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Technical: The Power Of Filters. Part One. HPF and LPF Filters.

Filtering frequencies to focus amplifier power and maximise system performance.

Filters are a great tool for maximising the power of any audio system. Matching the audio signal to the frequency response of the loudspeakers will maximise available amplifier power and minimise wasted energy.

All loudspeakers have a frequency response. In other words a range of frequencies that they are designed to reproduce. Sending a signal to a loudspeaker that contains frequencies that it does not reproduce just wastes amplifier power and energy.

By using filters to tailor the signal to the response of the loudspeaker we can ‘focus’ the  signal and maximise our amplifier’s available power.

Before now, the audio hardware required to filter every channel of an audio system may have been too expensive to implement, but many of today’s digital desks have filters available on every output so the tools required to focus our available amplifier power are more readily available. Also, power amplifiers are sometimes equipped with some very unassuming little switches on their back panels which are often ignored, but can be employed to greatly improve the performance of many sound systems.

The need for filtering of audio signals really stems from loudspeaker response. No one has yet invented a single high powered transducer suitable for high level PA applications that can reproduce the entire frequency spectrum. ‘Woofers’ reproduce low and low/mid range frequencies, ‘tweeters’ reproduce high and high/mid range frequencies.

For practical applications in modern portable PA systems, we really only need to deal with two types of loudspeaker. The full range speaker, sometime referred to as a mid/high speaker, and the subwoofer, or sub-bass speaker. ‘Full range’ speaker is a bit of a misnomer. We tend to label these speaker ‘full range’ because they provide useful output across most of the frequency spectrum and if we were just going to employ a single type of speaker, this would be it. These loudspeaker systems will usually reproduce most of the frequency spectrum but not the very lower frequencies required for genuine full range reproduction of modern rock, pop and dance music.

So to start with a simple system. A pair of full range speakers, driven from a stereo amplifier, from a mixing console. Typically, full range PA speaker produce useful response from around 55hz – 20khz for a speaker based around a 15″ drive unit, to 65hz – 20khz for a speaker based on a 12″ woofer. 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.

Amp rear panel filter switches

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.


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 these. 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 15″ and 18″ 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 150hz. This will sometimes require some experimentation. Use the manufacturers guidelines, but also your ears. I find 100-130hz is good for most 15″ subs, 80 – 120hz for 18″ boxes. By applying a 120hz filter we are reducing the output of 120hz 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.


This blog post was written by Simon Thompson. Soundman, musician, occasional DJ, and CEO/Proprietor of The Noizeworks. Live music equipment suppliers extraordinaire! Based around East London/Essex. UK. He hopes you find it useful and helps in your artistic and professional endeavours…..but……as with all his blog posts, this information is offered in an informal basis and he accepts no responsibility for any circumstances arising out of the use of this information or the inability to interpret it, etc.  



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