Reducing Noise from Forges and Foundries
The Handbook of the Black Country Forging and Foundry Project (Part 3)

Written by Bob Davis
Original version © 2002, ISVR University of  Southampton.  All rights reserved.

3  Reducing Noise - the principles

There is no 'standard' method of reducing noise from forges and foundries. Each site and its surroundings is different, and each will have different 'main' noise sources. This handbook offers examples of noise reduction solutions which have worked on some sites, and also explains the basic principles which lead to these solutions. The basic principles first
 

3.1  How does sound travel?

Noise travels through the air as a pressure disturbance. Moving further away from the source, the noise level generally decreases because the sound energy is distributed over a larger area. In a simple case, the sound level reduces by 6 dB each time the distance from the source is doubled (the 'inverse square law'). This rule does not always apply: for example, close to a large noise source such as the wall of a factory the noise level does not start to fall off with distance until you are some way from the wall.

This means that if a factory produces a noise level of 55 dB(A) at a house 200 metres away, the noise from the factory will be reduced to about 49 dB(A) at 400 metres. This is not a very dramatic reduction, even over quite a large distance. Relocating equipment or operations on a site to move them further away from houses (unless they are then screened by an intermediate building) may not be a very effective measure.
 

3.2  Can noise be contained?

If a source of sound is enclosed within a solid 'box', the sound energy emitted is reduced because a solid material has the property of sound insulation - only part of the noise energy striking one side is radiated from the other side. The heavier the material, the greater the sound insulation. Materials made up in the form of multiple layers (an example is double glazing) provide more sound insulation than a single layer of the same total mass. Values of sound insulation, like sound levels, are stated in dB or dB(A). Typical values of sound insulation are shown on Table 1 (overleaf).

Note that because decibels are logarithmic units (see Annex 2) the normal arithmetic rules of addition and subtraction do not apply. A noise reduction of 10 dB means that the original sound energy has been reduced by a factor of 10, a reduction of 20 dB means a reduction in sound energy by a factor of 100, 30 dB means a reduction by a factor of 1000 (that is to 0.1% of the original energy).

 

Single Panels

Weight
kg/m2

Sound
Insulation*
10 mm plywood / chipboard   5 15 dB
12 mm plasterboard / 1.2 mm steel / 4 mm glass  10 20 dB
3 mm lead  35 35 dB
100 mm lightweight concrete 100 40 dB
115 mm brick 200 45 dB
200 mm concrete 400 50 dB
Double Skin Panels    
0.9 + 0.55 mm steel. 150mm spacing. mineral wool infill  20 35 dB
Double glazed window, 6 mm glass. 100 mm airspace  30 40 dB

* figures are approximate average values for sound at mid-frequencies (250 - 1000Hz)

Table 2: Typical values of sound insulation - different materials


An enclosure can only provide good sound insulation if it is reasonably airtight. Noise will escape through any direct air path. An enclosure with holes amounting to 10% of its surface area will provide only 10 dB of sound insulation, however heavy the material the solid parts are made of. To provide 30 dB sound insulation, as well as being built of a suitably heavy material the total area of air gaps in an enclosure has to be less than 0.1% of the total area - a tall order.
 

3.3  Does sound travel in straight lines?

Sound is reflected from hard surfaces such as walls and roadways in much the same way as rays of light are reflected from a mirror. Some materials and surfaces - grassland and undergrowth, porous materials such as glass fibre blankets - absorb some sound and reflect only some of the sound striking them, as a dark surface reflects only some of the incident light. However, it can be misleading to compare the behaviour of sound with that of light. One significant difference is that sound is refracted round obstacles such as fences and buildings - it 'travels round corners'. It is a common observation that things (voices, cars etc) can be clearly heard even when they are not visible. There is some 'noise shadow' effect when a source of noise is hidden from view, but it is a limited effect. This is why attempts to reduce noise by building a wall or fence to hide a source of noise often produce disappointing results. For the same reason, noise radiated from the roof of a factory cannot be ignored even if the roof cannot be seen from a particular nearby house. Roofs are usually the largest part of a building, in terms of surface area. and are often the weakest in terms of sound insulation because the sheeting is light in weight and usually has openings for ventilation.

3.4  What levels of noise are we dealing with?

Noise levels in casting and finishing areas in mechanised foundries are generally around 85 - 90 dB(A). Shakeout machines (unless enclosed) can give higher levels, depending on the sizes of castings and core boxes.

Levels in forges where hammers are used are generally 95 - 100 dB(A). Because of the highly impulsive nature of hammer noise, this is the major problem facing forge operators.

How do these noise levels relate to levels outside the foundry or forge? Table 2 shows what noise levels might be expected at different distances from a typical building (with a floor area of 1000 m2) with average internal noise levels of 85 or 95 dB(A). The building is assumed to provide 15 dB(A) sound insulation this is a typical figure for a building with lightweight single-skin cladding and open doorways and ventilators.

 

Noise levels inside Noise level at distance outside
  100 metres 200 metres 400 metres
85 dB(A) 52 dB(A) 46 dB(A) 40 dB(A)
95 dB(A) 62 dB(A) 56 dB(A) 50 dB(A)


Table 3: Noise radiated from a lightweight building.

If these numbers are compared with the rough 'guideline' values on Table 1, it is clear that forges in particular present a major difficulty. Even 400 metres away, a forge in a lightweight building will give rise to a noise level 10 dB(A) above what might be judged reasonable in the early morning or late evening, and at distances of 100 metres or less is likely to produce unacceptable levels of noise during the day. Larger buildings will produce rather higher levels.

Noise levels in the range 50 - 65 dB(A) are commonplace in residential areas close to working foundries and forges.

Contents  |  Part 1  |  Part 2  |  Part 3  |  Part 4  |  Part 5  |  Annexes 
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Original version © 2002, ISVR University of  Southampton.  All rights reserved.