This paper describes a telephone receiver of the moving-coil type which is particularly adaptable to the horn type of loudspeaker and which represents a notable advance over similar devices at present available. Its design is such as to permit a continuous electrical input of 30 watts as contrasted with the largest capacity heretofore available of about 5 watts. In addition, measurements show that the receiver has a conversion efficiency from electrical to sound energy varying between 10 and 50 percent in the frequency range of 60-7500 Hz. Throughout most of this range, its efficiency is 50 percent or better. This contrasts with an average efficiency of about 1 percent for other loudspeakers either of the horn or cone type. Combining the 50-fold increase in efficiency with a 5- or 6-fold increase in power capacity, a single loudspeaker unit of the type here described is capable of 250-300 times the sound output of anything heretofore available. This device is in commercial use in connection with the Vitaphone and Movietone types of talking motion pictures. As commercially produced in quantities numbering several thousand, an average efficiency of the order of 30 percent has been realized.
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