Apparatus for the Quantitative Testing of Air and Bone Transmitted Speech.

Abstract

The usual test for the relative acuity of hearing air and bone transmitted sound is made with a series of tuning forks—the air vibrations coming from the prongs and the bone vibrations through application of the stem to the head of the examined individual. It is well known that the greatest disability of the deafened individual is found in his inability to hear conversations. The sounds from the fork are relatively pure tones while the voice sounds are complicated combinations of tones. For this reason the fork tests are a poor criterion of the sensitivity for speech. An air telephone with its large diaphragm and efficient production of air vibrations of large amplitude and little force necessarily shows an entirely different type of response to that elicited through a bone telephone which develops small amplitudes of considerable force. A quantitative method of producing voice sounds has been developed by the Western Electric Co. in the phonograph audiometer with its electromagnetic pick-up. This instrument may also be employed with a bone telephone provided the energy is stepped up with a 2 stage audio-amplifier. It is also possible to test both the air and the bone acuity with the same receiver under the same mechanical conditions. This quite eliminates the physical peculiarities of the receiver itself and makes a direct comparison possible. The air acuity is determined by holding the receiver by its mushroom stem against a resonant surface and the person examined writes the numbers heard which become progressively fainter and fainter. Next the record is changed and the bone telephone is held against the forehead in a similar manner. Again the numbers are written by the observer. If it is thought desirable to test the enhancement to bone acuity which is brought about by occlusion of the external canals, the ears are closed with plastocene and the test repeated the third time.