Adding Subwoofers To Your PA System

by Andrew Bishop

If you don’t already have some, adding subwoofers to your existing PA system is an upgrade that can pay off in one of two ways. The first and most obvious is that you can increase the level of low-frequency energy in the room – think of a nightclub or a reggae sound system. However, there is another, more nuanced benefit, in adding low-frequency impact to any PA system.

The vast majority of subwoofers are designed to reproduce frequencies below 150Hz. Driving the cone of a woofer in order to move the amount of air necessary to amplify low frequencies takes up a lot of amplifier power in a 2-way PA cabinet, so moving the reproduction of these frequencies down to a subwoofer “frees up” the top cab to concentrate on the mid and high frequencies.

This results in an increase in overall clarity and definition as the top cab’s amplification isn’t being drained by the effort of low-frequency reproduction and the midrange output of the cab’s woofer isn’t being compromised by the large cone excursions necessary to move the amount of air that low-frequency reproduction requires.

How Many Subs Do You Need?

All loudspeaker cabs (including subs) are basically Omni-directional at low frequencies, which makes it hard for the human ear to detect where low frequencies are coming from. Designers of small PA systems (and of home hi-fi) exploit this and so you’ll come across a good few 2.1 systems with small 2-way top cabs and a single sub that will work very well in small venues such as folk clubs or in other applications where clarity, but not necessarily volume, is required.

A good many of the available 2.1 systems are active, with the amplification etc. built into the sub. This integration is very convenient for transport and setting up, but can become a real liability if the sub goes down because all the amplification will usually go down with it. However, active systems have a lot of advantages in keeping down the amount of kit that needs to be transported, the cabling complexity and the time needed for PA setup. If your application doesn’t need a bass-heavy setup, a pair of active 15” top cabs, coupled with single 15” or 18” active sub will give you a system that would handle smaller venues and corporate presentation situations. If you find yourself looking for more bass, adding a second matching active sub could well be the answer.

If you’re a mobile DJ looking to create a club atmosphere, or a rock band with significant low-end requirements, you might be looking at running two or even four active subs per side and perhaps doubling the number of active tops. If you are looking at that level of performance, you might also want to consider a system made up of passive cabinets, say 15” or 18” subs (single or dual cabinets) coupled with 12” or 15” top cabinets.

Nowadays, the variety of active and passive cabinets that are available give you the opportunity to build a PA system that is tailored precisely to your needs. If you check out the BishopSound Big Rigs webpage, you’ll see our suggestions of tried and tested systems, but feel free to mix and match and to call us for advice on your choices.