Way back in the dawning days of sound reinforcement, when very few loudspeaker manufacturers existed and most sound rental companies built their own sound systems, the developing industry learned that low-frequency (LF) horn loading was important to keep the sonic energy focused on the audience and away from the walls and ceiling. But because horns must be many feet in length and very large in mouth area to properly control lower frequencies, most early loudspeaker designs had less-than-impressive LF output, especially in the deep bass region. Thus, the concept of the subwoofer was born.
The next four decades saw a lot of changes in loudspeaker technology. Measurement systems grew ever-more sophisticated, permitting the loudspeaker designer to more realistically understand what was taking place in a prototype system. The huge shift towards line arrays also meant that the ‘main' system on its own wouldn't always be suitable for providing deep bass. Subwoofers, which had never really lost significance, became more important than ever in meeting the demand for ever-increasing sonic intensity in the basement frequencies, particularly as Electronic Dance Music (EDM) became more popular.
Some line arrays are designed to fly subs along with the main speakers, some will locate them behind the main rig. Other system designers prefer the floor, which adds apparent power through coupling with the floor surface.
What Do You Do With Them
Subs are a welcome addition to the world of modern church audio, but along with the benefits come some inherent difficulties. Let's look at what this means:
To begin with, unless you're running a rather small event, the subs, if placed on the stage or floor, are probably 10 to 20 feet away from the ‘flown' or suspended main loudspeakers…if not further than that. Vector geometry shows that the different path lengths from the mains and the subs, to any given area of the audience, will only be geometrically equal in a small region of the room. Unequal path lengths mean that there will be a series of acoustic cancellations and additions between the subs and the mains. The system will exhibit a different sonic character in each area of the room, possibly in almost every seat.
Fortunately there a few things you can do to mitigate the issues. You can't fix the core geometric problem itself, but you can minimize the negative effect it will have on sound quality.
The simplest technique is to adjust the subwoofer-to-main crossover slopes to be as steep as possible. Most modern DSP-based loudspeaker controllers offer 6, 12, 18, 24, and sometimes 48 dB/octave crossover slopes. Some have even higher order options. Note: filter slopes are also referred to as “order.” Thus, a 6/dB per octave filter is said to be of a first order; 12dB/octave is a second order filter, 18 dB/octave is third order, and so on.
By steeply transitioning through the crossover region you'll be minimizing the bandwidth of the acoustical cancellations and additions that will occur due to the differing path lengths between the mains and the subs. In almost all normal situations, 48 dB/ octave slopes will prove to be great problem solvers.
Choosing Crossover Frequencies
Most subwoofers do not sound particularly good, nor do they perform well in respect to directional response, if they're crossed-over too high. The large cones become very narrow directionally as the frequency increases, and often sound quite harsh when trying to reproduce frequencies that are above their intended range. That's why you see 10-inch cones along with 18-inch cones in bass guitar amps. Something around 80 Hz to 100 Hz usually turns out to be about right, but it's important to listen to the sub by itself and see how much upper frequency harmonic distortion it's contributing to the overall system. That's because you won't hear much with the mains turned on, as they will mask any upper frequency distortion that the subwoofer is generating. But that doesn't mean the distortion isn't there, and shouldn't be dealt with. As a rule, the higher the crossover frequency, the more likely the sub will be adding some unpleasant coloration. Don't hesitate to crossover as low as 50 Hz if that's what it takes for your room and your music to sound their best.
Experiment with changing the crossover type, as well as the slope. Most loudspeaker processors provide a selection of crossover types such as Bessel, Linkwitz-Riley, Butterworth and sometimes more. Again, just as when setting the crossover frequency and slope rate, listen to the audible effect that the change in crossover types will produce. It's usually not subtle.
Is Symmetry the Holy Grail of Crossovers?
No. Definitely not with subwoofers. There are reasons that symmetrical crossovers, particularly of a low order, have characteristics that can apply to audiophile systems. In most cases they do not apply to professional applications.
