York HVAC Catalogue 2018 EN 2019 EN | Page 84

Weighting scales dB(A) Noise criteria (NC) curves The human ear detects the single frequencies with different intensity and it has therefore been necessary to establish a method that simulates human hearing. In order that a single overall sound output value of an object may be determined a weighted or averaged value, that assimulates human hearing, can be taken of the sound level in each frequency of the frequency band between, the lowest and the highest, which can be heard. The ear can only perceive the pressure variations of air pressure not the sound power itself. The radiated sound power is transformed into sound pressure, part of which is absorbed by the environment and such objects as carpets, clothes etc, that may be situated within the wave pattern, and the intensity decreases with distance. This effect is called “Room Effect”. The difference between the sound power and the received sound pressure can be read from a diagram. The noise dB curve at the single frequencies is plotted and compared with reference curves. The value of the highest reference line that touches the noise curve is the NC value. The generally accepted bands are centred on 62.5Hz which is then doubled each time to a peak of 8000Hz (8kHz). These are the octave bands . The most commonly used weighting curve is the “dBA” scale. Weighted dB differences are subtracted from the source frequency band values and the resulting dB(A) value will therefore be lower than the unweighted. The ratio sound pressure/sound power N/m The relationship between sound pressure waves and the sound power of the object producing them depends on the nature of the area around the object and the location of the person effected by the sound. In order to take accurate sound measurements it is preferable to have a “Free Field” environment. This is an area having constant properties, free of boundaries or objects or any other sound source which could effect measurements. The sound waves radiate outwards in concentric hemispheres from the source getting weaker as the distance increases. The average sound level at the surface of one of these hemispheres is directly related to the sound power of the machine which is assumed to be generated at a point at the geometric centre of the machine. If an object is large, relative to the distance from which sound measurements are taken, the sound cannot be considered to be coming from a single point and the sound level will depend on where the subject is standing. The sound level can vary considerably with a small change in position and it is not therefore possible to relate sound pressure to sound power using Near Field measurements. Frequency Hz Site installations are seldom a free field environment . Adjacent buildings, walls etc, affect the sound pressure waves and a wall in close proximity can increase the sound level on the opposite side of the machine by reflecting the sound back in that direction. 84