Российская академия наук

Frequency resolving power of hearing: methods of measurements and main data

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Frequency resolving power of hearing: methods of measurements and main data

Supin A.Ya.

Institute of Ecology and Evolution, RAS

Any sensory system is characterized at least by its sensitivity and discriminative (resolving) ability. Paradoxically, in practical audiology, instrumental audiometry implies measurement of only sensitivity, never resolving power – contrary to ophthalmology where the most important test is the measurement of visual acuity. i.e., resolving power. Meanwhile, experimental audiology elaborated a variety of masking methods of measurement of frequency tuning of hearing. However these methods are never used in practical audiology because they are time-consuming and provide results that are ambiguous for extrapolation to perception of complex natural sounds.

There are, however, measurement methods that allow asses directly the ability to discriminate complex auditory signals. Moreover, these methods are not time consuming that allows tu use them in for practical needs. These methods use complex test signals which, nevertheless, can be characterized by a limited number of exactly measurable physical parameters. One of the versions of these test-signals – sounds with so-called rippled spectra featuring alternation peaks and valleys. These “spectral grids” allow to estimate directly the frequency resolving power of hearing.

When these methods were applied, it appeared that the ability of the auditory system to discriminate complex sound patterns substantially differs from thet was predicted by previously known features of the auditory frequency-tuned filters. In particular:

1. The real frequency resolving power is markedly higher than thatr predicted by the frequency tuning of the auditory frequency-tuned filters.

2. A commonly accepted idea that frequency tuning depends on sound intensity (the higher the intensity, the lower frequency tuning) does not manifest itself in real resolution of complex sound patterns. Intensity-dependent variation of acuity of the frequency-tuned filters does not influence the resolution of complex spectrum patterns.

3. Traditionally in psychoacoustics, influence of background noise on perception of sounds considered as mostly energetic: addition of the noise to the signal decreases the spectral contrast of the signal, which results in poorer signal discrimination. It appeared that the real interaction of the noise and complex sound signals substantially differs from this model. A noise of lower frequency that the signal little influences the spectral contrast of the signal, however effectively suppresses discrimination of complex signals.

Majority of these effects can be successfully explained by lateral interaction within the auditory nerve centers that are well known in neurophysiology.

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