- by Vince Lepore
in Sound Sanctuary
Managing low frequency energy is a notoriously difficult proposition. The very nature of low frequencies is that they are large (see Fig. 1), difficult to control from a directional perspective and difficult to treat from an acoustical perspective. If you mix in rooms that have little or no acoustic treatment (as many of us do), trying to get your low-end under control can be a real challenge.
The Systems Perspective
At the system level, a common mistake is tuning too much low frequency energy into a system. Your days are then spent fighting low-end feedback and dialing low-end out of every channel you can find. Tune your system to be well balanced for the style of music you are mixing. If that’s contemporary worship music, you want a healthy amount of low-end in the system, but don’t overdo it or you’re just going to make more work for yourself. A few engineers out there will even tune their systems completely flat, without that typical bump in the low-end, and then add low frequency energy via their channel strip EQs when necessary.
If you have arrays of speakers in your space, whether it’s line arrays or point source arrays, remember that low frequencies couple between boxes, causing a build-up of LF energy as the number of boxes increases. Many speaker manufacturers provide equalization tools and guidelines specifically targeted at flattening out this LF coupling in arrays. If you’re having problems keeping control of your subwoofer energy, consider a directional subwoofer array. Cardioid subwoofers are out of reach for many churches due cost of the additional subwoofers and signal processing, but if you’re able to afford it, you can take control of your lowest frequencies and direct them onto your audience and off your stage. Directional subwoofer arrays can be achieved with ground stacked as well as flown subs, so don’t assume your subs must be on the floor to take advantage of directional subwoofers.
The Stage Perspective
Another way to clean up some excess low-end is to manage what’s happening on stage. Reducing the number of wedges makes a noticeable difference. It may not seem like wedges are producing a lot of low-end, but it is probably more than you think. If you ever have a chance to listen just to what is coming off your stage while muting the main P.A., there is a surprising amount of low and low-mid energy that comes off the back of a monitor wedge. Reducing the number of wedges and some clever use of high-pass filtering can make a world of difference. If you’ve got subwoofers onstage as part of a sidefill or drum fill system, consider replacing those with a tactile transducer such as a bass shaker. Not only do they produce significantly less low frequency energy, but they are also cleaner looking, lighter and easier to set up. Finally, whenever possible, isolate your guitar and bass amplifiers backstage.
The Mix Perspective
There are several mixing tools at your disposal to manage LF energy. Your mileage will vary depending on the mixing console you’re using. If you’ve got a high-end digital console, you’ll have access to many mixing tools that will help keep control of your low-end. However, even if you’re on a $500 analog board, there are still tools that can help. The obvious example is the almighty, all-powerful high-pass filter. The high-pass is so easily overlooked. It’s just so small, it must not be that important, right? I teach at a university where students are just learning the art of sound engineering. They don’t understand the vast importance of the HPF, so I must impress it upon them. I often have students ask me “what channels do you use a high-pass filter on?” I’m sure you can guess my answer. “What channels don’t I use a high-pass filter on?” Then, I proceed to list two or three channels and I’m done. It’s a short conversation. So even if you’re working on an 8-channel Mackie with high-pass filters fixed at 75 Hz, turn those puppies on! If you’re on a mixing console with variable frequency high-pass filters, or even fancier, HPFs with adjustable slopes, use that to your advantage.
Beyond high passes and EQ filters, a multiband compressor or dynamic equalizer can be an effective tool on select instruments. A bass guitar is a great example. If you’ve ever dealt with a bass player who could hit a low note that just seemed explosive through your P.A., applying a multiband compressor to manage those low notes while leaving the rest of the instrument’s frequency response intact can be very useful.
Finally, consider using the aux-fed or group-fed subwoofer technique rather than the stereo bus to drive subs. When the stereo bus is driving subs, every single input that’s assigned to it is making its way into the subs to some extent. Even when properly high-passed, an input channel — such as a vocal mic — bleeds muddy low frequencies into the subwoofer band. It may not seem like a big deal, but that energy adds up from many different sources on stage. Feed your subs from an aux (or a group), and you’ll gain the ability to selectively send input channels to the subs.
There isn’t one single solution for managing low frequency energy in your mixes or in your room. Careful consideration of your sources, your speaker placement and your signal processing will go a long way towards helping manage and control low frequencies, while still making it impactful for your listeners. Give these a try!
Vince Lepore is the technical director at St. Luke’s United Methodist Church in Orlando and teaches live production at Full Sail University.