Bass-ic Instinct: All You Need is Sub

Subwoofers are very special speakers. They beef up your soundstage, do the heavy lifting when your main speakers can’t, and provide subsonics like nothing else. Yet they’re not the easiest things to live with, as they benefit enormously from careful placement and setup.

I’m a big fan of the breed – my current (main) home theatre room has three subs on the go. There are two highly specialised Velodyne Acoustics SubContractor SC-IW in-wall models, plus a Monitor Audio Anthra W12, and I haven’t ruled out adding a fourth at some point. Why the addiction? Well, they move air in a way that’s physical and exciting. It’s one key reason why they’ve become integral to my home cinema experience. I can’t imagine listening without them.

That said, for all the visceral power of a serious subwoofer, it is actually doing its finest work when you’re barely aware that it’s there. If you think that’s a contradiction, then you’re right, which is why subs are essential, confusing, and, at times, infuriating! So, when do you need one, and where should you place it? And when is a subwoofer not really a subwoofer at all? Let’s take a deep dive into deeper bass…

What’s in a Sub?

In its simplest form, a subwoofer is a loudspeaker designed to reproduce low frequencies – i.e. bass that extends below what everyday conventional loudspeakers are capable of. Most of us would apply a subwoofer label to any boxes capable of reaching down to around 40Hz or below. This covers both dedicated enclosures and those sold along with soundbars in 2.1 (or multiple 1) packages.

Traditionalists may rightly point out that this definition is a little loose. After all, 40Hz still sits in standard woofer territory, and a true subwoofer should really flutter the envelope at 20Hz or lower. The situation is a bit woolly. I blame the TV industry, which has spent decades claiming to make televisions with ‘built-in subwoofers,’ which, in reality, struggle to produce anything below mid-bass. So I’ll use the term subwoofer in its most widely accepted, mainstream sense: a dedicated low-frequency speaker designed to extend bass beyond that of the main loudspeakers in a system.

Two Ways to Move Air

Subs fall into two camps, passive and active. The former is basically just a bass driver in a box, and so requires an external power amplifier and an external crossover to determine which frequencies to play. They’re particularly popular in custom AV and architectural installations, as amplification and electronics can be deployed in racks, leaving the sub itself to be placed in walls, unimpeded by heat or power requirements. Velodyne, Perlisten, KEF, Monitor Audio, Triad, Sonance and Bowers & Wilkins all supply such models. Passive subs also have a role to play outdoors in landscape audio, as electronics tend not to like inclement weather. Origin Acoustics, Sonance and Klipsch all have landscape models, although none have yet to make an appearance on Gardener’s World.

An active sub, by contrast, contains everything it needs in one package – the bass driver, an electronic line-level crossover, and a dedicated power amplifier. The latter can range from a few hundred watts to a kilowatt-plus. Crossover frequency, phase, and level controls are provided on the box, along with other possible refinements. Active subs are easier to use and integrate with everyday speakers. All they need is an LFE (Low Frequency Effects) input from the subwoofer output on an AV receiver, and they’re good to go. Key brands to look for include SVS, KEF, Monitor Audio, DALI, and REL Acoustics.

The Reflex

Subs can be sealed or ported – infinite baffle vs. reflex-loaded, to use the technical terms. The former type generally gives a tighter, tauter bass with less overhang or ‘chuffing’ from the bass port. The latter gives either deeper bass from a given cabinet size or better efficiency, so they’ll go louder with the same amount of power. Ported subs generally require smaller cabinets for a given output, so they’re a great way of achieving impactful bass from an affordable, accessible package. So to achieve a comparable performance from a sealed box, you’ll likely need a larger enclosure, more power and probably more money. On the plus side, sealed units tend to have tauter bass, and you can position them closer to a wall than ported types.

Star Power

The advent of home cinema changed the role of subwoofers virtually overnight. Modern film soundtracks include a dedicated LFE (Low Frequency Effects) channel – the “.1” in 5.1 or 7.1 systems, which is reserved exclusively for deep bass information. Explosions, earthquakes, jet rumbles, and T. rex footsteps are all mixed with the expectation that a subwoofer will be present. Without one, a significant part of the soundtrack’s intent is compromised. When that water ripples in the cup, foreshadowing the arrival of Jurassic Park’s star attraction, that deep bass LFE stomp instils genuine fear…

Subs also improve the overall system sound because they take the load off the main speakers, in terms of bass reproduction. They free the main amplifier and remaining speakers from having to reproduce energy-hungry low frequencies. Benefits include greater overall headroom, more dynamic range and often cleaner mid-range clarity.

Stereotypes

This is especially noticeable with high-quality 2-channel stereo systems, where it’s common to connect a subwoofer using its speaker-level (high-level) inputs, taking the signal from the same amplifier terminals that feed the main speakers. This ensures the subwoofer inherits the same tonal character and timing as the rest of the system. As many hi-fi amps don’t have a dedicated subwoofer output or bass management, speaker-level corrections are the only elegant way to add a subwoofer without replacing the amplifier. Check your sub’s manual for specific connection advice, but usually it involves doubling up the speaker cable on your amp’s binding posts.

