THE ESSENTIAL GUIDE TO SUBWOOFERS

What is a subwoofer?

A subwoofer is a specialized speaker designed to reproduce low-frequency audio signals, delivering the deep bass tones that standard speakers often struggle to handle. By focusing solely on these lower frequencies, subwoofers enhance the overall sound experience, adding depth and power that would otherwise be missing from the audio mix.

For any loudspeaker to play low-end frequencies in a way that goes loud and deep, it needs to fulfil certain requirements, namely:

• A cabinet large enough to house the components and provide space for the driver(s) to “breath” (read on for a note about bass horns and open-baffle designs)

• A driver large enough to move sufficient air to produce the deep, powerful low-end frequencies

• An amplifier with enough power to drive the relatively large driver – deep and plentiful bass requires lots of amplifier current.

Let’s take a closer look at each of these...

Large speaker cabinet

Bass frequencies have long wavelengths, and in a hypothetical system consisting of a completely free-standing bass woofer, the bass waves from the front and back of the diaphragm, being out of phase with each other, would partially cancel each other out. Bass horns and open baffles solve this problem to some degree but, as they’re not commonly used in modern loudspeaker design, they are outside the scope of this article.

A cabinet provides a significantly more compact and practical solution to the challenge of recreating deep bass. There are two primary cabinet designs: sealed and ported.

Sealed cabinet

As the name suggests, a sealed subwoofer is completely enclosed. As the driver moves back and forth, the pressure inside the cabinet increases and decreases, creating a damper or spring effect that results in a faster transient response and tighter bass that’s well suited to music. The pressure within the cabinet means, though, that more power is required to generate powerful lower frequencies. Sealed subwoofers should ideally be larger than ported cabinets, as they need a larger volume of air to produce the same level of sound. However, they’re generally smaller, which makes them better suited to smaller rooms where available floorspace is often limited and the level of bass produced by a larger, ported subwoofer could be overwhelming.

Ported cabinet

Also known as a bass-reflex cabinet, a ported cabinet includes one or more precisely engineered tubes, or ports. Above and below the port’s resonant frequency – the frequency at which it vibrates the air inside it the most – the port isn’t active. However, at its resonant frequency, the port takes over from the driver to produce most of the sound. Ported subwoofers require less power to drive and, pound for pound, deliver more powerful bass than a sealed design – though at the expense of transient response. More powerful bass means that ported subs are best suited to home cinema setups, though their larger size tends to make them less suited to smaller rooms.

Powerful speaker drivers

A subwoofer requires more energy (electrical current) to reproduce low frequencies at the same volume than mid and high frequencies. This also means that the speaker driver must be able to convert that current into motion rather than heat. To enable this, a woofer must have a powerful magnet motor system capable of driving the speaker cone to the long excursions required for loud sound without going into mechanical distortion.

The size and weight of the driver diaphragm also matter, as it must be large enough to move enough air to reproduce the bass and sub-bass frequencies with sufficient energy. A large diaphragm must also be relatively stiff or it will flex uncontrollably at large excursions, leading to distortion. A good bass speaker cone will be both stiff and lightweight – too heavy, and the result will be sluggish response, which itself will cause a kind of distortion.

A powerful amplifier

As we’ve already noted, moving lots of air at low frequencies requires lots of power. Exactly how much power depends on how sensitive the speaker is and how loud and deep you want to go. When it comes to subwoofers, exactly how much depends on the size and design of the model.

Here’s a quick breakdown of amplification requirements based on subwoofer design and size:

Sealed

• Small (8–10")

  • Lower efficiency

  • Requires more power to reach same volume as larger models

• Large (12–15" and above)

  • Moderate efficiency

  • Still power-hungry, but moves air more effectively

Ported

• Small

  • Moderate efficiency

  • Needs less power than sealed, but bass-reflex ports must be carefully tuned

• Large

  • High efficiency

  • Can achieve high volume levels with less power

Paradoxically, small bookshelf speakers are often less sensitive than speakers with larger enclosures, as it’s more difficult to move the air within the confines of a small bookshelf speaker enclosure.

What is a subwoofer used for?

One reason for using a subwoofer is to provide more bass than your main speakers can deliver alone. In a home cinema setup, a surround sound amplifier will often have bass-management functionality that can be set up to route the lowest frequencies to the subwoofer, relieving your front (and other) speakers of deep-bass duty. This lets even large floorstanders breathe more easily and distort less.

In surround sound mixes, such as 5.1, the subwoofer fills a dual role: first, it takes over responsibility for reproducing the deep bass frequencies that would otherwise need to be generated by the main speakers. Second, it handles the sound specifically tailored to a subwoofer via an LFE (Low-Frequency Effects) channel – these are the epic rumbles of thunder and room-shaking explosion type sounds that make movie-watching experiences even more visceral and believable.

