Appendix 2: Introduction to Parametric Equalisers
Almost all sound systems offer
bass
and
treble
adjustments for the sound –
these are basically coarse versions of a
more general tool called an
equaliser
that is often used in recording studios.
Once upon a time, if you made a
long-distance phone call, there was an
actual physical connection made
between the wire running out of your
telephone and the telephone at the
other end of the line. This caused a big
problem in signal quality because a lot
of high-frequency components of the
signal would get attenuated along the
way due to losses in the wiring.
Consequently, booster circuits were
made to help make the relative levels
of the various frequencies more
equal
.
As a result, these circuits became
known as
equalisers
. Nowadays, of
course, we don’t need to use
equalisers to fix the quality of
long-distance phone calls (mostly
because the communication paths use
digital encoding instead of analogue
transmission), but we do use them to
customise the relative balance of
various frequencies in an audio signal.
This happens most often in a recording
studio, but equalisers can be a great
personalisation tool in a playback
system in the home.
The two main reasons for using
equalisation in a playback system such
as the BeoLab 90’s are personal
preference and compensation for the
effects of the listening room’s
acoustical behaviour.
Equalisers are typically comprised of a
collection of filters, each of which has
up to 4 “handles” or “parameters” that
can be manipulated by the user. These
parameters are
•
•
•
•
15.1
Filter Type
The
Filter Type
will let you decide the
relative levels of signals at frequencies
within the band that you’re affecting.
Although there are up to 7 different
types of filters that can be found in
professional parametric equalisers, the
BeoLab 90 contains the three
most-used of these:
•
•
•
15.1.1
Low-shelving Filter
In theory, a
low-shelving Filter
affects
gain of all frequencies below the centre
frequency by the same amount. In
reality, there is a band around the
centre frequency where the filter
transitions between a gain of 0 dB (no
change in the signal) and the gain of
the affected frequency band.
10
100
1,000
10,000
−6
−4
−2
0
2
4
6
Frequency (Hz)
Gain (dB)
Figure 15.1: Example of a low-shelving
filter with a positive gain. Frequencies
below approximately 80 Hz have been
affected.
10
100
1,000
10,000
−6
−4
−2
0
2
4
6
Frequency (Hz)
Gain (dB)
Figure 15.2: Example of a low-shelving
filter with a negative gain. Frequencies
below approximately 80 Hz have been
affected.
Note that the low-shelving filters used
in the BeoLab 90 define the centre
frequency as being the frequency
where the gain is one half the
maximum (or minimum) gain of the
filter. For example, in Figure
, the
gain of the filter is 6 dB. The centre
frequency is the frequency where the
gain is one-half this value or 3 dB,
which can be found at 80 Hz.
Some care should be taken when using
low-shelving filters since their affected
frequency bands extend to 0 Hz or DC.
This can cause a system to be pushed
beyond its limits in extremely low
frequency bands that are of little-to-no
consequence to the audio signal. Note,
however, that this is less of a concern
for the BeoLab 90, since it is protected
against such abuse.
15.1.2
High-shelving Filter
In theory, a
high-shelving Filter
affects
gain of all frequencies above the
centre frequency by the same amount.
In reality, there is a band around the
centre frequency where the filter
transitions between a gain of 0 dB (no
change in the signal) and the gain of
the affected frequency band.
49