Page 23 - Summer 2008

Fig 8. Left–“Pseudoscopic Thompson” by Nicholas Wade. Silvanus Phillips Thompson is shown with his left and right eyes superimposed. Right–his illustration of the pseudophone which he described in the following way: “The simple instrument for which the author suggests the name Pseudophone consists of a pair of ear-pieces, A A, furnished with adjustable metallic flaps or reflectors of sound, C C, which can be fitted to the ears by proper straps, D and E, and can be set at any desired angle with respect to the axis of the ears, and can also be turned upon a revolving collar about that axis so as to reflect sounds into the ears from any desired direction.” 52
parts.”45 The stethophone made such experiments both easier to perform and with more precise separation of the sound sources than had been possible beforehand. Alison then applied these principles to the practice of auscultation in medicine.
46
Interference beats could be heard binaurally, but tones differing in phase could not. He confirmed these findings in a second paper,48 and described the experience that the combined tone was local- ized within the head. In a further article,49 he described his invention of the pseudophone, which was modeled on Wheatstone’s pseudoscope.50 He wrote: “The Pseudophone is an instrument for investigating the laws of Binaural Audition by means of the illusions it produces in the acoustic perception
of space.”51
At about the same time, Anton Steinhauser (1802–1890)
presented his theory of binaural audition in a short mono-
53
It was applied to his differential stethophone, which he subse- quently referred to as a bin-aural stethoscope. Later, Silvanus Thompson (1851–1916), shown in Fig. 8, used the term bin- aural in the title of an article concerned with similar experi- ments to those of Fechner34 with tuning forks (though he was unaware of this earlier work). He presented a tone of slightly fluctuating pitch to one ear (using an India-rubber tube) and
47
graph.
It was translated into English by Thompson, and it
Alison soon afterwards introduced the term binaural.
a standard tuning fork to the other ear.
proposed that the directions of sounds were determined
principally by differences in intensity between the ears. This
is essentially the same conclusion as had earlier been reached
6-9
by Venturi . Steinhauser stated that the area of binaural
hearing was a novel one: “The second branch of the subject \[binaural audition\], which has never, to my knowledge, been yet developed, has to discuss the general question of hearing,
with respect in particular to the circumstance that it is per- formed with two ears. It is concerned, further, in deciding what part binaural hearing plays in the various phenomena of hear- ing in general, and the various advantages thereby gained.”54 (The novelty of this area was attested by the absence of any references in Steinhauser’s earlier monograph.)
Alison’s invention of the stethophone did not have the
impact of Wheatstone’s stereoscope— there was no sudden
surge of studies in the way there were after the announce-
ment of the stereoscope. On the other hand, Thompson’s
experiments did herald a new departure. That is, rather than
the stethophone, it was Thompson’s use of rubber tubes to
deliver sounds separately to each ear, and his invention of the
pseudophone, that transformed the experimental study of
binaural hearing. The characteristics of the monaural stimuli
could now be specified and delivered more precisely, and the
factors involved in sound localization could be fractionated
(see also the later work of Thompson55 and Rayleigh56). The
renewal of interest in matters binaural even led to the intro-
duction of a specific terminology for stimulating the ears.
Carl Stumpf (1848–1936) distinguished between dichotic
which referred to the stimulation of each ear with a different
stimulus and diotic—the simultaneous stimulation of each
16,57
Related twentieth century observations
The development of electronic technology at the turn of the twentieth century enabled researchers for the first time to study binaural interactions with the precision required to arrive at firm conclusions. The early speculations and informal experiments described above addressed at least five separable issues concerning binaural integration: The integration of input from the two ears to localize sounds; the integration of
ear with the same stimulus.
22 Acoustics Today, July 2008