Synopsis: This paper describes a telephone receiver of the moving coil type which is particularly adaptable to the horn type of loud speaker and which represents a notable advance over similar devices at present available. Its design is such as to permit of 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 per cent in the frequency range of 60 to 7,500 cycles. Throughout most of this range, its efficiency is 50 per cent or better. This contrasts with an average efficiency of about 1 per cent for other loud speakers 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 loud speaker unit of the type here described is capable of 250 to 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 per cent has been realized.
Electrostatic Transmitter of Constant Sensitivity,-(i) Characteristics. This instrument is the same in principle as that described in 1917, but certain changes have been made which, as proved by actual tests, render the sensitivity independent of changes of temperature, pressure and humidity. The sensitivity is also found to be constant over a long period of time. By means of a piston-phone and a thermophone, for which corrected formulae are available, both the absolute sensitivity and phase lag were determined for frequencies of from 10 to 12,000 cycles. Eight transmitters similarly constructed give the same curves within 20 per cent. With a steel diaphragm 0.0051 cm. thick having a natural frequency of 7,000 cycles, and with an air gap of 0.0025 cm., the mean sensitivity is about 0.35 millivolt/dyne. (2) Use with an amplifier for measurement of sound intensity. Combined with an amplifier of ordinary design the instrument has an overall sensitivity which is practically uniform from 25 to 8,000 cycles. It is therefore remarkably well adapted for the measurement of the intensities of complex tones and tones of changing pitch and for use with an oscillograph for recording sound waves. On the other hand, if sounds of a definite pitch are to be measured, the apparatus can be made highly selective and almost any desired sensitivity can be obtained by using a tuned amplifier in connection with a vibration galvanometer.
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