7.4.4 Noise Reduction

To maintain efficiency and for good communication, the noise levels in control rooms should be kept as low as possible. There are three main points at which noise can be reduced before it is perceived by the ear: the source, the pathway, and the receiver:

Source ^ Pathway ^ Receiver

The following are three ways in which noise levels can be reduced:

1. Reduce the noise level at the source: i. e., take action at the source.

2. Prevent the noise spreading from the source: i. e., take action at the pathway.

3. Give the person affected some form of hearing protector: i. e., take action with the receiver.

Hearing protectors should not be used in the control room. The work is such that the use of hearing protectors will cause communication problems and could also disturb the work. An attempt should be made to prevent noise from being created. The following methods may be used to reduce the noise generated:

1. The forces that cause the vibration of the different machine or equipment parts should be localised as much as possible, reduced, or, if possible, elim­inated. Rotating parts such as fan blades should be well balanced, and fans should be mounted in heavy casings with flexible mountings to minimise the transfer of vibration to other surfaces.

2. Resonance of the various parts of the machine and plates should be stopped. This can be achieved by damping material or increasing their stiffness or weight.

3. The noise may be reduced by reducing the area of the vibrating parts.

4. The noise-radiating characteristics of the noise source can be changed by changing the fixing positions of the vibrating surfaces.

5. Noise from air movement should be reduced using acoustically-absorbent lin­ings in ducts or by using an ‘acoustic tunnel’ to absorb the noise from the unit.

It is less common in modern control rooms to find noisy items such as impact printers. Where these or other noise sources are found, however, a common method of noise reduction is to prevent the spread of noise from the source by using an acoustic enclosure. This is often also a sufficiently good way of preventing noise disturbance. Any noise-reduction enclosure installed should not interfere with the control room work to be carried out. Most types of enclosure, for example, hinder visual tasks. Enclosure of printers used in computer installations often makes it considerably more difficult to read the printed paper. Such a situation may sometimes be ameliorated by installing special lighting inside the enclosure. Enclosures should be provided with sufficiently large windows that have some form of antireflection treatment.

The walls between ordinary offices have a sound reduction effect of about 30 dB. The walls of control rooms should be provided with at least 35 to 40 dB reduction, and restrooms where privacy is required should have a noise attenuation of around 50 dB.

One general background noise that often occurs in control rooms is the noise from fans used for ventilation of both the computer equipment and the room. In the worst cases, the noise from the fans can rise to over 70 dBA. The equipment and the control room itself should be ventilated to ensure that noise levels do not exceed 35 dBA. This is usually easily arranged by providing the ventilation ducting with internal sound-insulating material or in some cases by installing special acoustic baffles.

Infrasound—that is, sounds at frequencies below the audible range—may cause subjective problems and perhaps also have a negative effect on the level of alertness. Infrasound may occur, for example, in air-conditioning systems, water turbines (such as in a power station), silo installations, and dryers.