On the construction of hearing and speaking instruments / by John Bishop.

  • Bishop, John, 1797-1873.
Date:
[1856]
Catalogue details

Licence: Public Domain Mark

Credit: On the construction of hearing and speaking instruments / by John Bishop. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    speaker removes a few feet from the aperture, his voice becomes totally inaudible by the deaf person whom he addresses. To find a remedy for these defects, so as to enable a deaf person to hear at any other distance sounds which are audible by him with an ear-trumpet of a given aper- ture at a given distance from the speaker, is the pro- blem we propose to investigate. We know by the theory of acoustics, that a sound diminishes in intensity in proportion to the square of the distance of the organs of hearing from the origin of the sound; and we also know from geometry, that the areas of similar figures are as the squares of their corresponding lines. From these acoustic and geometric laws we can easily determine the area of an ear instrument, when- the distance between the speaker and the hearer is altered. Let A be the diameter of the aperture of the instru- ment through which the patient can hear a person speaking at the distance D from the external aper- ture, then, at any other distance ?nD, the intensity of the sound at the aperture will, by the first of the above-mentioned laws, be to its intensity at the original distance D, as D2 : m2D*, or as 1 : m2. From this cal- culation, as well as from experience, it is clear that the same instrument cannot collect the same amount of sound at different distances; and, since the quantity of sound collected depends upon the area of the aperture, we must, in order to collect the same amount of sound, vary that area in exactly the same proportion as the in- tensity of the sound has been changed j and therefore