Work Study and Ergonomics

Introduction to Noise

Any unnecessary sound which creates disturbance to human is generally treated as noise. Thus any unnecessary sound is known as noise that results in disturbance and lowers quality of life which leads to sleep and mental/auditory fatigue. The annoying effect of noise, whether from a factory, motorway, marriage palace or nightclub, varies from person to person. Secondly, it can divert concentration and audible caution signals or interfere with occupation that may result into accidents. Finally exposure to excessive noise can result in hearing loss, provided the exposure period is of sufficient duration; for example, loud music may lead to hearing disorders.

Sound

The pressure difference in air, water or any other medium that is detected by human ear is known as sound. Physically, sound is a vibration of gas, liquid or solid medium particles.

The nature of sound

Sound is a mixture of many waves or fluctuations (sound waves) impinging on the eardrum. It is generated from any energy source setting up hasty and varying pressure in the nearby air. The rate at which variations occur, is called frequency or pitch, and is expressed in hertz (Hz), i.e., cycles per sec. One hertz means the number of complete air waves passing a fixed point per sec. The normal human ear is sensitive to frequencies between about 20 and 20,000 Hz being particularly sensitive in the range 2,000–6,000 Hz. (with maximum sensitivity at 4000 Hz.) that is less perceptive at elevated and lower frequencies.

Characteristics of sound waves

Sound having one frequency only is called as pure tone, such as sound produced by the tuning fork (see Figure 13.1 A). As components distributed over a wide range of frequencies, most of industrial noise are very complex, called broad band (see Figure 13.1 B). Noise produced by looms, an air jet industrial machinery or printing presses, are few examples.

There is a significant noise due to impact between metal parts in steel industry setup. Due to a wide range of impacts per sec, noise is treated as broad band noise, as in a riveting machine. On the other hand noise having widely spaced impacts termed impulse noise, as drop hammer-operated hand tool (see Figure 13.1 C).