Abstract:
Introduction: Chirp stimuli have been shown to have an advantage over traditional click and
toneburst stimuli in evoking larger amplitude Auditory Brainstem Responses (ABRs). In
Aotearoa, broadband (click) and frequency specific (toneburst) stimuli are calibrated using a
unique set of reference values which result in a higher level stimulus being presented than when
calibrated using internationally agreed values (ISO 389.6). This study aimed to determine
whether the Chirp Advantage still exists in Aotearoa, or if the higher reference values negate
any potential advantage. It also aimed to investigate whether any clinically significant
differences in gain prescribed in the fitting of hearing aids may be caused by the unique
calibration protocols.
Methods: ABRs were measured monaurally at 40 dB nHL on 12 normally hearing participants
using three stimulus types (CE-Chirp, Aotearoa click/toneburst and ISO click/toneburst) in five
conditions; broadband, 0.5 kHz, 1 kHz, 2 kHz and 4 kHz tonebursts. The stop criteria for data
collection was 40 nV residual noise. Response amplitude and Fmp (a statistical indication of
response presence) were compared across stimulus conditions. Also, hypothetical audiograms
derived from the different calibration references were run through hearing aid fitting software
and both hearing aid response curves and Speech Intelligibility Index (SII) data were recorded.
Results: Results varied between frequencies. Broadband chirp amplitudes were statistically
significantly greater than both versions of click stimuli, 2 kHz chirp was greater for the ISO
toneburst but not Aotearoa and at 0.5 and 4 kHz the chirp was not statistically significantly
different from either version of toneburst. Chirp Fmp values were statistically significantly
larger than for both versions of the 2 and 0.5 kHz tonebursts but were not statistically
significantly different from either version of click stimuli, and were only larger than the ISO
toneburst at 4 kHz. The data for the 1 kHz condition were inconclusive. Hypothetical
audiograms and hearing aid fittings showed a direct effect of calibration differences on
outcomes of hearing aid fittings for infants with moderately-severe or worse hearing losses,
producing below average SII values.
Conclusion: At some frequencies the Chirp Advantage is reduced but not eliminated in
Aotearoa by the higher stimulus levels set at calibration of toneburst stimuli. The higher level
tonebursts in Aotearoa have a direct effect on the audiogram which, in some cases, will lead to
clinically significant differences in the gain prescription with negative impact on hearing aid
outcome measures.