Subwoofer Guide: Exploring Different Types of Active Subwoofers

Is your audio system lacking oomph or pizzazz? Are you trying to add extra bass to your full-range speakers with EQ, but not achieving acceptable results? Are you in awe of large-scale club systems and festival shows? If you are looking to increase the low-end of any PA or sound system, read on!

A subwoofer is a loudspeaker designed to reproduce low-pitched audio frequencies known as bass and sub-bass. These frequencies are often much lower than those that can be (optimally) generated by a woofer in a full-range speaker. Even full-range speakers that generate frequencies from 45 Hz to 20 kHz can benefit from a dedicated sub cabinet. By adding a matched subwoofer with a properly calibrated crossover to your system, your full-range speaker will run more efficiently and without additional intermodulation distortion and resonance. In fact, you may find that your full-range speaker can output more SPL before distorting, making the high-mids and highs clearer and more precise, while the low-mids and lows have a tighter bass response.

Deciding on whether you need a subwoofer or what type of subwoofer will inevitably come down to its intended purpose—how low does it need to play and how loud does it need to be at a given distance? Much of this is dependent on the source material. In general, you wouldn’t need as much low end for a rock show as you would for an EDM event. Even jazz and classical are highly dynamic and will often require a wide frequency response, but with a balanced / flat frequency response.

Be aware, there are significant tradeoffs to size, output, and extension. If you are looking for extreme output with low-frequency extension, you won’t find it in a compact box. If you need something compact, then be prepared to settle for less output and/or less bass response. Also, take into consideration the type of drivers used (12, 18, 21") and their power requirements and, remember, you can always scale up for additional output. Meaning if you add more subwoofers, you will increase the overall output and, in some cases, extend the lower-frequency response.

By doubling the power (or boxes), you will achieve an additional 3 dB of output.

Example: If Subwoofer A has a Sound Pressure Level (SPL) of 140 dB and Subwoofer B has an SPL of 146, you would need to combine four subwoofer A’s to achieve the same acoustic output as a single subwoofer B. Four smaller subwoofers may be a better bet, because you can scale them up or down depending on the size of the event. If you are aiming for a larger-scale setup, get the largest, most powerful sub you can afford.

There is a ton of different cabinet designs, from the standard and ubiquitous to the rare and exotic. Powered versions have built-in amplifiers, while passive versions require an outboard amplifier and additional processors.

In this article, we will focus on the most popular designs, such as bass reflex (a.k.a. direct radiating, ported, vented), bandpass, folded horn, and tapped horn. We’ll also be focused on powered subwoofers, which have the benefit of the manufacturer already matching an internal amplifier to the driver. Most will come with built-in DSP, which may include advanced speaker processing, EQ, crossover settings, limiting, and more.

Reflex / Direct Radiating

These enclosures have a vent or hole cut into the cabinet and a port tube affixed to the hole, to improve low-frequency output, increase efficiency, or reduce the size of an enclosure. Vented or ported cabinets use cabinet openings to transform and transmit low-frequency energy from the rear of the speaker to the listener.

The QSC KS118 is a single 18" 3600W powered subwoofer that offers a frequency response of 41 to 98 Hz (-6 dB) and a maximum SPL of 136 dB (at 1 m). The wood enclosure is coated with rugged paint, capable of withstanding the rigors of on-the-road wear and tear. With a manageable weight of 104 lb, it is relatively compact and easy to carry or lift. A set of four heavy-duty casters is included, which aids in transporting. Onboard DSP optimizes and protects system performance, as well as offers advanced capabilities such as the ability to array two units in a cardioid arrangement (more on this later). An M20 threaded pole receptacle accepts a 35mm pole to easy deployment with full-range loudspeakers.

The JBL PRX818XLFW is another compact 18" powered subwoofer with similar performance. The 1500W (peak) Class-D amplifier pushes the sub to a maximum SPL of 134 dB with a frequency response of 35 Hz to 87 Hz (±3 dB). The onboard DSP includes dynamic limiting, crossover, and component optimization. What’s more, the unit features built-in Wi-Fi, which allows you to use a remote-control app for your portable device. Control and configure your show from anywhere in the room. At 80 lb, the sub is easy to transport. The smaller PRX815XLFW provides less output (131 dB peak SPL) and frequency response (40 to 87 Hz ±3 dB) but in a much lighter (58-lb) form factor, making it ideal for small and ultra-portable setups. For similar performance, network capabilities, and a more advanced DSP, check out the JBL SRX818SP. It efficiently provides a frequency response of 35 Hz to 120 Hz (-3 dB) at 135 dB (max SPL), by way of a 1000W peak Class-D amplifier that utilizes Crown’s Drive Core Technology. With HiQnet Network Integration, the SRX818SP features automated control-interface configuration. Wired control is provided via etherCON connector, while third-party wireless router capability is included; control options include Audio Architect, a HiQnet Motion Control iOS app, and stand-alone applications for iOS and Android. If the price tag on these JBL subs is too much, check out the JBL EON618S 18" powered subwoofer with a simple DSP feature set, Bluetooth control, and lightweight (72-lb) design, it’s easy to set up and deploy.

If money and size are not a concern, you might check out these powered subwoofers from JBL, RCF, dB Technologies, D.A.S. Audio, and Turbosound.

Bandpass

A bandpass design is often used to achieve an extremely efficient sound pressure level, with a maximum bass output within a specific frequency range. It uses a compound enclosure with two chambers. The dividing wall between the chambers holds the driver; typically, only one chamber is ported. Essentially, it is a modified vented box where the rear of the driver is in a sealed box, while the radiation from the front of the cone is directed into a ported chamber. This design effectively creates a fourth-order electrical bandpass filter. A sixth-order bandpass can be achieved by adding a port to both sides of the driver, resulting in a powerful, low-distortion bass.

