09 Mar 2020 - by Graham

At the bottom of every speaker stack, under every stage, and tucked in the corner of every hi-fi enthusiast's room, you will usually find one or more subwoofers. These large box-like speakers are a recognisable feature of every PA system and at every disco or nightclub with a good sound you will hear someone at the bar declaring that the place has "great subs!" to anyone who will listen. Subwoofers are apparently an integral part of any sound reinforcement or PA system, but do you always need them? And if you do, what is the best way to set them up?

Subwoofers are speakers that are specifically designed to reproduce extra low frequencies towards the bottom end of the audio spectrum, generally somewhere below 100Hz. This frequency range includes the kick drum, the bass guitar, synthesised low notes from keyboards, and all the other instruments and effects that make sounds in this range. Reproducing low frequency sounds clearly can present a few complicated physical challenges for speaker designers, so normal "full range" speakers tend to be a bit inefficient in this range, hence the need for subwoofers. They fill in the space where your main PA leaves off, and create depth in the sound that would otherwise be lacking.

But if your particular usage case does not include anything that creates sound in those frequencies, then adding a subwoofer can reduce clarity, rather than improving it. Voices, acoustic guitars, wind instruments, most orchestral stringed instruments, and a lot of percussion, all produce the most important part of their sound at higher frequencies, and it is only by accident that they produce sound in the subwoofer range. Handling noise, breath leakage, wind noise and similar all have strong low frequency components, and so amplifying that range when your instrument does not need it will only help to muddy the soundscape and hide the subtleties of your playing or singing. So when do you use a subwoofer and when do you leave it out of your system?

Do I need one?

If you are putting together a sound system for a single guitar and a vocalist, or for two stringed instruments and an announcer, or anything that does not have a strong deliberate low frequency presence, then no. Save yourself some money and a whole lot of effort in lugging the thing around and stick to some decent full-range speakers for your PA. If you have a subwoofer in your system and this is your sort of music then at best it will never make a sound, and at worst it will do a perfect job of amplifying your microphone fumbles and any wind that blows across the stage, all of which you would rather not hear.

If you are a classic guitar / bass / drums combo with or without a keyboard player, and are mostly gigging in pubs and small clubs, then you probably don't need one either. The bass player will have his or her own combo-amplifier that is designed for those frequencies, and the kick drum will probably be too loud already (they usually are) so does not need amplification. There is an argument that the keyboard might benefit from a subwoofer if your player is into heavily synthesised low-frequency beats and sweeps, but the soundscape in pubs and clubs is usually extremely muddy even before you set up the PA system, so it is probably best not to add to the chaos with a sub. Keep it for the stage.

Note that if you use an all-in-one system then a lot of the high-end ones have a subwoofer built in to them anyway. This is not a problem, but do your best to use the mixer part of it to keep unwanted low frequencies out of the subwoofer channel as much as possible. We will post an article about how best to do this this soon.

Adding a Subwoofer

Assuming you have decided you need one, or have been asked to set up a PA that includes them, how do you set up a subwoofer properly so that it helps rather than hinders the sound? Can you just throw it under the stage and hope for the best? Should you always have two, to match your main PA speakers?

Positioning

If you buy a subwoofer for your HiFi system they often come with a small note saying that "as low frequency sounds are non-directional you can place this speaker anywhere in the room". Whilst some of that is correct and low frequencies are indeed omni-directional so you don't need to be standing in front of the speaker to hear it, positioning is a lot more critical than just "put it anywhere", and this is down to something called  'phase'.

Link arms with your friend and start walking down the road. If you walk in step it is generally pretty smooth and comfortable. but if you get out of sync you will soon find that you are pulling each other around and making a right mess of things. This is because you are walking "out of phase", and the two walking rhythms - the waveforms - are interfering with each other. The same happens with sound, and so you have to think of the subwoofer as 'linking arms' with the main PA system and trying to walk in time.

You achieve this with a PA system by making sure that the subwoofer is perfectly in line with the main PA speaker, and that they are the same distance away from the listeners. If you have one subwoofer per side then the traditional way to do this is to stack the main speakers on top of the subwoofers, and carefully line up the front grille of the mains with the front grille of the subs. You can stack them directly on top of each other, or some systems allow the use of a separator pole to raise the mains up to a more appropriate listening height. In systems with just one subwoofer, or if you want to set them up in the middle for some other reason, then draw an imaginary line between the fronts of the two main speakers and put the subs on the same line.

This keeps the subwoofers "in phase" with the mains, and helps avoid any loss of clarity due to phase canceling.

Crossover

Subwoofers are generally designed to work with sound below about 100Hz, whereas your full-range main speakers will likely work best with sound above about 80Hz up to 15-20kHz. What they do with information that falls outside these ranges is unpredictable and it can sound messy or distorted if you let it get to the wrong speaker. To prevent this you use a Crossover, a device that separates the low frequencies from the high and sends the right range to the right speaker.

There are a number of ways of doing this and the best way will depend entirely on the complexity and configuration of your individual installation, but getting started with crossover is usually fairly simple. Most smaller subwoofer units are active, and include an adjustable low-pass filter on them somewhere. This allows you to set the top-end of their response to match the bottom end of your main PA's response. Then on your mixing desk, speaker driver, or main EQ, apply a low-cut / high-pass filter to everything that comes out of your main speakers, making sure that the signal you are sending to the subwoofers comes out of the system before this adjustment.

This is a simple crossover that will preserve the clarity of the sound coming out of your speakers and get the best advantage from having a subwoofer in your system. As your system grows you may find you need to add in a professional crossover unit to your amplifier stack, but keep it simple at first until you understand the principles.

Conclusion

Adding a subwoofer to your system can be hugely beneficial to your sound, but only if you actually need it. Take the time to set it up properly and understand how it works in conjunction with the rest of your PA system and it will be a valuable tool.