It seems that the laws of physics are unkind to live sound engineers, especially when it comes to the lowest frequencies. Case in point: traditional omnidirectional subwoofers. It seems that no matter where you place them, you get tons of sound leakage where you don’t want it and not enough bass where you do want it. The solution to this conundrum is to deploy a directional subwoofer array. In this article, we’ll explore the benefits of directional subwoofers and discuss how you can leverage subwoofer placement, digital processing, and purpose-built enclosures to take command of your subwoofer array and coax the absolute best low-frequency performance possible out of your PA system.
The Nature of Low-frequency Sound
Why are low frequencies such a problem to get under control? Again, it all comes down to physics. Low-frequency sound waves are gargantuan — the lower the frequency, the larger the sound wave. High-frequency sound waves, on the other hand, are smaller and therefore more manageable. A 40Hz sound wave, for example, is nearly 30 feet long. A 10kHz soundwave, by comparison, is barely longer than one inch.
The long wavelengths inherent in bass frequencies make them extremely difficult to aim, as building a loudspeaker large enough to contain them simply isn’t practical. For example, you’d need a 15-square-foot speaker enclosure — at minimum — to give you even an iota of control over the directivity of the aforementioned 40Hz sound wave. Moreover, you’d need to get much closer to the full size of the sound wave if you really wanted to gain full control over it. The smaller wavelength of the 10kHz sound wave, conversely, can be easily contained by a realistically sized loudspeaker.
The uncontainable nature of low frequencies becomes glaringly obvious when deploying a subwoofer in a live-sound scenario. Take, for example, a typical speaker configuration in which a full-range PA speaker is positioned on top of a subwoofer at the front of the stage. In this arrangement, the full-range speaker will project most of its sound forward. The subwoofer, on the other hand, operates omnidirectionally, meaning that it aims almost as much sound back toward the stage as it does toward the audience.
In addition to their out-of-control directionality, low-frequency sound waves also contain significantly more energy than their higher-frequency brethren. This is why you need more wattage to drive a large woofer than you do to drive a comparatively small tweeter. This also explains why low frequencies travel easily through solid materials.
Why Would You Want to Aim Low Frequencies?
So, why would you want to control the directivity of your PA system’s lowest frequencies? Well, there’s a whole host of reasons:
#1: Prevent Feedback from Stage Microphones
Because typical subwoofers focus as much energy backward as they do forward, they tend to bleed into — and wreak havoc on — your stage microphones, giving rise to rampant, out-of-control feedback. Live sound engineers will usually compensate for this by using highpass filters on the mic channels. That said, wouldn’t it be easier if these errant bass frequencies weren’t projecting toward the stage in the first place?
#2: Minimize Low-frequency Sound Leakage
When your subwoofer spills out bass in all directions, it inevitably produces low-frequency buildup in random areas throughout the venue. This is especially problematic when the sound waves interact with reflective surfaces and neighboring structures. Things get even weirder if you deploy more than one subwoofer, as their respective sound waves tend to interfere with each other. This means that some areas will be bass deficient as frequencies cancel one another out while other areas will have too much bass as frequencies combine. If you could aim the lower frequencies in the right direction, however, then you’d be able to achieve a more balanced and controlled result.
#3: Tighten Up the Mix
With a typical subwoofer, your audience will not only hear the low-frequency sound waves that are aimed toward them but also those that are shooting out the back of your enclosure as they bounce off the reflective surfaces behind it. This results in a muddy, indistinct sound. Aiming the low frequencies in a single direction, on the other hand, allows more of the direct sound — and less reflected sound — to be heard by the audience. This not only means that the low frequencies will be easier for the FOH engineer to mix, but it will also lead to better sound overall, allowing the audience to enjoy greater note clarity and potentially higher SPLs.
How to Make Your Subwoofers Directional
Okay, so we’ve established that the omnidirectional nature of subwoofers can be problematic. The good news is that building a directional — a.k.a. cardioid — subwoofer array is relatively easy, so much so, in fact, that live sound engineers have been creating these setups since the 1950s. They use deliberately positioned enclosures combined with an amalgamation of delay, polarity, and output-level adjustments to attenuate the sound that’s projected toward the stage.
Historically, engineers were required to calibrate these adjustments by hand and implement them by using outboard processing units. These days, however, many modern subwoofers, such as the QSC KS118, RCF SUB 8003-AS MK3, LD Systems Maui 44 G2, and Yamaha DXS15mkII, enable you to achieve cardioid operation automatically by feeding two or more enclosures the same audio signal, matching their gain settings, and leveraging built-in digital processing to calibrate their delay and polarity settings.
So, where — and how — should you position your subwoofers onstage to attain directional performance? Most typical stage layouts limit you to three possible choices:
#1: Side by Side
Placing two subwoofers side by side, one of them facing the audience and the other facing the stage, doesn’t take up a ton of stage real estate. It also allows for decent visibility of the stage from the audience’s perspective. That said, this configuration gives you very limited control over the resultant cardioid pattern.
#2: Stacked
Stacking two subwoofers on top of each other, one of them facing the audience and the other facing the stage, offers slightly more control than a side-by-side arrangement. It also clears up more floor space. Despite this, the greater height of this setup will obscure the stage from the audience’s perspective if you’re performing on a smaller stage.
#3: Back to Back
Placing two subwoofers back to back, one of them facing the audience and the other facing the stage, offers the best-sounding results with the most rear sound attenuation but at the expense of floor space. Hence, you need a large amount of stage real estate to make this configuration feasible.
It’s also possible to include additional subwoofers in your array to increase the amount of front-facing output, but doing so will net you slightly less efficient rear attenuation. For example, you can build an array of three stacked subs, two facing the audience and one facing the stage. Or, for more effective rear attenuation, you can also arrange three subs back to back, two facing the audience and one facing the stage, but at the cost of floor space.
The Simplest Solution: A Single-box Cardioid Subwoofer
An even easier way to achieve directional subwoofer operation is with a single-box cardioid subwoofer, such as the QSC KS212C. This purpose-built loudspeaker houses both front- and rear-firing drivers along with onboard DSP, providing you with effective cardioid operation from a single, easy-to-manage enclosure.
Cardioid subwoofers really offer the best of both worlds, supplying you with performance on par with a back-to-back subwoofer array — an impressive 15dB of attenuation, for example, in the case of the KS212C.
Not only does a single-box cardioid sub offer easier deployment than a multi-enclosure array, but you’ll also minimize the subwoofer’s incursion into your floor space and optimize your audience’s line of sight to the stage.
Trust Sweetwater to Help You Maximize Your PA
Tired of excessive stage noise and out-of-control feedback? Then give your Sweetwater Sales Engineer a call at (800) 222-4700. We’ll apply our expert knowledge and extensive experience to help you get the most out of your PA system.