The effect of noise on auditory steady-state response (ASSR) has not been systematically studied, despite the fact that ASSR thresholds are sometimes measured in noisy environments. This study examined the effects of noise (speech babble) on the ASSR thresholds obtained from 31 normal hearing adults aged from 17 to 36 years (mean = 25 years). The ASSR thresholds at 0.5, 1, 2 and 4 kHz were measured in the right ear only using the Biologic MASTER system twice in quiet and in the presence of 55 dB A and 75 dB A of speech babble. The results showed no change in mean ASSR thresholds across the test-retest conditions in quiet. The mean ASSR thresholds obtained in the quiet conditions were 23.8, 22.5, 18.2 and 20.4 dB HL at 0.5, 1, 2 and 4 kHz, respectively. No significant shift in ASSR thresholds across all test frequencies was found when 55 dB A of speech babble was presented. However, when 75 dB A of noise was applied, the mean ASSR thresholds were significantly shifted by 9.5, 3.8, 4.2 and 5.8 dB at 0.5, 1, 2 and 4 kHz, respectively.
An increase of 30 dB in the sound-pressure level at a 10-kHz difference frequency, measured at a range of 1.83 m, was obtained by introducing bubbles into the region of interaction of a parametric projector. The bubbles, produced by forcing air through a microporous filter, formed a thin screen that was situated perpendicular to the acoustic axis of the source transducer. The primary frequencies were 45 and 55 kHz. The difference frequency beam pattern was measured and appeared to be governed by the broadside directivity of the insonified region of the bubble screen. A theoretical model was derived that demonstrates the effects of bubble size and primary frequency on the difference frequency level. It was shown that the maximum difference frequency level is obtained from bubbles resonant at the primary frequencies. The results of this investigation are consistent with those of Clynch and Rolleigh [J. Acoust. Soc. Am. 55, S51(A) (1974)], who reported difference frequency level enhancement due to a small volume of bubbles produced by cavitation. [This work was supported by ARPA and the Naval Sea Systems Command.]
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