Sistema de movilidad, energía y conectividad

I Introduction

In the previous chapter, two distinct DPOAE emission mechanisms were shown, with a transition occurring in the lower sideband at a ratio o f about fi/fi^l .l. It is believed that this change also relates to a change in the location of the emission source, with the

wave fixed emission type being emitted directly from the Ï2 region and the place fixed

emission being emitted via a reflection in the region o f the DP frequency place.

Clarification of the location of the sources of DPOAE on the basilar membrane is of interest because it determines the frequency specificity o f CAE and because this would yield information regarding the emission mechanisms. Suppression is one method to explore the place responsible for an emission.

Published DPOAE suppression tuning curves suggest that the Zfi-fî DPOAE source is around the stimulus peaks or more specifically around fi on the basilar membrane. It is arguable whether there also may be a contribution from DP region (Brown and Kemp, 1984; Kummer et al, 1995). Conversely, tuning curves for the 2fi-fi DPOAE suggest that the emission source is exclusively in the region of the DP frequency place (Martin et al, 1987; Kemp, 1998; Martin et al, 1998).

Phase sweep data shows that there are two distinct emission mechanisms: a ‘wave fixed’ emission which dominates lower sideband DPOAE when fi/fi is large, and a ‘place fixed’ emission which dominates the lower sideband when f^/fi is small and always dominates the upper sideband. As discussed in section (A II i) these emissions may be from different sites, the ‘wave fixed’ emission being from the ^ region and the ‘place fixed’ emission being from the region o f the DP frequency place. However the place fixed emission source has only been shown in the literature to be at the DP place in the case o f upper sideband DPOAE.

As discussed in section (A II e), the effect o f suppression by a third tone on a distortion product derived from the other two stimulus tones can be used to imply where along the basilar membrane the DPOAE might be generated, on the grounds that the maximum suppression by f] occurs at the fg travelling wave peak. This method is intended to provide information regarding the site o f the DPOAE emission but doesn’t directly indicate the mechanism o f emission. In other words, the method doesn’t indicate whether the emission is wave or place fixed.

Mechanisms o f otoacoustic emission E: DPOAE with triple tone stimulus 114

Previous workers exploring DPOAE suppression tuning have tended to avoid a close stimulus spacing because it is then harder to resolve which significant frequency the tuning curve is centred on. A wide frequency spacing spreads out the stimulus peaks and the DP frequency place, aiding resolution of the suppression tuning curve. However the wave fixed mechanism is known to dominate the Zfi-fi DPOAE at wide ratios so the published data are not representative of all stimulus conditions and may not be relevant to place fixed DPOAE.

Therefore, performing DPOAE suppression tuning curves with frequency ratios below

^ /fi=1 .2 may be o f interest as the emission mechanisms may be different with smaller

frequency ratios. However there are various factors which may complicate the results. If there is more than one DP emitting region or a distributed source region, interference effects are likely as in general the various contributions will not summate in phase. Suppression of only a part of the total source will change the balance o f the DP source contributions with unpredictable effects depending on the phase relationship between the suppressed part of the DP and the total summed DP. For example, suppressing an out of phase part of the source region can result in enhancement o f the DPOAE amplitude.

Specifically, if there are sources of DP in the region o f fi and the DP frequency place, the ear canal DP depends on the phase relationship between the two sources, particularly if the two contributions are of similar magnitude. Suppression of just one source will result in either suppression or enhancement of the ear canal DP depending on whether the two sources were summating in phase or out o f phase.

If the DP frequency place contributes significantly to the DPOAE, it must be a reemission of DP generated in the region of fi because fi and fi don’t reach the DP frequency place. Therefore the emission site is different to the generation site. In this case, a suppressor tone would suppress the emission whether it was added at a frequency corresponding to the generator site or the emission site.

The effects o f suppression are in themselves quite complex - the third tone suppresses the DP, but also can suppress the primaries and the primaries can suppress the third tone. Therefore the travelling waves of each of the three stimulus tones can be altered by the presence of the other two tones. This can alter the shape of the DPOAE suppression tuning curve.

Suppression by third tone is a ‘blunt instrument’ in that the envelope o f the third tone covers (and may have a significant effect over) an extended part of the basilar membrane, and not just at the peak of the travelling wave. Therefore there may be an

Mechanisms o f otoacoustic emission E: DPOAE with triple tone stimulus 115

effect on both the DP frequency place and the primary tone region o f the basilar membrane at the same time. The shape of the third tone travelling wave envelope will also change shape depending on its level - the frequency resolution that can be applied to the results deteriorates at higher levels of stimulus.

As indicated at the end o f section (A II i), addition of a third tone also introduces other possible additional sources of DP generation. These are discussed by Martin et al. (1999b), Martin et al. (2000) and Fahey et al. (2000).

Nevertheless, in this study the effects of suppression by a third tone on DPOAE production will be investigated using intermediate and small frequency ratios, at which there should be a significant ‘place fixed’ emission. Such measurements may provide evidence for the location(s) of DPOAE source(s) under these stimulus conditions and specifically determine whether indications of a contribution from the DP place can be identified.