Some simple rules

• For each doubling of distance from the source, there is a 6 dB reduction in the level of sound pressure.
• Addition and subtraction sound levels is logarithmic. The total sound level (either force or pressure) two separate machines with sound levels of L1 and L2, when they work together, is defined as:

For the case of two identical machines, adding a second machine increases the noise level by 3 dB and adding a third machine will increase the sound level by 4.8 dB.

I f the source is set on a flat surface sound level will be equivalent to an empty area on the level + 3 dB (see Fig. 31.1). In the case when the source is also close to the wall there will be 9 dB increase in the level of sound emitted in comparison with free field case.

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