decades later, Schouten (1940) formulated his “residue theory,” which suggested that the missing fun- damental pitch was based on the temporal amplitude enve- lope of the missing fundamental stimulus that would exist in a high-frequency region after the sound was transformed by inner ear filtering processes. Helmholtz’s spectral approach to explaining pitch perception and Schouten’s temporal ap- proach characterize today’s debate as to the probable audito- ry mechanisms accounting for pitch perception (Yost, 2009).
Perceptual scientists in the 19th century observed that the source of a sound could be spatially located based entirely on sound despite the fact that sound has no spatial proper- ties. So how could sound provide information about spatial location? Lord Rayleigh (James William Strutt, 3rd Baron Rayleigh, 1842-1919, Nobel Prize in Physics 1904) and oth- ers reasoned and observed that a sound presented to one side of the head would be more intense at the ear nearest the sound than at the far ear, especially because the head would block the sound from reaching the far ear (the head forms an acoustic shadow). This would produce an interaural level difference (in the horizontal or azimuth plane), which would increase as a sound source was moved from in front toward one ear or the other. Thus, Rayleigh (1876) proposed his “binaural ratio” explanation for sound source localization.
Rayleigh was aware that others (e.g., Silvanus Thompson, 1878) used tuning forks delivering sounds of different fre- quencies to each ear to suggest that the interaural phase might also provide a cue for sound source localization. When one calculates the interaural time difference (ITD) as- sociated with the perceptible interaural phase difference, the ITD is often less than one millisecond. Rayleigh and others felt that a difference this small could not be detected by the auditory system and, besides, Helmholtz had shown that the
“ear was phase insensitive.” Thus, interaural phase (time)
localization, have had a lasting impact on psychoacoustics. Helmholtz was influenced by Georg Simon Ohm’s (1789- 1854) “acoustic law” that stated that the ear performs a lim- ited Fourier analysis by determining the sinusoidal compo- nents of complex sound. Ohm is perhaps better known for his work on electricity and for whom the unit of resistance is named. Fourier (1768-1830), a French physicist/math- ematician working earlier in the 19th century, established a theorem regarding complex fluctuations of heat over time. Fourier’s theorem can be applied to sound pressure varying as a function of time. Helmholtz used Fourier’s theorems to describe a resonance theory of frequency analysis per- formed by the inner ear as the basis of pitch and argued that the resonance place with the greatest magnitude would be a determining factor in pitch perception. Because his inner ear resonators were more sharply tuned at low frequencies, low frequencies were likely to be a dominant factor in pitch perception.
In 1844, August Seebeck (1805-1849) constructed a siren (Figure 4) through which air passed as the siren rotated (see Turner, 1977). The siren produced a pitch based on a series of har- monic tones, each an inte- ger multiple of a fundamen- tal tone the frequency of which was the value of the
Figure 4. Seebeck’s siren. 48 | Acoustics Today | Summer 2015