A new, powerful, magnetostrictive bone conduction (BC) vibrator is described. At low output levels, it has a wide frequency range (5 Hz to above 20 kHz), and for midfrequencies undistorted outputs reach up to 50 g in terms of acceleration. BC thresholds and SL’s (the latter determined by AC/BC cancellations) were measured with this vibrator. Both varied with contact force, but much more markedly with contract area, in unoccluded as well as in occluded ears. Sound attenuation boxes, after von Békésy, were used in the unoccluded state. Results of the AC/BC cancellations permitted determination of the external-ear and the inner-ear BC components. With the ear unoccluded, variations with contact area (and/or force) were mainly related to changes of the inner-ear responses, but with the ear occluded, they were related to those of the BC-generated sound pressures in the ear canal for f<2 kHz. At AC and BC threshold, SPL’s in the ear canal had identical values and did not vary with contact area or force for f<2 kHz. For a comparable small contact area, BC acceleration thresholds were similar to those specified in ANSI S3.13−1972. For larger contact areas, however, they were lower, by as much as 25 dB, at some frequencies. These results indicate that acceleration is not a valid indicator of the BC input to the skull. Subject Classification: [43]65.22, [43]65.80, [43]65.64.
An experimental bone conduction vibrator was used to measure force and acceleration directly at the point of contact, the subject's forehead. Force and acceleration at threshold were measured for six subjects over a frequency range of 250 to 6000 Hz and over a contact area range of five to one. These measurements suggest that for any test subject, the variation in force of threshold with contact area is much smaller than the corresponding variation in acceleration at threshold.
Bone-conduction (BC) thresholds measured in terms of acceleration had shown large systematic variations, mainly with contact area [J. Acoust. Soc. Am. 55, S62 (1974)]. Sound pressures generated in the occluded ear canal by BC signals of threshold strength were now found to be independent of contact area and, below 2 kHz, equal to those required for AC thresholds. Air/bone cancellation experiments revealed that, below 2 kHz and with the ear canal occluded, the BC stimulus is really given by the sound pressure generated in the ear canal, i.e., one measures the response to an air signal instead of that to a bone signal. [Essentially the same conclusion was reached for the cat; cf. Acta Otolaryng. Suppl. 132 (1966).] The cancellation experiments permitted the assessment of BC components unaffected by the sound pressure in the ear canal. These true inner-ear BC responses also varied with the contact area, demonstrating once more that acceleration is a poor indicator of BC responses. Experiments now in progress will determine if the above findings apply also to the unoccluded ear canal. [Supported by NIH and DRF grants.]
Previous studies [J. Acoust. Soc. Am. 55, S62(A) (1974), EE 8; 56, S4(A) (1974), B8] showed acceleration to be a poor indicator of bone conduction (BC) responses. Using the same magnetostrictive vibrator, unoccluded BC thresholds were measured in terms of acceleration at the input (forehead) and force for frequencies between 100 and 10 000 Hz and for four different-size contact areas. In terms of force, thresholds were independent of the contact area or its place, while in terms of acceleration they varied considerably. Békésy (1939) had already monitored force at the input, realizing this to be the only quantity defining cochlear input, measurable outside the skull. Being the equivalent of electrical current, it is constant throughout a series circuit: i.e., skin, bone, skull content, cochlea. A second vibrator (Radio-Ear B 70 A), calibrated in terms of force, was also employed. Results, when compared with those obtained with the magnetostrictive vibrator, for a comparable contact area, varied by as much as 20 dB. Reasons for these discrepancies will be discussed. [Supported by NIH grants.]
A magnetostrictive vibrator was used that was described earlier [J. Acoust. Soc. Am. 51, 124 (1972)]. It has wide frequency and intensity ranges. BC thresholds as well as sound pressure readings in the ear canal (to assess vibratory skull responses) were taken with reference to acceleration measured at the input, i.e., at the forehead. Response variations were reduced by a number of improvements in the suspension of the subject's head in relation to the vibrator. In terms of acceleration at the forehead, BC thresholds and sound pressures in the ear canal still varied considerably and systematically with changes in contact area and, to a lesser degree, with contact pressure. The latter kind of variation was reduced, and overall sensitivity improved, when a relatively stiff grease was applied between vibrator and skin in an effort to improve coupling. The results indicate once more that monitoring acceleration at the contact point is not a reliable way of predicting BC responses. [Supported by NIH and DRF grants.]
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