BRUEL & KJAER 1618 - Octaves and Third Octave; Specified Filter Frequencies

acteristic of Fig .4 .1 is equal to the area below the ideal filter characteristic when the
ideal and practical filters have the same Effective Bandwidth .
4.1.3. Octaves and Third Octaves
An Octave is the interval between two frequencies having a ratio of two, i.e.
f
= 2 f
u ,
(4 .1 )
or
f
u
- =2
(4. 2)
f,
where f u and f, are upper and lower frequencies respectively.
Similarly, fractions of octaves are defined by:
f
u
-=2
n
(4 .3)
f,
where n is the fraction of the octave.
Therefore,
third octaves are defined by:
f
_u_= 2'/3 = 1 259
f, '
(4.4)
Since the centre frequency is defined by:
(4 .5)
and the bandwidth :
B = f - f
u ,
(4 . 6)
it follows that :
B = Kf
m
(4 . 7)
where K is a constant depending on the ratio between f u and f, . For third octaves K =
0 ,231 , and for octaves K = 0 , 707 . The bandwidth of the filters in percent is given by
K x 100 .
4 .1 . 4. Specified Filter Frequencies
The octave and third-octave filters in the Type 1617 have centre frequencies on the
scale recommended by lEe Publication 225 . This relates the frequency scale to 1 000 Hz
reference since the centre frequencies are calculated according to :
3n
f = 1000 x 1 0'
0
for Octave fi Iters
m
(4 . 8)
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