Stages In Noise Control

The basic stages in the control of noise are:

 Definition of the problem
 Specification of the noise reduction required
 Design of the optimum noise control treatment

Definition

To define a noise problem, the noise source must be identified. The noise level and its frequency spectrum at the point where control is required must be measured. All noise transmission paths between sources and control point must also be clearly identified.

Accurate identification of the source can occasionally be difficult. In cases of doubt, the true source can usually be identified either by successive removal of alternatives or by “fingerprinting” using a detailed frequency analysis. The level at the control point must be measured in accordance with the relevant Standard, e.g. to produce a Leq value for assessment of hearing risk.

The frequency spectrum must be measured since the performance of most noise control treatments depends heavily upon frequency.

The transmission paths between source and control point must be identified. Failure to control a single major path will seriously impair the value of any treatment. It should be noted that paths may be airborne or structure-borne or both, and direct or indirect such as reflections from room boundaries.

Specification

The noise reduction required is the difference between the level measured at the control point, and that defined as acceptable by the relevant Standard or Code of Practice. The reduction should be specified at all frequencies to provide sufficient control system design information.

As a general guide, reductions of 5-10 dB(A) are usually readily achievable, whilst reductions of 10-20 dB(A) often require extensive treatment. Generally, high frequency noises are more easily controlled than low frequency noises. Where reductions exceeding 20 dB(A) are involved it is probable that professional advice would be needed.

Design

Noise Control may be achieved by:

 Source Control

e.g. change of process, relocation of noise source, modification of components.

 Receiver Control

e.g. use of ear defenders, relocation of operative.

 Path Control

e.g. enclosure, screens, ceiling treatment, wall treatment, floor treatment.

Although Source and Receiver Control must always be investigated, many noise problems are solved by Path Control.

Transmission Path Control

Path Control employs two complementary techniques:

Reflection and Absorption

Reflection utilises acoustically opaque barriers to reflect noise away from the control point. Systems may range from partial height screens to complete enclosures, depending on the scale of the problem. Reflection alone provides little actual dissipation of noise energy. This can be most effectively achieved by use of absorption techniques.

Absorption normally entails the conversion of noise into thermal energy. It is usually achieved by friction losses of sound waves passing into materials containing networks of narrow internal passages such as Glass, Rock and Foam. For maximum effectiveness, careful physical design of the material is essential.

The sound absorption performance of a material is defined by its absorption coefficient.

Performance should always be presented as a function of frequency, and in terms of the thickness of the material. Whilst open cell materials have good sound absorption characteristics at all frequencies, the inclusion of an airspace between the insulation and the surrounding structure will usually improve performance at the lower end of the frequency range.

The performance of reflecting surfaces as sound barriers is expressed as a Sound Reduction Index (SRI) or Transmission Loss. (The difference in acoustic energy on either side of the material, measured in dB).

It is common practice to express the overall performance using single value, usually the average SRI over the frequency range 100-3150 Hz.

The SRI expresses the maximum attainable performance under ideal conditions. In practice this can be reduced, sometimes considerably, by “flanking transmission” and other effects.