13. Eliminating interfering sounds in a telephone-transmitter stethoscope. R. B. ABBOTT, Purdue University.--By the use of a telephone transmitter with its diaphragm tuned to some low natural frequency, very faint sounds of low frequency have been detected and amplified to audibility with a vacuum tube amplifier, without interference from higher pitched tones. This type of transmitter is very sensitive to sounds of low frequency and unresponsive to those of higher frequency. A very thin aperiodic diaphragm attached to the opening of the mouthpiece converts this transmitter into a very efficient stethoscope for listening to faint sounds of the human body, because these sounds are of low frequency. In fact, by tuning the diaphragm to frequencies of 100 per second or less, the troublesome interference due to sounds of extraneous sources, heretofore experienced, is almost entirely eliminated and the sound of heart beats becomes very clear and distinct. By combining such an apparatus with a three-step vacuum tube amplifier and a loud speaking telephone, the sound of heart beats has been made audible so that hundreds of persons assembled in a large audience room heard it clearly and with ease, phonograph records of heart beats have been made which reproduce the beats with perfect accuracy and clearness, and heart beats have been broadcasted by radiophone and heard with perfect ease by hundreds of persons listening in at receiving stations.
[Abbott, R. B. Physical Review, 1923, 21, 200]