Set-up Secrets

The most important thing you can do with any subwoofer is to set the crossover point. This allows you to dictate the frequency at which the subwoofer takes over from the main speakers. Typical options will be 80Hz, 120Hz and 180Hz. When setting up a home cinema system, this is the easy part, as 80Hz is the default. That said, much depends on the loudspeakers that the sub is working with. If you have more specialised enclosures, you might need to tweak. When installing some Monitor Audio on-wall Shadow speakers, I needed a higher crossover point as the on-walls had a limited frequency range. Sub-sat speaker systems, once very popular, usually coupled a sub to often tiny, lifestyle enclosures. Again, the crossover would be higher here.

Lower crossover points are generally preferable for larger speakers, while smaller speakers may benefit from a slightly higher subwoofer setting.

Experimentation is worthwhile. Set too high, and your sub becomes localisable as a separate source – which is not what you want. Set too low, and there’s a gap in the soundstage, leaving bass thin and underpowered. If you’re going manual, aim for a smooth transition between speakers and subwoofer. Play a CD test disc with frequency sweeps, and determine at which point your main speakers fail to convey bass. That’ll signpost your optimum crossover point.

Remember, a good subwoofer extends the soundstage, not drawing too much attention to itself – until it wants to slam you in the chest to emphasise a particular beat. A badly configured sub will announce its presence at every opportunity, adding unnatural weight to door slams or honking during dance tracks. You may also need to tweak the phase. Phase control allows you to align the timing of the subwoofer’s output with the main speakers. If your bass sounds weak or hollow at the crossover point, incorrect phase may be the culprit. Simply adjust it until the bass sounds full and coherent.

Placement

Room acoustics play a huge role in bass performance, and often the best solution is not one subwoofer but two, or in my case, three. Subwoofers excite room modes, creating standing waves that can exaggerate or cancel out certain frequencies. By exciting different room modes, dual subs will smooth bass response across the listening area. Many AVRs offer two LFE outputs for twinned subwoofers. This isn’t a stereo signal, but the same mono signal doubled. Take the hint!

Subwoofer placement can be determined by several factors, often out of your control. Smaller listening rooms may simply not have that much floor space to play with, so just try to avoid placing the thing too near a wall to avoid boundary reinforcement. If you have a certain amount of latitude, you can determine the ideal spot for your listening space by getting on your hands and knees and literally crawling around the room. Not dignified, but it does work. The trick is to haul your subwoofer to your preferred listening position, play some music (try James Blake’s Limit to Your Love, which has a 30Hz pressure wave masquerading as melody), and then crawl around possible locations as it plays to identify where the bass sounds most effective. Then swap the subwoofer into that exact spot, and reclaim your listening throne.

Subwoofer Shortlist

Q Acoustics Q SUB

The smallest Q SUB80 features an 8-inch driver, and can be accommodated in most rooms; stablemates include the correspondingly larger Q SUB100 and Q SUB120. Each model is very modestly priced and comes in a sealed cabinet designed to sit close to walls or furniture. A low-pass filter covers 40Hz to 250Hz, so it should match most loudspeaker styles. Drive units are tailored for each size, and two finishes are available: Satin Black with a black grille or Satin White with a grey grille. The Q SUB80 sports a 150W amplifier, while the Q SUB100 has 200W, and the Q SUB120 has 300W.

Elipson Premium

Comprising the SUB 10, 12 and 15, these new affordable subs from Elipson are designed with both music and movies in mind. The Premium SUB 10 combines a 250mm fibreglass cone with a 300W Class D amplifier and a front-firing bass-reflex port. The SUB 12 raises the ante with a 300mm fibreglass cone and a 500W Class D amplifier. The flagship of the range is the Premium SUB 15, intended for high-end home cinema systems and critical stereo listening. It shifts air with a large 380mm driver and 650W Class D amplifier.

Monitor Audio Vestra

You might not expect a subwoofer to be particularly stylish, but that’s what Monitor Audio has achieved with its very affordable Vestra series. Comprising three models, the Vestra W10, W12, and W15. The W10 and W12 models are sealed, while the W15 model is a ported design for an extended low-frequency performance. Sporting C-CAM (Ceramic-Coated Aluminium Magnesium) drivers, amplification ranges from 250 watts for the W10 to 500 watts for the W12 and W15.

SVS 5000 R|Evolution Series

At the higher end of the market, you’ll find the SVS 5000 R|Evolution. Ohio-based SVS has developed quite a reputation for high-performance subwoofers, and this 15-incher, with dual monoblock amplifier and 295MHz Analog Devices DSP, explains why. There are two iterations – the SB-5000 is a sealed cabinet design, while the PB-5000 is ported. Providing the power is a formidable 2,000 watts RMS, 5,000+ watts peak dual monobloc Class D amplifier. Each amplifier powers one of two voice coils. Niceties include advanced digital audio processing and DACs with a 120dB signal-to-noise ratio. They come in Black Gloss or Black Ash finishes.

DALI V-16 F

Sometimes you just have to unleash the beast that is the DALI V-16 F! This magnificent monster features a 16-inch driver, with Constant Surface Surround licensed from sister brand Purifi, and a 1,500W Class D power amplifier, able to peak at 2,500W. Connections include XLR and are hidden away within a cavity in the cabinet. This very premium-priced sub features four continuous flare reflex ports, allowing it to drop down to an astonishing claimed 18Hz. You might need some help getting it into your listening room, though, as it weighs 65kg.

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