If your surround system doesn’t include a subwoofer, your main speakers will have to handle LFE as well as the “normal” bass content. The graphic below shows the relative output as the subwoofer starts rolling off at around 50 Hz while the woofer ramps up to maintain a constant frequency response until the handover at 80 Hz. The shaded area is the frequency range carried by the LFE channel.

Most stereo amplifiers don’t provide bass-management functionality – perhaps a separate subwoofer output if you’re lucky. Still, stereo music can benefit greatly from the addition of a subwoofer for essentially the same reasons as a surround system: deeper, louder bass. This may seem like an obvious thing to do with small bookshelf speakers that may be bass-light, but you will also hear the difference with larger speakers when matched to the right subwoofer.

Let’s consider these two use cases in more detail...

Subwoofers for surround sound

Since a subwoofer is a common component in a surround sound system, virtually any surround receiver and processor are likely to feature subwoofer outputs and bass-management features. This also means you should be able to adjust certain subwoofer levels using the amplifier’s remote control, while you’re seated at the listening position.

Here, we recommend going for the best-possible low-end response and reproduction, as most movies will benefit from it. Major cinematic releases are designed to sound fantastic in movie theatres with large sound systems, taking advantage of the added realism that convincing low-frequency sound brings to some scenes. In other words, bass is important in cinema.

If you have a large listening room, choose a large subwoofer with a powerful amplifier or, if it looks or sounds better, two medium-sized subs with powerful amplifiers. Most of the more fully featured surround receivers will allow you the freedom to connect multiple subwoofers, and it’s certainly worth considering if you have the space.

Subwoofers for stereo music

For many stereo enthusiasts, no speaker is too big to benefit from a little help from a subwoofer. It’s just a great way to get deeper and more powerful bass. Before you go shopping for a subwoofer, though, bear in mind that there is such a thing as too much bass. Perhaps your room is relatively small, in which case a lot of deep bass can sound overwhelming. Or perhaps you’ll find that you don’t really like extra bass or that your favourite music genre doesn’t really need it.

As a rule of thumb, choose a subwoofer with a lower frequency response than your main speakers. If you own a pair of bookshelf speakers with a lower limit of 50 Hz, aim for a subwoofer that will reach down to 30-35 Hz. On the other hand, if you own a big pair of floorstanders that can reach down to 35 Hz, a sub that goes below 30 Hz will likely be a better fit. The key is to add something you don’t already have – to extend the bass response of your setup.

If your stereo amplifier is fitted with a subwoofer or preamp output, that’s where you connect an active subwoofer (keep reading for a comparison of active and passive subs below). A good active subwoofer will also let you adjust gain, phase and crossover frequency to help you tailor the sound to your room and system. Here’s a definition of these three terms:

• Gain controls the volume of the subwoofer in relation to the main speakers.

• Phase can be thought of as a very short delay in the signal, which is useful for getting the timing of sounds just right – again in relation to the main speakers.

• Crossover frequency is the frequency at which the subwoofer starts to fade in and the main speakers start to fade out, as the chart from earlier shows.

Active vs. passive subwoofers

Here is a summary of the differences between an active subwoofer and a passive subwoofer. In general, there are far more active subs available on the market, but there’s no denying that there are instances when a passive subwoofer is the best option.

Active subwoofers

• Onboard amplification – Having the amplification integrated into the cabinet means the subwoofer doesn’t need to draw power from your existing amplifier. As mentioned earlier in the article, relatively large subwoofer driver(s) take a lot of power to drive effectively, so onboard amplification means that you don’t need to think about whether your existing amp is up to the task.

• Optimal pairing – Subwoofer manufacturers can tailor the amplifier to the subwoofer driver(s) to deliver the best-possible sound reproduction.

• Configurable controls – With integrated gain, phase and crossover controls, you can fine-tune the subwoofer to your setup.

Passive subwoofers

• Suited to custom installations – As a passive subwoofer doesn’t include onboard amplification, the driver(s) can be packaged into a much lower profile cabinet and mounted readily into the cavity of a wall. In this custom install setup, you still of course need to provide power to the sub using external amplification and control it externally. If you want to learn more about custom installations, check out this guide to custom speaker installations.

• Pairing required – Not having onboard amplification means that your existing amplifier may not be the best match for your new passive sub. Your only option may be to change out your amplifier, which adds to your investment.

• External integration – Not having integrated controls like gain, phase and crossover means you need to provide them via an external device, such as an audio/video receiver or DSP.

What’s the best subwoofer for me?

