The Relative Sensitivity of the Ear at Different Levels of Loudness

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Abstract
Alternation Phonometer for Balancing the Loudness of Tones of Different Pitch.—It has been found that if two tones alternate in the ear at a suitable rate, for instance 25 times a second, the interruptions of the louder tone are the more conspicuous, and it is possible to adjust the relative intensities until the interruptions are equally conspicuous. Two tones when balanced in this way are said to be equally loud. This definition is justified by the fact that if two tones each balance a third tone they are found to balance each other and are also accepted by the ear as equally loud. The electrical circuit, commutating relays, and other apparatus used in making a loudness balance are described. To enable the mean pressure of the sound at the ear drum to be calculated from the electrical energy input, either a thermophone, whose theory is known, or an electromagnetic receiver which had been calibrated (rather imperfectly) by means of an artificial ear cavity and an electrostatic transmitter, was used next the ear. Successive readings usually agree within 5 per cent.

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