Hearing Devices

Hearing loss caused by cochlear damage is permanent. Hearing impaired (HI) listeners can, however, use hearing devices to aid auditory perception and restore the hearing. Two main types of hearing devices that exist are cochlear implants (CIs) and hearing aids (HAs)

1.2.1 Hearing Aids

Hearing aids (HA) are devices that delieve the amplified sound to user. They are only useful when a region of hair cells in the cochlea is still intact and some residual hearing remains. In the

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case of severe to profound hearing loss residual hearing is often limited to a small frequency range, for example only up to about 1 kHz. Only the frequencies within this range can be usefully amplified. For individuals with dead regions towards the base of the cochlea, amplification of high frequencies is reported to sound distorted or noise-like [3]. The parameters of the HA are in this case set to attenuate rather than amplify high frequencies.

One of the biggest challenges concerning amplification of sound for people with hearing impairment is loudness recruitment. Absolute thresholds are elevated, whereas the level of uncomfortable loudness remains the same. The dynamic range of people with hearing loss is thus reduced. If all sounds were amplified to the same degree, high level sounds would be uncomfortably loud. Several strategies have been developed to compensate for loudness recruitment.

The most commonly used strategy in current HAs is automatic gain control (AGC). This strategy reduces the dynamic range of the signal by means of compression. A wide range of levels at the input is compressed into a smaller range at the output and low-level sounds are amplified more than high level sounds.

An important aspect of HA designs, especially with respect to the perception of speech in noise, is the microphone. Many HAs have omnidirectional microphones that amplify sounds from all directions to the same degree. Directional microphones, on the other hand, are more sensitive to sounds that come from a particular direction. An important benefit of this type of microphone is that it can improve the signal-to-noise ratio (SNR) and thereby aid perception of speech in noisy environment.

1.2.2 Cochlear Implants

People with severe to profound hearing loss in both ears, who do not benefit sufficiently from HAs, may be provided with a CI which is a prosthetic device can partly restore hearing. The

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device rely on electric current to stimulates the auditory nerve directly via an electrode array which implant into the cochlea, thus bypassing malfunctioning parts of the outer, middle and part of the inner ear. In 1957, Djourno et al. reported a sense of environmental sounds, which was the first device for electric stimulation of the auditory nerve. However, speech could not yet be perceived [5]. The first commercial implant was developed in the 1980s. The House-3 M single electrode implant was developed in 1984 and had several hundred users [6].

Since the 1980s several different CIs have been developed, but all have certain features in common, including: (1) a microphone that picks up the acoustic signal; (2) a signal processor that converts the input signal into a waveform appropriate for electrical stimulation; (3) an external transmission system and an internal receiver which are connected by means of a transcutaneous link and are responsible for the transmission of the electric signal to (4) multiple electrodes inserted into the cochlea [7-8]. Figure 1.3 provides an overview of the components of the CI. The electrode array

Figure 1.3 An overview of the components of the CI. Reprinted from Medical Electronics GmbH, of Innsbruck, Austria.

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is usually implanted in the scala tympani, which is located close to the auditory nerve, as figure 1.4 shows. The stimulator is connected to the electrodes and transmits the electrical signals to the appropriate electrodes. The mapping between the channels generated by the signal processor and the electrode array is fixed.

The electrodes near the base of the cochlea convey the high frequencies and the electrodes nearest the apex the low frequencies. The number of implanted electrodes varies among different designs, but is usually between 12 and 22 [9]. The electrodes are generally inserted equidistantly up to 25-30 mm from the base of the cochlea, thus stimulating only nerve fibres with characteristic frequencies above 250-500 Hz. The frequency selectivity of the system depends on the number of electrodes as well as the distance between these electrodes. In addition, the limited effect of increasing the number of electrodes may also be explained by distorted frequency-to-place mapping associated with implantation. There is generally a mismatch between the frequency outputs of the speech processor and the CF of the nerve fibres stimulated by the electrode array. The frequency

Figure 1.4 The electrode array is usually implanted in the scala tympani along BM, which located close to the auditory nerve. Reprinted from [13].

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alignment of the electrodes and the auditory nerves is an important factor contributing to speech understanding. A shift in frequency-to-place map leads to degeneration of speech intelligibility [10-11]. It should be mentioned, however, that listeners can to some degree adapt to this shift in frequency-to-place mapping [12].

The signal processor forms an important aspect of implant design. The function of the processor, also known as “signal processing strategy,” is to decompose the signal into different frequency bands, thus attempting to simulate the frequency analyzing functions of a normal cochlea.

In comparison with a healthy cochlea, however, the decomposition of the signal into its frequency components is far less precise. The implant provides a limited set of frequency components with a wider bandwidth, resulting in spectral degradation of the signal. The signal processing strategy extracts the envelope for each band by means of rectification and smoothing. Compression is subsequently performed on the output of these channels. This is necessary because the range of the acoustic amplitudes present in the input is larger than the dynamic range of the CI users [7]. The selected envelopes are then used to modulate trains of electric pulses and transmitted to the electrode array.