Augment Your P.A. with a Subwoofer

A Subwoofer Can Extend the Power of Your Current Setup

By Jon Chappell

More and more people are recognizing the benefits of a powered subwoofer, and not just DJs or techno musicians who need to pound out an earthshaking beat. A subwoofer handles low-end frequencies very well, and can offload some of those responsibilities from your P.A.'s midrange-oriented cabinets. You can buy a three-speaker system that includes a subwoofer (or will accommodate one when the time comes to upgrade), but anyone can use a subwoofer in any system. The trick lies in the mixer, and the feature called an aux send or aux bus.

Using your mixer’s aux bus, you can selectively route low-end signals to a dedicated subwoofer, leaving your top boxes, or midrange cabinets, to handle everything else--guitars, vocals, keyboards, and the drum kit minus the kick. For bands who have been getting along with their current P.A. and are starting to play larger venues, or just realizing they need to kick up the output a notch, investing in a subwoofer will often get you the additional power you need at an economical price. And you don’t have to scrap your current system!

The trend toward using powered speakers makes the mixing and matching of top boxes with subs much more versatile and commonplace. A powered speaker is one that doesn’t require an external power amp, either as a stand-alone unit or as built in to a powered mixer. But even if you do elect to use passive cabs, most modern power amps, such as those by Crown, QSC, and others, are allowing their amps to be split, so that they can drive midrange speakers and subwoofers separately but simultaneously. A self-contained, powered subwoofer is a tool that you don't always need  to bring out, but is handy to have for special occasions.

TAKE THE SUB WAY

Here are some of the advantages of bringing a subwoofer into your system.

• You maximize your total power by directing midrange and treble frequencies to the top boxes and low end to the subwoofers. This means that your system will be louder at the same power output, or that your maximum output power will be the same as a larger system’s.

• You can be more directional with your midrange cabs. This is because subwoofers are non-directional; you can place a sub almost anywhere near the stage (within reason) and achieve a similar effect as if it was directly under (or connected to) the midrange boxes. This way you can tilt and angle the top boxes any way you like.

• You can keep stage rumble out of open microphones. This is especially important in outdoor gigs with makeshift stages that often ring and rumble when people walk across them or when vehicles pass by in close proximity. For rumble reduction, you need a high-pass (low-cut) filter, but they’re fairly inexpensive, and if you do a lot of outdoor venues and festivals, they’re well worth the money. And even when you can’t always hear it, without a high-pass filter your midrange components are expending energy inefficiently in attempting to transduce these extraneous and spurious low-frequency energy impulses in your system.

• The two different signal paths can be processed separately. You can obviously apply individual effects and EQ to each channel, but sometimes it’s helpful to process the entire bus, such as when using feedback eliminators, cross-overs, or delay compensation. In these cases, a separate subwoofer path allows you to modify just the mid/high signal, just the low-end signal, or both. Want more bass? Don't EQ, just turn up the Aux Send control!

Adding an aux sub to your system is easy. First, decide on what instruments will go to the subwoofer. That's usually the kick drum and bass (electric or acoustic), though you could additionally add a floor tom and split keyboard part. Fig. 1 shows a drum kit whose kick mic is set up to go to Channel 1 in the mixer.

Fig. 1: Take the kick drum mic and run it into Channel 1 of the mixer.

SUB MIXING

Once the mics and inputs are taken care of, you have to set up your mixer so that the drum is sent to the subwoofer. This is done using an aux send. The aux send volume control is on the channel strip. The output of the signal is a back-panel jack, usually to the right side of the mixer (as you face the mixer during normal operation). Run a cable of the appropriate length from the Aux Send output jack to the subwoofer's input (see Fig. 2). If you have other instruments, run those at the appropriate level using the channel's Aux Send level as well. It will be grouped with the kick and output to the subwoofer as well (also shown in Fig. 2). You can now think of the aux send control as a low-end EQ. Turning up the level increases bass response.

Fig. 2. Use the Aux Send level control (on the channel strip) and the Aux Send output to feed the subwoofer.

CONCLUSION

Deciding whether a dedicated sub will suit you is a simple test to make, as it's pretty easy to borrow a subwoofer, and you should hear the results almost immediately. Routing gets more complicated if you're going to hook up a feedback eliminator or a high-pass filter. But the first step in maximizing your available power is to get the subwoofer into the system and feed it the signals it's best suited to reproducing. That leaves the midranges boxes free to do their thing, achieving the same volume level with less power, or using the same power to boost the output a little. And with a small P.A., every little bit of clean, efficient boost helps.

Jon Chappell is a guitarist and the Senior Editor of Harmony Central. He has contributed numerous musical pieces to film and TV, including Northern Exposure, Walker, Texas Ranger, All My Children,  and the feature film Bleeding Hearts, directed by actor-dancer Gregory Hines. He is the author of  The Recording Guitarist: A Guide for Home and Studio (Hal Leonard), Essential Scales & Modes (Backbeat Books), and Build Your Own PC Recording Studio (McGraw-Hill), and has written six books in the popular For Dummies series (Wiley Publishing).