Noise Rating (NR) Curves

These curves are developed in Europe to
assess community noise complaints. They are shown in Figure 9. Their use is similar to
that for the NC and PNC curves.

(b) __The Weighted Scales__

__
__The weighted scales are designed to
quantify sounds or noises by one single value and yet do not have to refer to graphs or
curves. The single numerical values are called sound levels.

The octave-band pressure levels are
adjusted individually before they are combined to form one single number. The
normalization is shown in Fig. 10, and is intended to give a better subjective evaluation
of the impact of noise or sound upon the human ear.

Four weighting scales: A, B, C and D were
introduced. These weighting curves are in fact the inverse of equal loudness curves and
taking the fact that the equal loudness curves get flatter as sound pressure level
increases. The A-weighting was for sound pressure levels below 55 dB; B-weighting for
levels between 55 and 85 dB; C-weighting for levels above 85 dB; and the D-weighting for
even higher levels. Nevertheless, the A-weighting is now used almost exclusively in
measurements that relate directly to human responses, both from the view point of hearing
damage and of annoyance.

The formula for converting octave-band
sound pressure levels into sound levels on the X-weighting scale, X being A, B, C, or D,
is:

(1)

*LX-weighting* =
sound level on the x-weighting scale, dB(X)

*Lpi* = sound
pressure level for the ith octave band, dB

*F* = correction factor, dB

The values of the correction factors are
given in Table 4.