Noise can mask the various signals that it may be necessary for the operator to hear as it raises the threshold of audibility of such signals. The effect of masking is very complex, and the particular masking effect caused by a noise depends on its level and spectral composition. Where a noise consists mainly of a single tone, the greatest degree of masking will occur at a frequency one octave higher. It can be seen from Figure 7.9, for example, that a tone or a noise with a main frequency of 200 Hz and a level of 60 dB will completely mask a signal of 1000 Hz at 25 dB (everything under the continuous line is masked).
On the other hand, if the level of the masking noise at 200 Hz is dropped to 40 dB, the signal will be heard, as only sounds under the dashed line will be masked. Figure 7.9 also shows the masking effect of 60 and 30 dB tones at 3500 Hz. Broadband noise (for example, ‘white’ or ‘pink’ noise) will cause masking over most of the auditory spectrum, although higher frequency noise from fans and air-conditioning will affect the intelligibility of speech more than lower frequency noise.
Figure 7.9 shows that the masking effect is greater for frequencies higher than the frequency of the masking noise itself. Different types of noise are associated with different degrees of disturbance:
High frequencies High noise levels Intermittent noise Unexpected noise Unexplained or unreasonable noise
FIGURE 7.9 Masking produced by pure tones. (VanCott and Kinkade, 1972. With permission.)
Noise is known to cause increased levels of stress in the human body, but although the effect of noise on accident frequency has been studied, no clear-cut results have emerged. There is a tendency, however, for accidents and mistakes to occur more frequently in noisy workplaces, especially where a certain degree of concentration (on
machines, screens, and so forth) is required. Noise, especially intermittent or high frequency noise or noise at a high level, will cause more errors in the operator’s work.