They do not effectively reproduce frequencies below this. So
in order to maximise our available amplifier power, we would
employ a filter that removes these frequencies. This filter is
called a ‘high pass filter’ or HPF. In other words it is a filter
that passes high frequencies and removes low frequencies
from a signal. It can take many forms. Many modern digital
mixing consoles allow you to apply filtering to every output of
the mixer. So we can apply a HPF to the mixer output. We
could use a modern digital loudspeaker processor. We could
place a parametric or graphic EQ across the output of the
desk, or we could use a ‘crossover’ system (more on
crossovers later). But something that often gets overlooked is
that power amplifiers are often equipped with filters, so check
the rear panel of your amps.
These filters may not be variable, they may not offer exactly
the cutoff frequency you require, but they can be really useful
in removing low frequency signals that your speakers can’t
reproduce thereby focusing your available amplifier power
and maximising your systems performance.
This is a graphic representation of the sort of high pass filter
we may employ for use with a typical mid/high loudspeaker
system.
This will sometimes require some experimentation. Use the
manufacturers guidelines, but also your ears. I find 1 00-
1 30hz is good for most 1 5″ subs, 80 – 1 20hz for 1 8″ boxes.
By applying a 1 20hz filter we are reducing the output of
1 20hz and above and therefore really focusing and
maximising our amplifier power.
Again, we can use a range of tools to achieve this. The
processing features of a digital mixing console, a dedicated
loudspeaker processor/controller, a dedicated analog
crossover/filter or the filtering features of a power amplifier.
Many modern power amps have on board low pass filters
(LPF) designed specifically for running subwoofer speakers.
Here’s a graphic representation of a typical low pass filter:
Use filters wherever possible to match the signal to the type
of speaker at the end of the signal chain. You’ll be using
your power amps to their full potential, create more
amplifier ‘headroom’, and increase the point at which you
begin to see the dreaded red clip lights.
In part two, we’ll take a look at filter slopes, and crossover
filters.
Filtering unused low frequency information is the most
important element in maximising system performance, since
these are the frequencies that contain most energy and will
waste more amplifier power if filtering is not employed. You’ll
be turning the signal energy into heat rather than sound!
Subwoofer Systems
Once we add a pair of sub bass speakers to enhance low
frequency reproduction, we need to apply filtering to t hese.
We need a filter that passes low frequencies and removes
higher frequency information. We need a low pass filter.
(Some sub bass speakers may contain a passive filter
network that filters high frequency information after the power
amplifier. These do not tend to be found in high powered pro
systems as they come after the power amplifier and so don’t
help to focus amplifier energy, and they are inefficient,
introducing what we call insertion losses. There are also limits
to how much power passive networks can handle. )
Large 1 5″ and 1 8″ sub bass woofers can reproduce
frequencies up to 1 – 1 .5khz, but our mid/high speakers will
be covering this frequency range. So it’s crucial that we
apply a low pass filter to remove these and really focus the
energy and power into frequencies below around 1 50hz.
Gigging Musician
Page 9
Proel EBN Series XLR multicore
stageboxes.
Compact dimensions. High quality,
minimal diameter CBR Series multicore
cable. Neutirk XLR chassis stagebox
connectors.
Heavy duty Proel metal cable XLR
connectors.
Colour coded cable returns for easy
identification.