14.07.2008
Click and chirp signals with identical amplitude spectra have equal behavioural hearing thresholds for audiometric routine. This might simplify calibration procedures.
For brainstem electric response audiometry (BERA), belonging to the objective audiometric methods, the cerebral responses to sound are measured by means of scalp electrodes. Usually, standardized short-duration signals ("clicks" and "bursts") are employed as stimuli. However, recent clinical research has shown, that brain responses, called auditory evoked potentials (AEP), become even more pronounced when short "chirps" are used.
Both clicks and chirps consist of many single-tone components, the so-called acoustic spectrum. Each single-tone component has its individual sound pressure level and, moreover, its individual start time of the sinusoidal oscillation. When producing chirps whose frequency components all have the same respective sound pressure levels as the click frequency components, but have start time lags increasing with frequency, then those chirps feature both an increasing instantaneous frequency and the higher efficiency regarding the auditory evoked potentials, as mentioned above. The physiological cause of this property is the nature of wave propagation within the inner ear. The chirp's impact is greatest when its high-frequency start time lags are in fact calculated from propagation times in the cochlea.
Figure 1: Earphone terminal voltages of click and chirp
The higher efficiency can only be proven when it is shown that those chirps have a higher effect not because they are generally louder than the clicks, but even when they have exactly the same loudness level.
In order to demonstrate this fact, the hearing thresholds for both signals need to be measured.
Measurements on groups of subjects, conducted within a research project of the PTB section "Sound in Air" have shown that in fact signals with very different time courses, but equal amplitude spectra, such as the clicks and chirps described above, produce statistically equal hearing thresholds when conventional audiometric earphones are used. The above figure shows the respective earphone terminal voltages of both click and chirp. Note that when both signals were presented 30 dB above threshold, the chirps shown there caused significantly greater brain responses (auditory evoked potentials) than the clicks.
The benefit of this result for practical audiometry is as follows: Within a group of short-duration test signals having different start time lag spectra, but identical amplitude spectra, it will no longer be necessary to calibrate each signal separately. Instead, it will suffice to have just one of the signals calibrated for a hearing level. This means a great reduction in the number of expensive and time-consuming hearing threshold measurements with subject groups for the determination of reference hearing thresholds.
Contact person:
Johannes Hensel, FB 1.6, AG 1.61, e-mail: Johannes.Hensel@ptb.de