Most noise sources radiate uniformly in all directions, commonly referred to as hemispherical radiation. However many factors can affect the noise level such as walls, buildings, trees or any object in the noise path. Hard surfaces tend to be reflective, soft surfaces tend to be absorptive. The noise level will decrease with distance, as a rule of thumb the noise level will decrease by 6dB each time the distance is doubled. This is referred to as distance divergence. Direction also affects the noise level, (0°) direct line of sight noise level will be higher than (90°) direction line of sight to a stack emission point.
Background noise, that is the ambient existing noise level, is important, a noise level cannot be reduced further than +3 dB of the background noise level.
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