Specification of Noise Emission from Machinery and Machinery Noise Regulations (The European Union Machinery Noise Regulations)
According to a simple application of the inverse square law, a portable electricity generator with a sound power level of 110 dBA will create a sound pressure level (Lp) of 92 dBA at a distance of 5 m away outdoors on soft ground (grass) well away from any sound-reflecting surfaces. If placed on hard ground (e.g., tarmac or concrete), the sound pressure level from the same source at the same distance increases by 3 dB to 95 dBA because of the reflections of sound from the ground to the reception position. If placed at the base of a vertical wall on the same hard ground (i.e., now close to two reflecting surfaces) it produces another 3 dB increase. If the same machine is measured at a distance of 5 m in a room such as a workshop, the sound pressure level will now be increased because of the influence of the floors, walls and ceilings and any other sound-reflecting surfaces nearby, and the measured sound level will depend on the nature of the room, whether it is large or small and whether it is ‘live’ or ‘dead’ (i.e., depending on the amount of sound absorption in the room to control the reflections).