Following a discussion of the acoustical load to which a telephone receiver is exposed when held to an ear, an account is given of measurements of the mechanical impedances of diaphragms. Since the measurements were made under circumstances for which the effects of the air on each side of the diaphragm were known, it has been possible to isolate the mechanical impedance of the diaphragm alone. Consequently modifications of the frequency characteristic of a receiver, resulting from known changes of the acoustical load, can be estimated. It is shown that the presence of the steady flux is responsible for most of the mechanical resistance of the diaphragm.The magnetic forces by which the diaphragm is actuated have been studied by measurements of both the direct and alternating flux leaving the pole-pieces, and of the pull on the diaphragm due to the direct flux. Different diaphragms, spacing distances, and receivers, were used for these tests. It appears that, with the very adequate magnet strength used in modern receivers and the resultant degree of magnetic saturation of the diaphragm, the sensitivity of the receiver is not critical to variations either of magnet strength or of air-gap.Typical examples of frequency characteristics of receivers, taken on an artificial ear, are shown, illustrating the effects of various simple modifications to the construction of the receiver. The sensitivity as recorded by the frequency characteristic of a receiver is in good agreement with that calculated from measurements of flux, pull, and mechanical impedance.Some measurements are recorded of both amplitude distortion and non-linear distortion introduced by telephone receivers; the magnitudes are such as would not ordinarily be perceived by the ear.