4.2 Noise Rating (NR)
curves are used in the IMO Code on Noise Levels on board Ships and
in statutory codes and were incorporated into ISO 1996. They are a
set of empirical curves relating the linear octave band pressure level
to the centre frequency of the octave bands each of which is characterised
by a Noise Rating number which is numerically equal to the sound pressure
level given by the curve at 1000 Hz,
Figure 3.4.1 Noise rating curves
. The subjective effect of the noise is predicted
from the NR number. The octave band sound pressure levels are plotted
on the NR curves. The NR number is that NR curve to which the highest
plotted octave level is anywhere tangent.
4.3
Narrowband
analysis of noise
The narrowband analysis of noise is useful in locating sources
which are dependant upon speed of rotation and give rise to pure tones,
for example, fans at blade passing frequency.
Figure 3.4.1 Noise rating curves