Different operations or different noise sources cause the sound changes over time. Noise is intermittent if there is a mix of relatively quiet periods and noisy. Impulse or impact noise is a very short burst of loud noise which lasts for less than one second. Gun fire or the noise produced by punch presses are examples of such noise. The sensitivity of the human ear to sound depends on the frequency or pitch of the sound.
People hear some frequencies better than others.
If a person hears two sounds of the same sound pressure but different frequencies, one sound may appear louder than the other. This occurs because people hear high frequency noise much better than low frequency noise. Noise measurement readings can be adjusted to correspond to this peculiarity of human hearing. Table 2 lists examples of typical noise levels. The use of dB unit makes it easy to deal with the workplace noise level data provided we use a set of simple rules as summarized in Table 3.