Effects of notch noise bandwidth on the auditory brainstem response to clicks.

The effects of notch width of notch masking noise on the auditory brainstem response (ABR) were investigated with ten normal-hearing subjects. Wave V latencies and amplitudes were measured to a click in quiet and in the presence of noise with notches centered around 1000 Hz and 4000 Hz. Notch width was either 0.5, 1.0, 1.5, 2.0, or 2.5 octaves. A 95 dB SPL broadband noise was necessary to mask a 65 dB nHL click at an effective level of 73 dB nHL. All ten subjects at both 1000 Hz and 4000 Hz yielded identifiable responses for the click in quiet and for the 2.5 octave and 2.0 octave conditions, and eight or nine subjects responded to the 1.5 octave notch noise. In contrast, no responses were observed to the 0.5 octave condition, and only three or four subjects responded to the 1.0 octave notch noise. These findings suggest that when ABRs are obtained to 65 dB nHL clicks in notch noise, the notch width should exceed 1.0 octave. Moreover, because of the relatively low amplitudes elicited with the 1.5 octave bandwidth, it appears preferable to select notches that are 2.0 to 2.5 octaves wide. Considering the wide bandwidth required, the high noise levels necessary to mask clicks, high ABR thresholds, and the difficulty setting the signal-to-noise ratio, it appears that tonepips are more promising than clicks in notch noise for assessing frequency specific ABRs.