Recommendation regarding Strategies for Noise Control

First of all noise levels have to be measured or predicted and compared with the relevant standard, so that a noise problem is known to exist then adopt three strategies which entail few noise control procedures. These strategies are:

(i) Control at the source

(ii) Control along the paths of a sound from sources receiver

(iii) Control at the Receiver

By considering possible noise control measures related to each of these strategies in turn, the best single measure or combination of measures can be identified. In general the most effective way of tackling a noise problem is at source. Control of both road and air traffic noise at source is possible by quietening individual vehicles which are subject to legal noise linits. The main concern, usually is the techniques should include specially constructed noise barriers in the case of road traffic noise. Take-off and landing restrictiona and "Minimum Noise" routes in the case of aircraft, and double glazing of noise effected facades for both road and air traffic noise. Other noise sources, with whose control we shall be concerned include construction sites and industrial premises.

The Control of Noise at the Source: In a situation containing several noise sources and where a noise problem has been shown to exist, there each source should be identified, and the most dominant located, so that priorities for noise reduction can be established.

A. 1. Control of Noise by Design or Choice of Process

In the design of machines and mechanical systems a great deal can be achieved with regard to reduction of noise. For any particular operation or precess complete elimination of the noise producing forces is impossible, since these forces (such as combustion forces in engines, magnatic forces of motor, impact forces in punching and hammering operations, and cutting forces in many metal working processes) are essential to the operation or process. Quiet designs requires that impact and rattle between components should be avoided ; reciprocating movements should be braked gently; metal component should be exchanged with quieter plastic, nylon or composit components ; noisy components or processes should be enclosed; power sources and transmissions which give quiet speed regulation should be chosen ; vibration sources within the macine should be isolated ; cover panels and inspection hatches should be stiffened and well damped and machines should be provided with adequate cooling fins which reduce the need for air flows, and therefore the need for fans which are noisy. Processes that produces impulsive forces, such as hammering, punching and pressing operations, are very important class of noise produces. The most effective method of control is to replace the inpulsive force by a steadily applied force that will achieve the same effect. Examples are the relatively quiet breaking of concrete where a bending action is used rather than percissive methods and the use of steady squeezing forces rather than impulsive hammering to insert riverts into sheet metal work.

A.1.2 : Vibration Isolation :

A typical source of noise problems, in addition to those due to sound directly radiated from a machine is the consequence of vibration transmitted from the machine into the building structure (in whish machine is housed) at the places where the machine is in contact with the building.

A.1.3 : Enclosures :

Use of an eclosure to attenuate the noise from a machine or part of a machine is an obvious way of solving a noise problem at source.

A. 1.4 : Frequency Dependence Reduction :

As stated above vibration isolation is effective mainly at low frequencies of the order of 100 Hz or less. Both transmission loss and absorption are frequency dependent.

A. 2.1 : Control Between Source and Receiver

Methodological Effects on Out-door Sound Propagation : The propagation of sound out-doors is sensitive to atmospheric conditions. Effects due to wind, temperature gradients, air-turbulance and humidity nust be considered.

A.2.2 : Indirect Sound Paths

Although much of noise control is based upon the sound paths of the type shown in figure-1 (below). These are not only paths. Airborne sound results from any vibrating surface. However, the housing of a machine is not necessarily the only vibrating surface. The vibrations of a machine can cause the radiation of noise at places quite remote from the machine itself by causing the structure on which the machine is mounted, to vibrate. The vibration in turn can be transmitted to adjacsent structures which can radiate air-borne sound.

Figure-1

Air-borne sound is detected by its ability to cause vibration in some component of the detector. In the same way as direct mechanical impacts on a surface, air-borne sound is able to cause vibrations in any surface with which it comes into contact. The sound energy can then be carried further as structure-borne sound to re-radiate air-borne sound further. Such indirect paths of sound are known as flanking transmission. These include any areas of low transmission loss, such as open windows creating outdoor paths from window to window. Other air-borne paths lie through connecting duct-work, ventilation grilles, openings under doors, gaps at walls, poor seals at floor edges, back to back electical sockets, false ceilings and open troughs in floor-ceiling structures.

A.2.3 : Absorption and Absorbing Materials :

When considering larger spaces, such as factories or machine halls, it is necessary to consider the possible use of absorption as a means of controlling noise between source and receiver.The main effect will be on the reverberant sound. Thickness of an absorbent can reduce reverberant sound by 10 dB in a noisy room.

A.2.4 : Barriers

Several noise control methods depend upon putting an obstacle in the way of an air-borne sound path. The effectiveness of such an obstacle depends upon how much sound can get through or around the obstacle and how much is reflected or absorbed by it.

A.2.5 : Control at the Receiver:

Sound Insulating Swellings: People are most often distributed by road traffic noise , aircraft noise or noise from nearby industrial premises, when they are in their own homes. If the disturbance occurs during the evening or night homes, then probably people are at their most sensitive to noise as well. If every one lives in a cavity wall beick structure containing no windows, doors, airbricks, roof joistsor, fireplace, they can count upon average sound insulation's od 50 dB(A). The physical characterstics needed foe a wall to offer good transmission loss are (i) that the wal should be made from nonporous dense meterial and (ii) that there should be no flanking paths. Sound reduction offered by a wall increase withits mass. This is called Mass Law.

Possible options and procedures for controlling noise indoors and outdoors are summerized in figure-2 (below).

Figure-2 : Noise Control Methodology

Next>>>commendations regarding different methods of Protection Against Noise

Back to Cover Page