You may well find that the best crossover solution is an asymmetrical one. The mains may perform best with a gradual LF roll-off, perhaps 6 dB/octave or 12B/octave at a corner frequency of about 140 Hz. That's just an example, but a realistic one. It will keep them from sounding too thin in areas of the room that are not well covered by the subs.
Then, you may want the subwoofers to be crossed over at 80 or 90 Hz with a 48 db/octave slope. If you have measurement equipment available to look at the combined response, you can learn a lot. That said, very low frequencies are the absolute hardest to measure and interpret accurately. With the exception of cinema and studio playback systems, subwoofer volume levels, EQ, and other settings will need to be arrived at largely by ear. This is especially true when subwoofers are used more for effect than for pure musical accuracy. Earthquake effects and other AV-style enhancements may call for making subjective decisions, rather than attempting to achieve a perfectly flat response.
Other Methods
Incremental digital delay can be used and is sometimes able to better align the subs to the mains – but not always. Delay may or may not help the addition/cancellation problems between the subs and the mains. It depends a lot on the shape of the room and how the loudspeakers are focused. Sometimes delay does little more than just change things, not necessarily improve them. That said, if audience members are seated right in front of a stack of subs on the stage, it becomes very important to delay them so that the LF isn't the first thing they hear…followed by the upper frequencies some 20 or 30 milliseconds later. That's a recipe for disaster. So even if you can't improve the summation between the subs and the mains, at least you can remove the time-smear for the people who are in close proximity to the subwoofers.
Going Minimal
Another technique is to assign as few instruments as possible to the subwoofers – only those that really need extra depth such as kick, bass, floor toms, etc. This assumes that you are using an aux send or other routing arrangement like a submaster to feed the sub bus, a very common practice. Depending on how much LF energy is available from the main loudspeakers this can be a good fix. By minimizing much of the bandwidth overlap between the subs and the mains, you can eliminate – or at least reduce – the typical room-wash that destroys intelligibility and clarity. In a highly reverberant room, the region between 50 Hz to 200 Hz is highly susceptible to producing an ugly LF wash that masks all the good things that are happening in the mid and upper frequencies. I remember a Dire Straits concert in an arena. Mark Knopfler changed guitars about 15 times or more during the two-hour set. It was impossible to hear any significant difference among the guitars due to the badly aligned and poorly utilized sound system. BTW, the lighting was terrible too. Rapidly moving lights during quiet ballads. Not everyone is cut out for this type of work.
I ‘Heart' Cardioid
Next up is the cardioid subwoofer configuration. This has been getting a lot of attention in recent times and with good reason. Various ‘dedicated' cardioid subwoofers are available in today's market and there are also many conventional subwoofers that can be configured to behave as part of a cardioid array. The basic idea is to put one (or more) subwoofers facing rearward and out-of-phase with the front-facing subwoofers, thereby cancelling some of the acoustical energy that's radiated towards the rear.
Cardioid configurations can be powerful problem solvers, especially in rooms that are acoustically difficult. By reducing the intensity of the rear energy wave from the subwoofers, energy that would otherwise bounce off the rear walls - drastically out of time with the front wave – will be minimized by virtue of the cardioid configuration.
When planning cardioid configurations, it's highly recommended to consult with the loudspeaker manufacturer. Some have put a lot of time and effort into working out the best ways to derive a cardioid pattern from their products. Tables, charts and advice are available that show how to deploy various cardioid arrangements, based on a given number of subwoofer modules and the extent of the directional control that is desired (and affordable). Deploying cardioid subs is not rocket science, but should be approached carefully. Each unit in a cardioid array must be wired with the proper polarity, and typically there is incremental digital delay needed in order to achieve the manufacturer's performance specifications.
Reaching New Lows
Personally, I love good, clean sub bass when it's not exaggerating the musical spectrum. Hearing a deep pedal tone on a pipe organ, or an accurately reproduced crescendo on a 60-inch concert bass drum, is one of the great aural experiences that our industry can provide --- one that goes well beyond that of an unamplified symphony orchestra. When subwoofers add that extra octave or two in the low frequencies, the part of the spectrum that stirs the soul, it's a pure delight!
