Bone-conduction receiver with an extended frequency range for measuring the threshold of audibility

Abstract

The present-day standardized threshold audiometry provides a measurement of the audi- tory sensitivity from 0.125 up to 6.0-8.0 kHz. Yet the human ear, as is well known, per- ceives tones of up to 20 kHz frequency, while for bone conduction it responds to sounds up to 200 kHz, i.e., to ultrasounds (1-3). It has been found in practice that an investigation of the audibility thresholds over an extended frequency range (i.e., up to 20 kHz) permits the diagnostic possibilities of audiometry to be considerably enhanced. The diagnostic significance of audibility thresholds at frequencies of I0, 12, 16, and 18 kHz for air-borne sounds was made clear in (4-9). However examinations with air- borne stimuli substantially restrict the diagnostic possibilities of audiometry, even over the extended frequency range because under air-conduction conditions the upper limit of fre- quencies perceived is lowered. It is necessary to develop threshold-measuring methods at frequencies above 8 kHz for bone-conducted sounds. In most cases the detection of early signs of deafness is needed for patients having a symmetrical auditory abnormality (presby- cousis, otosclerosis, the various forms of cochlear neuritis, etc.). Under these conditions it is of paramount importance to develop a bone-conduction receiver that can reproduce sounds of suitable frequencies, and to calibrate the same. The bone-conductor receivers currently used in audiometric practice are of the electro- magnetic type. To a considerable extent this is due to their relatively low electrical im- pedance that permits a receiver to be matched to generating apparatus without the use of special transformers.