Chirp Evoked OAEs

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Parent Category: Basics of OAEs
Category: TEOAEs
Created on Wednesday, 05 December 2012 17:03
Last Updated on Saturday, 29 December 2012 14:58
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Editor's Note:Despite the fact that chirp-OAEs were not applied widely in the clinical OAE field, they represent a considerable evolution in the TEOAE protocols. This material is mirrored from the NATASHA project web pages. The material has a technical nature and we recommend to the readers to visit Dr. Newmann's web-link for more details on this topic.

The use of chirplet stimuli are useful for evoking TEOAE in, e.g., clinical applications. Due to the higher level of the chirplet versus the click the signal-to-noise ratio can be improved. Therefore the detection of an TEOAE and the observation of saturation with increasing stimulus level is facilitated, especially when using the ''bark'' chirplet signal. This holds especially in the case of very weak TEOAE. In addition, the stimulation with narrowband stimuli allows a specific evaluation of the functionality of selected basilar membrane regions in a single measurement (Neumann, 1997, Neumann et al., 1994)



Equipment and Stimuli

a) Set-up (equipment, test environment, etc.):

The measurement of transient evoked otoacoustic emissions TEOAE using chirplets are almost similar to the procedures for TEOAE measurement using clicks. They may depend on the specific setup and instrumentations. Some important aspects for the measurement of TEOAE with chirplet signals are pointed out here:



    • A sound source with a flat frequency response should be used for the acoustical stimulation of the ear (e.g. Etymotic Research ER-2 insert ear phone). Otherwise the chirplet stimuli may need to be adapted to compensate for the frequency response of the sound source.



    • The amplitude of four successive stimuli need to be multiplied with the factors 1,1,1,-3, respectively. This nonlinear averaging suppresses the linear response and thus provides a better separation between stimulus and TEOAE (Bray and Kemp, 1987).



    • The chirplet signals should not be longer than 3 ms to allow a separation of stimulus and ear response.



    • The acoustic response of the inner ear to the chirp stimuli should be recorded in the sealed ear canal by using an insert ear probe (e.g. Etymotic Research ER 10 A).



    • A artefact rejection and noise reduction procedure should be processed to the data, e.g. by using a weighted averaging technique which utilizes the inverse of the respective RMS value and/or rejecting segments with high RMS values.



    • The stimulation with the chirplet signals might be carried out with the same setting of the maximum amplitude as a click used instead. This results in an higher stimulus level of the chirplet signal in comparison to the click stimulus due to the longer duration of the chirplet signals.

 

b) Stimuli:

Fig. 1c shows a chirplet signal after applying the described correction to the instantaneous frequency. It can be seen that the resulting chirplet signal matches the target spectrum much better than before. In order to uniformly distribute the stimulus energy over the entire basilar membrane, a non-uniform target spectrum can be used in the design of the stimulus. The chirplet signal in Fig. 1d was therefore designed for a target spectrum that is uniform on the bark scale using the formula according to ZWICKER and TERHARDT (1980). This stimulus is in the following referred to as the ''bark'' chirplet.



Fig. 1: Time signal and spectrum of chirplet signals with increasing instantaneous frequency from 500 Hz to 6000 Hz: a) rectangular window, linear increase of instantaneous frequency, b) Hanning window, linear increase of instantaneous frequency. c) Hanning window, compensation of the window effect by a nonlinear increase of the instantaneous frequency. d) same as c) with a target spectrum uniformly distributed on a bark scale.

Background information:



The basic idea to generate a chirplet signal for a given power spectrum is that the spectral energy at a certain frequency increases if the instantaneous frequency of the chirplet takes values close to this frequency during a longer period of time. Thus, by suitably selecting the time course of the instantaneous frequency of a signal, it is possible to manipulate its spectrum. Therefore, we specify the chirplet signal in terms of its instantaneous frequency F(t) and its envelope A(t):

The distribution of the instantaneous frequencies can be selected to be identical to the prescribed power spectrum L(f). Within the limitations discussed below, the spectrum of the generated signal would match the desired spectrum. In addition, the correspondence between the desired power spectrum and the observed power spectrum of the signal is better if the instantaneous frequency changes slowly with time. This implies that the instantaneous frequency function F(t) should be monotonic.

To obtain a distribution of the instantaneous frequency identical to the desired spectrum L(f), the rate of change of the instantaneous frequency F(t) must be inversely proportional to the spectral energy L(f) at this frequency. For example, a high energy of a particular frequency component in the desired spectrum of the signal should lead to a slow change of the instantaneous frequency at this particular frequency:

The envelope A(t) is not specified yet. To demonstrate the significant effect of the envelope A(t) on the resulting signal spectrum, a rectangular and a Hanning window is used to generate the chirplet signals shown in Figs. 1a and 1b. The signals in these figures were calculated for a duration of 3 milliseconds with a flat target spectrum between 500 Hz and 6000 Hz, with no power falling outside this range. Therefore, the instantaneous frequency function increases linearly from 500 Hz to 6000 Hz. The abrupt transitions of the signal in Fig. 1a (rectangular window) leads to an undesirable widening of the spectrum.

This widening effect can be reduced by using a Hanning window of length T as the envelope function A(t). The resulting signal is shown in Fig. 1b. Although the power spectrum is now limited to the desired range, the signal still does not match the target spectrum since the power is not uniformly distributed over the desired frequency range. This new difficulty is caused by the weighting introduced by the Hanning window. The reduction in signal amplitude at the beginning and at the end of the chirplet also reduces the contributions of these regions to the power spectrum.

To compensate the power loss in the low and high frequency regions the distribution of the instantaneous frequency is modified: Until now, the time variable used in the instantaneous frequency function F(t) was a linear function of time. The substitution of this time variable with a function t(t) enables a change in the instantaneous frequency distribution. This function t(t) compensates the effect of the weighting Hanning window by increasing with the integral of the window function. Since a loss in signal power should be compensated, the new function t(t) is given by the normalized integral of A²(t):

 

References:

 

Neumann, J., S. Uppenkamp and B. Kollmeier (1994). "Chirp Evoked Otoacoustic Emissions." Hear. Res. 79: 17-25

 

J. Neumann, Recording techniques, theory and audiological applications of otoacoustic emissions (BIS Verlag, Oldenburg, 1997) http://www.bis.uni-oldenburg.de/bisverlag/neurec97/inhalt.html

 

P. Bray and D. T. Kemp, "An advanced cochlear echo technique suitable for infant screening," British Journal of Audiology 11, 191-204 (1987)