These calculations are based on the noise prediction methods contained in Annex F of BS 5228-1:2009. They assume that the intervening ground is hard, and use the simple barrier attenuation method.
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Noise level is either a sound power level (SWL) or sound pressure level (SPL). If noise level was measured on site it is likely a sound pressure level. Enter the distance it was measured at.
Attenuation of noise due to barriers.
If the noise source is visible from the receptor there is no attenuation, while a barrier that completely obscures line-of-sight is assumed to cause an attenuation of -10dB.