numerical value assigned
to
the
perceived noisiness
of
the
1000
Hz
critical
bandwidth
of
the ear (910
to
1090
Hz)
at
a band pressure
level
of
40
dB re
2
x
10-
5
Nfm2.
Since
the
calculation procedure for PNdB requires analysis
of
the
sound
and tedious weighting and addition
to
the
Noy levels, Ref. 1 also suggested a
frequency weighting characteristic for a sound
level
meter. Referred
to
at
that
time
as
the
"N"
weighting network it was chosen
to
be
the
inverse
of
the
40
Noy contour.
The
reason for this was
that
the
40 Noy
contour
is
fairly representative
of
the
band width and
level
of
aircraft fly-over noise.
Also aircraft noise spectra biassed by this weighting gave good agreement
with perceived noise
level
calculations except
for
a constant 10 dB
difference between
the
absolute values. A practical electronic weighting
network
to
approximate
the
N-weighting
is
expected
to
be standardised in
the
near future. This will be known as
the
"D-weighting". Although more
convenient
in
use
than
the
perceived noise level procedure,
the
D weighting
should
not
be expected
to
accurately measure the relative noisiness
of
two
sounds which are
of
different bandwidths. On
the
other
hand
the
perceived
noise level scale should
do
this since
in
this case frequency bands are
individually weighted rather
than
the
block weighting
of
the
whole spec-
trum.
A weighting curve based on
the
inverse
of
the
40
Noy
contour
(The
D-weighting)
is
preposed
in
ISO
Recommendation
507
(second edition) as
an approximation
to
PNdB
in
which
is
suggested
that
the
dB
(D)
level
+ 7
dB
approximates.
REFERENCES:
1.
K.D. Kryter: JASA Vol.
31
No.
11
( 1959)
p.
1415
2. K.D. Kryter and
K.S.
Pearsons: JASA Vol.
35
No.6
( 1963)
p.
866
3. S.S. Stevens: JASA Vol. 28
No.5
(1956) p.
807
8
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