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Abstract
Understanding speech in a moderately noisy environment occurs because speech is a highly redundant signal. If part of speech signal is masked by noise other parts of the speech signal will convey sufficient information to make the speech sufficiently intelligible. This allows effective speech communication. However, a person with hearing loss will have difficulty in detecting speech signal that is less redundant. This occurs because the speech is either inaudible or severely distorted (Levitt, 2001).
Enhancement of signal over noise generally leads to improvement in an individual’s ability to recognize speech in a background noise. Moore (1989) reported that individuals with normal hearing require a SNR at least +6 dB for satisfactory communication. On the other hand, individuals with sensory neural hearing loss generally require a significantly greater SNR to obtain speech recognition performance equivalent to listeners with normal hearing (Schum, 1996; Killion, 1997).
Although hearing aids improve the audibility of the speech signal, the background noise will be amplified together with speech. Accordingly, the signal to noise ratio (SNR) is not increased resulting in poorer speech intelligibility due to the effect of increased upward spread of masking at high listening levels and distortion caused by the hearing aid (Launer and Moore, 2003). As a consequence, newly developed techniques in hearing aids are directed towards enhancement of speech intelligibility through noise reduction.
Advanced microphone technology help in improving SNR. In 1970s the directional microphone technology has been applied in the field of hearing aid aiming to improve the speech intelligibility in noisy environment. Directional microphones improve the satisfaction of hearing aid users with limited value in reverberant environment (Ricketts, 2003).
Noise reduction algorithm has been added to digital signal processing hearing aids in order to improve the performance in noisy environment. These algorithms try to select speech and cancel noise depending on the acoustical parameters of speech versus noise (Donald, 2003).
Several studies were conducted comparing the directional microphones versus omnidirectional microphones in improving speech intelligibility especially in noisy environments (Ricketts and Dhar, 1999) and (Thompson, 1999). Other studies investigated the role of noise reduction algorithms in improving speech intelligibility in noisy environments (Alcantara et al.,