acteristic of Fig.5.1 is equal to the area below the ideal filter characteristic when the
ideal and practical filters have the same Effective Bandwidth.
5.1.3. Octaves and Third Octaves
An Octave is the interval between two frequencies having a ratio of two, i.e.
f = 2 f
Or
fu
fi
where fu and flare upper and lower frequencies respectively.
Similarly, fractions of octaves are defined by:
u
—= 2-
fl
where n is the fraction of the octave.
Therefore, third octaves are defined by:
fu
-- 21/3 = 1,259
fi
Since the centre frequency is defined by:
fm = \/fu x
and the bandwidth:
it follows that:
B fu — fI
B = Kfm
(5.1)
= 2 (5.2)
(5.3)
(5.4)
(5.5)
(5.6)
(5.7)
where K is a constant depending on the ratio between fu and fl. 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.
5.1.4. Specified Filter Frequencies
The octave and third-octave filters in the Type 1618 have centre frequencies on the
scale recommended by IEC Publication 225. This relates the frequency scale to 1 000Hz
reference since the centre frequencies are calculated according to:
3n
fm = 1000 x 1010 for Octave filters
(5.8)
24
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