Nb Ce-Chirp® For Assr - Interacoustics Eclipse Additional Information

Eclipse Additional Information Page 128
With the design in the frequency domain the spectral characteristics of the stimulus can be accurately de-
signed (see below), however, the temporal characteristics may be less precise.
Whereas design in the time domain is well known and has been preferred for the construction of traditional
ABR-stimuli, the design in the frequency domain is not well known. In principle this design calculates the sum
of a number of pure tones (cosines) having fixed frequencies corresponding to whole number multiples of the
stimulus repetition rate (i.e. the number of stimuli per second). For a broad band stimulus the cosine frequen-
cies cover a large bandwidth (e.g. from 100 to 10kHz). For a narrow band or frequency specific stimulus the
cosine frequencies cover a narrow band (for example a constant bandwidth e.g. 200Hz, or a relative band-
width e.g. one octave, one third octave or the like).
With a given stimulus repetition rate both the amplitude and the phase spectrum can be analysed because
the frequencies at which the ASSR has components are known and therefore all other frequencies will con-
tain background noise only. It is important to realize that both spectra (amplitude and phase) carry infor-
mation about the ASSR as well as the background noise and therefore the information from both spectra
should be included in order to optimize the analysis and the detection of the ASSR.
6.2.2 NB CE-Chirp® for ASSR
The travelling wave set up in the cochlea by a brief stimulus takes a considerable amount of time to reach
from the base of the cochlea to the apex i.e. from the highest to the lowest frequency responding area. The
individual areas along the cochlear partition, the corresponding hair cells and nerve fibres of the auditory
nerve will therefore not be stimulated at the same time and the compound neural response will therefore be
temporally smeared. This can be counteracted by allowing the lower frequencies of the stimulus to enter the
cochlear before the higher frequencies or in other words to delay the higher frequencies relative to the lower
frequencies. Such a scheme has to be based on an appropriate model of the cochlear delay.
A stimulus that tries to compensate for the cochlear delay can be designed in the time domain; this is tradi-
tionally called a Chirp (Wegner & Dau, 2002). However, in the frequency domain the design is straight for-
ward because the phase of each cosine can be modified in accordance with the cochlear delay at that fre-
quency.
Example If a specific cochlear latency model (Elberling et al, 2007) is used to modify the phase of the fre-
quency components of a broad band click with a repetition rate of 90 stimuli/s (or 90Hz) the corresponding
stimulus is called a CE-Chirp®, with a waveform as shown below.
By adjusting the amplitude of each cosine, compensation for the frequency spectrum of the final acoustic
stimulus can be controlled (for instance to compensate for the frequency characteristics of the earphone or to
choose between a flat, a rising or a falling frequency spectrum).