Changes that occur in the ABR when utilizing an in-situ calibration procedure for click stimuli

A broad band click is the traditional stimulus of choice when recording the Auditory Brainstem Response (ABR). The click elicits synchrony in neural firing along auditory pathways which evokes a robust ABR. However, common transducers do not accurately reproduce the click stimulus. A transducer will not respond to an electrical stimulus with a single brief displacement of its diaphragm. Instead, a transducer's diaphragm will oscillate at its resonant frequency. This acoustic ringing is evident in the spectrum of the stimulus. Since the exact nature of the ringing is transducer-dependent, the spectrum of a click will depend on the particular transducer employed. A click is further influenced by resonant characteristics of the ear canal. It is well documented that ear canal resonance can offer a sound pressure boost to the stimulus of 15-20 dB. Thus, earphone and ear canal acoustics will modify an acoustic signal, resulting in enhancement of some frequencies and attenuation of others. These modifications will result in an acoustic signal that differs across individuals and test sessions.; The effects of these differences on the resultant ABR have not been reported. This study utilizes a calibration procedure that corrects for the individual transducer and ear canal changes. The calibrated click present in the ear canal is virtually identical to the original signal generated in the computer, and possesses nearly a flat frequency spectrum from 500 to 10,000 Hz. The uncalibrated signal has a preponderance of energy in the 1000 to 4000 Hz range. ABRs were recorded from normally hearing subjects using calibrated and uncalibrated signals. Results of this study indicate the additional high frequency energy present in the calibrated click coupled with its abrupt stimulus onset effectively shortens ABR component latency, increases component amplitude, improves waveform morphology, and results in a greater number of identifiable responses at perceptual threshold in comparison to ABRs evoked by uncalibrated clicks.