The organs of hearing are in certain respects better adapted to receiving signals than are those of sight. This applies particularly to the recognition of complex patterns. Even in the presence of noise and complex sounds from a machine installation, a trained ear can easily detect deviations and diagnose faults. The ability of the ear to differentiate frequencies is shown in Figure 10.7. This shows the smallest change in the frequency of the sound that can be distinguished against the frequency of the original sound. Figure 10.7 shows that it is easier to distinguish changes in frequency at lower frequencies. Changes in loudness, however, can be heard better at higher frequencies.
In order to hear a sound in a noisy* environment, it is clear that the sound must be a louder noise than when there is a quiet environment. Background noise masks the sound signals. Low-frequency noise masking occurs when the masking sound is of the same frequency. The masking effect is increased if the background sound has a lower frequency than the sound to be heard. The masking effect of a noise on speech is expressed in speech interference level (SIL) units. SIL values are based on octave band measurements and are often the same as the current masking noise. The SIL value is the average of the sound pressure levels (in dB) in three octave bands with centre frequencies of 500, 1000, and 2000 Hz. Table 10.7 shows the masking of speech at different distances between speaker and listener.
A hearing aid can improve reduced hearing ability. However, if there are wide variations in noise levels, the hearing aid can actually increase the problem by amplifying all the sound equally.
The need for amplification for those with hearing damage may vary with the intensity and frequency of the sound. Certain people with hearing impairments can
hear loud sounds relatively well, while not hearing softer sounds at all. Amplification of the sound can thus mean that the hearing-impaired person would certainly be able to hear the weak sounds, but at the same time the loud sounds would all become too loud. In work situations, therefore, earmuffs are to be preferred. They will attenuate the surrounding noise while the relevant signals will be heard clearly.