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that any experiments using two ears should take note of the ear to which stimuli are presented.)
The invention of the stethophone and beyond
An important advance in the study of binaural hearing occurred with the invention of the stethophone (Fig. 7) by Somerville Scott Alison (1813—1877). His communication of this invention to the Royal Society was read by John Tyndall (1820—1893) on 22 April 1858 and it was printed in their Proceedings
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graduated from its Medical School in 1833. He moved to London in 1841, where he set up practice and specialized in diseases of the heart and lungs. It was in this context that he invented the stethophone, which was derived from the stethoscope—an invention of René Théophile Hyacinthe Laennec (1781–1826).38 Laennec’s stethoscope was a simple tube or cylinder that could amplify sounds from the chest when placed between ear and chest. The cylin- der could be made of paper, but wood proved more durable. Adapting a single tube which then connected to the two ears appeared shortly afterwards, although its adaptation was more for convenience than for any binaural bene- fits: “The instrument adapted to this pur- pose consists of a tube, connected at its middle at right angles to the cylinder, to be applied to the patient, and connected at its moveable extremities to two tubes.”39
Alison’s stethophone had independ-
ent ear tubes, so that different sounds
could be listened to with this instru-
ment: “The differential stethophone is
simply an instrument consisting of two
hearing-tubes, or trumpets, or stetho-
scopes, provided with collecting-cups and
ear-knobs, one for each ear respectively. The two tubes are, for convenience, mechanically combined, but may be said to be acoustically separate, as care is taken that the sound, once admitted into one tube, is not communicated to the other.” 40
In describing his experiments using the stethophone, Alison referred back to those of Wheatstone: “Mr. Wheatstone shows that a vocal sound is heard louder in that ear that is closed, say with the finger, than in the other. He also shows, that the sound of a tuning-fork placed upon the head is heard louder in that ear which is closed than in the other which remains open, even though the tuning-fork may be brought nearer the open ear than the closed one.”41 Here, Alison was
in the following year.
Alison was born in Edinburgh, and
Fig. 7. Alison’s stethophone as illustrated in his origi- nal article: “The tubes are composed of two parts nearly equal in length, one near the ear-knob, made of metal (C); while the other part, near the collecting- cup, is made of metal wire (B), to impart flexibility. The ear-end is curved, so as to approach the ear, and is supplied with an ivory knob (D) for insertion into the meatus externus. The other end of the tube, being intended to collect sound, is supplied with a hollow cup, or receiver (A) made of wood, or some such material.” 37
referring to Wheatstone’s report of experiments in which he described the microphone, and his binaural investiga-
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This was the only study that Alison found relevant to his own, using the stethophone. He pre- sented sounds of different intensity to each ear and found that the more intense one was heard in the ear receiv- ing it. “Sound, as is well known, if applied to both ears in equal intensity, is heard in both ears; but it will be found, if the intensity in respect of one ear be moder- ately yet decidedly increased, by bringing the sounding body nearer that ear than the other, or otherwise, as by the employ- ment, in respect to one ear, of a damper or obstructor of sound, or in respect to the other ear, by the employment of some intensifier, or good collector or conductor of sound, the sound is heard in that ear only which is favoured and has the
tions with tuning forks.
advantage of greater intensity.”42
The principal sound sources used for
Alison’s experiments employing the stethophone were watches (although he did not cite Weber’s30 or Fechner’s34 reports using pocket watches). Most of Alison’s studies involved a single watch, the intensity of which was varied to each ear. The watch was heard in the ear receiving the more intense sound. When two different watches were employed, one to each ear: “the sounds of both watches are heard, but the sound from one is heard in one ear, and the sound from the other is heard in the other ear.” 43 On the basis of the experiments he formulated two laws: “1st, that sounds of the same character are restricted to that ear into which they are conveyed in greater intensi- ty, and 2nd, that sounds differing in char- acter may be heard at the same time in the two ears respectively, even if they be made to reach the ears in different degrees of intensity.” 44
Alison also conducted an experiment that can be related to those proposed by Wells10 and also to that conducted by Wheatstone27 in which components of sounds were present- ed to different ears, both simultaneously and in succession: “In order to effect a division of a compound sound, it is only necessary that the two sounds of which it is composed may respectively be heard at certain points, in greater and lesser intensity, and that the respective cups of the stethophone be placed at these points...For example, a compound sound com- posed of the two sounds of two watches placed together upon a table, with the unassisted ear is distinctly heard in its com- pound state, and cannot be divided into its constituent two
Binaural Hearing and the Stethophone 21