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.
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