band of frequencies will be affected.
This can be seen in the examples in
Figures
and
10
100
1,000
10,000
−6
−4
−2
0
2
4
6
Frequency (Hz)
Gain (dB)
Figure 15.6:
Example of two low-
shelving filters. The black curve shows
a filter with a Q of 0.4, the red curve
shows the a filter with a Q of 1. For both
filters, the centre frequency is 100 Hz
and the gain is +6 dB.
10
100
1,000
10,000
−6
−4
−2
0
2
4
6
Frequency (Hz)
Gain (dB)
Figure 15.7: Example of two peaking fil-
ters. The black curve shows a filter with
a Q of 0.5, the red curve shows the a
filter with a Q of 8. For both filters, the
centre frequency is 1 kHz and the gain
is +6 dB.
It should be explained that the Q
parameter can cause a shelving filter
to behave slightly strangely. When the
Q of a shelving filter exceeds a value of
0.707, the gain of the filter will
“overshoot” its limits. For example, as
can be seen in Figure
, a filter with
a gain of 6 dB and a Q of 4 will actually
have a gain of almost 13 dB and will
attenuate by almost 7 dB.
This over- and undershooting of the
filter’s magnitude response is the
reason the Q of the high-shelving and
low-shelving filters in the BeoLab 90’s
parametric equaliser have been limited
to a maximum value of 1.
Note that, when the Q is set to a value
of 1, then the resulting overshoot of
the filter, in decibels, is approximately
7.4% higher than the stated gain of the
filter. For example, if Gain = 6 dB and
Q = 1, then the maximum actual gain
of the filter will be 6 * 1.074 = 6.44 dB.
If Gain = 3 dB and Q = 1, then the
maximum actual gain of the filter will
be 3 * 1.074 = 3.22 dB.
10
100
1,000
10,000
−10
−5
0
5
10
Frequency (Hz)
Gain (dB)
Figure 15.8: Example of low-shelving fil-
ters with a Q of more than 1. The black
curve shows a filter with a Q of 0.7 for
reference, the red curves shows filters
with Q’s of 1, 2, and 4. The centre fre-
quency of this filter is 100 Hz and the
gain is +6 dB.
Note that some of these
values are not possible in the paramet-
ric equaliser in BeoLab 90.
51