The QSC KS112 is a sixth-order bandpass ultracompact subwoofer well suited for smaller installations and portable entertainment applications. The single 12" woofer is powered by a 2000W (peak) Class-D amplifier to achieve a frequency response of 41 to 108 Hz (-6 dB) with a maximum SPL of 128 dB. With QSC’s Intrinsic Correction tuning and loudspeaker management, the KS112 provides smooth bass response. Its lightweight (about 62-lb) design makes for easy portability and is further enhanced by the included four heavy-duty casters. Deployment in either vertical or horizontal positions with dual M20 speaker pole fittings provides flexibility, while an optional KS-LOC security cover prevents unauthorized tampering of controls.

The TurboSound Milan M18B and M15B subwoofers also employ a sixth-order bandpass design. Both speakers utilize a 2200W Class-D amplifier to power the respective 18" or 15" drivers. The M18B can produce 40 to 100 Hz at ±3 dB with a max SPL of 134 dB, while the M15B can produce 45 to 100 Hz at ±3 dB with a max SPL of 134 dB. These Milan subwoofers feature Klark Teknik DSP and a switchable 6 dB bass boost at 40 to 90 Hz. For other notable bandpass subwoofers, check out offerings from Turbosound, Avante, and Yamaha.

Horn-Loaded Subwoofer

A horn subwoofer is designed to improve the coupling between the speaker driver and the air. It often gives the impression of amplifying the sound, but in reality, a properly designed horn has the effect of making the speaker cone transfer more of the electrical energy in the voice coil into the air, appear to have higher efficiency. Most modern horn-loaded subwoofers employ a folded design, which allows for a long horn path, while minimizing the size. Horns are highly efficient, meaning less power for more output and, when multiples are coupled, they exhibit the 3dB gain per doubling of power, but also have be run at a lower frequency with an increased directivity. The design also helps eliminate the resonance introduced by the enclosure and reduces nonlinear distortion.

The dB Technologies DVA KS20 Active Horn Loaded Subwoofer uses two Digipro G3 power amplifiers rated at 3600W to power two 18" woofers with a frequency response of 41 to 120 Hz (-10 dB) at 138 dB. A 56-bit DSP controller manages limiting and delay, while the compact housing offers a black metal grille to protect the housing. Another powered horn-loaded subwoofer to check out is the Cerwin-Vega Earthquake EL-36DP, an 18" folded horn subwoofer powered by a 2000W amplifier. With a frequency response that reaches as low as 37 Hz at an SPL of 136 dB, it’s worth checking out.

There aren’t a lot of powered horn-loaded subwoofers, but some of the passive designs can offer stunning performance. For example, the Turbosound Flashline Series TFS-900B is a large horn-loaded enclosure with two carbon-fiber 18" woofers in a large 23.6 x 49.1 x 44.9" enclosure and a weight of 338 lb. While it’s not a small box, its power and efficiency are remarkable. When powered by 6400W peak, the TFS-900B can produce a maximum SPL of 151 dB from 35 to 150 Hz (±3 dB).

Tapped Horn

A tapped horn design utilizes both sides of a high-powered, long-excursion driver in an enclosure that is ported to the horn itself, with one path length long and the other short. The two paths combine in phase at the horn’s mouth within a desired frequency range. This design is highly efficient since both sides combine to increase the output, but in a relatively small enclosure. Tom Danley of Danley Sound Labs holds the patent to the “true” tapped horn; however, there are several designs that employ a rear tap or reverse-tapped horn.

Yorkville makes several powered rear-horn loaded horns, which offer good value for performance. The Elite Series ES18P features a single 18" driver that’s powered by a 3200W peak amplifier. The frequency response is 43 to 150 Hz at 140 dB peak. With a size of 24.1 x 22.8 x 34.7" and a weight of 137 lb, the subwoofer is easily transportable. The enclosure features built-in casters in the rear and a handle. Stacking 2 x 2 on their side to couple the horn mouth creates a powerful bass wave that is sure to impress. And because they are modular, they can be used in pairs or singles for great coverage and performance for whatever-sized event you should need. These are my top picks for the best bang for the buck. 140 dB peak output rivals most double 18 reflex subwoofers, but at a fraction of the cost, size, and weight.

Cardioid Subwoofers

Most subwoofers are omnidirectional with energy being transmitted in all directions, leading to a buildup of bass on a stage, which can lead to turntable or microphone feedback, as well as poor stage monitor performance. A cardioid subwoofer is typically a configuration of two or more subwoofers, in which one subwoofer is reversed and phase-inverted to produce a cancellation at the back of the subwoofer. It works well and can be a real problem solver in live sound situations. Most professionals will deploy three stacked subwoofers, with the bottom providing the cancellation.

There are many powered subwoofers that include cardioid settings in their DSP, but nearly all are designed for use with two or more enclosures. However, QSC has come up with a nifty design that employs two 12" drivers in a sixth-order bandpass configuration that creates a cardioid pattern from a single enclosure. The QSC KS212C provides all the benefits of a cardioid subwoofer array, but without the headache of multiple enclosures and outboard processing. The single subwoofer can be deployed in a vertical or horizontal position and features M20 speaker pole fittings in either position. The rear panel offers an LCD control module with variable crossover and savable and recallable scenes, as well as Intrinsic Correction tuning and loudspeaker management.

As you can see, there are a ton of powered subwoofer options in various designs and budgets. I highly recommend demo’ing a subwoofer before buying, so if you are in the area, come on down to the SuperStore or hit us up on the Web. Do you have a favorite subwoofer design? Let’s hear about it in the Comments section, below.