Ok, so you need a subwoofer, but which one? The answer is, of course, it depends. To help you decide, let’s consider three scenarios:

“I listen to music and want better bass”

A sealed subwoofer, with its tighter, more controlled bass, makes a great addition to your existing stereo speakers. Sealed subs are generally more compact than ported designs, which also makes them well-suited to smaller rooms. However, remember that they’re also harder to drive so, if you have a larger room, a ported subwoofer may be a better choice. And unless you’re a seasoned audiophile with a finely tuned ear, a ported sub is still likely to give you the improvement you’re looking for.

“I want epic movie-night experiences!”

If you’re looking to shake the room with bangs, booms and blasts, a powerful ported sub is generally the way to go, at least in large rooms. Here’s why: In movie sound, there’s a ‘reference level’, which is understood as the loudest intended playback level. This is a calibration level at 75 dB at the listening position, and the goal is to be able to hit maximum peak levels of 105 dB for each speaker channel without distortion. But remember the LFE channel we mentioned earlier? The reference level for LFE is 10 dB higher – a whopping 115 dB! So, if you’re serious about home cinema, this is what your subwoofer should ideally be capable of. Now imagine trying to coax a smaller subwoofer to reproduce the rumble of an earthquake at 115 dB in a larger room: it’s bound to distort and sound awful.

It’s no wonder, then, that home cinema enthusiasts like powerful ported subwoofers, and often use more than one in their setups. More speakers and amplifiers equal not just loud, but also clean sound.

Granted, not everyone wants to play surround sound at reference levels in their home. But, if your goal is a more immersive movie experience, you should think twice before investing in an inadequately specced subwoofer that you’ll likely end up regretting.

“I listen to music and watch movies”

In this scenario, you should prioritise your movie-watching experience. There’s no point having a great music listening experience just to be disappointed when you fire up your home cinema to rewatch that epic, explosion-filled climax to your favourite movie.

Budget and room size

Let’s talk budget. There are some very expensive subwoofers on the market, so while you should invest in the right subwoofer for your needs, we certainly don’t recommend blowing your life savings on the ultimate subwoofer if a more affordable alternative will give you the performance you’re looking for. So don’t be tempted by huge cabinets with multiple oversized drivers, if all you need is a compact sealed subwoofer.

When it comes to room size, it’s one of the more important factors to take into account. Below, you’ll find a rough guide to help you identify which type of sub (sealed or ported) is best for your room. Note that the dimensions are given in cubic feet and metres, as you need to consider how much air the subwoofer will need to move, not just the two-dimensional floorspace:

Small room (up to ~1,500 ft³ / ~42 m³) – A good-quality sealed sub can usually deliver the depth and impact you need for music and home cinema experiences, especially in smaller rooms where the limited dimensions give low-end frequencies a natural boost.

Medium room (~1,500-3,000 ft³ / ~42-85 m³) – Either a sealed or ported design can work — sealed for tighter bass, ported for more effortless LFE impact. Your listening habits (music vs. movies) should influence your choice.

Large room (above ~3,000 ft³ / ~85 m³) – A ported design makes more sense for LFE‑heavy home cinema experiences, as the more efficient design can produce higher volumes in the deep bass without needing vast amounts of amplifier power.

To learn more about setting up a home cinema, check out this guide to setting up a surround system.

Where to position a subwoofer?

The subwoofer placement needs to be right for your listening room. In general, placing any speaker near room surfaces will enhance the bass response (most subwoofers are already designed with their drivers close to or even firing directly at the floor). Placing it close to one wall adds the bass-enhancing effect of a second room surface, while putting the sub in a corner adds a third. But beware: the result can be a welcome bass boost or a big boomy mess.

Standing waves

A position in which your main speakers sound good will not necessarily work well for a subwoofer. This has to do with an acoustic phenomenon known as standing waves, which is most pronounced in the lower frequencies, where subwoofers operate. Standing waves are resonances that occur at specific frequencies – depending on the size and dimensions of the room – that result in some bass frequencies sounding louder than others. To find out more, check out this guide to improving acoustics in your room.

Monaural sound

Your main speakers, whether in a stereo or surround setup, must be placed for the best-possible imaging. High frequencies, in particular, contain many spatial clues to sound imaging, which is why you can’t place stereo and surround speakers just anywhere. However, since frequencies become increasingly monaural (harder to pinpoint where they’re coming from) below 80 Hz, you can focus on positioning the subwoofer to mitigate standing waves, rather than sound imaging.

In reality, the layout and furnishing of your room may not let you place a subwoofer exactly where you want it. This is why being able to adjust properties like gain, phase and crossover is important, as it lets you put the subwoofer where you have room for it and then fine-tune the sound.

In some cases, it makes sense to have more than one subwoofer in a setup. Perhaps you want the most powerful bass you can squeeze into your room, or you want the power of a very large sub but prefer to achieve it using two or more smaller subs. Surround systems in large rooms can also benefit from multiple subs spaced apart to distribute bass evenly around the room.