guide to subwoofers
A subwoofer is a type of speaker designed specifically to produce deeper sounds, allowing them to deliver bass and other low-frequency effects (LFE) that traditional speakers can’t handle.
The addition of a subwoofer can help with the low-end in a traditional two-channel speaker setup, but as soon as you move to a multi-channel system at least one subwoofer becomes essential. A sub not only shoulders the low-frequency heavy lifting, reducing the load on the other speakers in the system, but also reproduces the dedicated LFE channel used in movie soundtracks.
Subwoofers come in all shapes and sizes, and the quality of low-frequency sound they produce is determined by a variety of factors, including driver size, enclosure design, and power handling.
Here’s our non nerdy guide to the key points you need to consider when buying a subwoofer.
You may have a pair of floorstanding speakers that are capable of producing a decent amount of bass, but unless they are insanely big they’ll struggle to generate frequencies below 60Hz. There’s a simple reason for this: driver size.
There’s a popular saying in the world of subwoofers: there’s no replacement for displacement. A speaker makes sounds by moving air – so the larger the driver, the more air it can move, and the lower the frequency it produces. Subwoofers, by their nature, require a very large driver.
The actual size can range from 8 inches to at least 18 inches, with some subs going even bigger. Since driver size dictates bass production, you want to choose the largest that’s practical, but it’s worth remembering that the bigger the driver, the heavier the enclosure, and the greater the levels of amplification power needed – all of which adds to the price.
As with any speaker, a subwoofer requires an enclosure to produce sound. There are two types: sealed and ported.
Sealed: As the name suggests, this type of subwoofer houses its driver in a sealed box. Generally, this approach results in a smaller enclosure, which makes installation easier and placement more flexible. A sealed sub also tends to be more accurate, with better control, preciser timing, greater responsiveness, and tighter bass – making them better suited to music.
The downside to a sealed enclosure is that it can’t dig as deep as a ported version, thus lacking the kind of brute force trauma required from today’s blockbuster movie soundtracks. To increase the bass extension of a sealed sub, some manufacturers add a passive radiator that responds to the movement of the driver, thus shifting more air and generating lower frequencies.
Ported: This type subwoofer has one or more ports (sometimes referred to as vents) that allow air to escape from the enclosure. This results in a bigger and louder sound because there are two different mechanisms moving the air – the port and the driver. As a result, these subs can move a lot of air and fill up a very large room with bass, making them ideal for film soundtracks.
The problem with ported enclosures is that they tend to be less precise, and often slightly sluggish when compared to a sealed alternative. They can also be harder to install due to their sheer size and weight, and tricky to position and tune correctly. But if you’re the kind of person who enjoys unsociable levels of bass, a ported subwoofer is just the infrasonic ticket.
While there are passive subwoofers that require the addition of an external amplifier, most models are the active variety that use a built-in amp to generate the necessary power. When looking at subwoofer specifications, you’ll probably be faced with two different power measurements: RMS and Peak Power. While both are similar, there are a number of important distinctions.
RMS stands for Root Mean Square, and is a measurement of how much power a subwoofer can handle on a continuous basis. Conversely, peak power does exactly what it says, representing the maximum amount of grunt a sub can manage in short bursts.
Since RMS is a measurement of the amount of power a sub can handle without damage on an ongoing basis, it’s more representative of its actual capabilities. The peak power number will be higher, but only achievable for short periods of time, whereas RMS reflects your experience for the majority of time. So always check the claimed RMS measurement in a sub’s specs, and beware of excessively high and often misleading peak power numbers.
The crossover is the frequency point at which the subwoofer takes over from the other speakers in the system. It is essential to set a crossover that ensures the subwoofer's low-frequency output doesn’t interfere with the high-frequency sound of the other speakers.
How you set the crossover is going to depend on your AV processor, amplifier or receiver, but as a general rule-of-thumb 80Hz is a good starting point. If you can set crossovers for each individual speaker, the best approach is to check the claimed frequency response of the speakers, and set the sub to take over just before they hit the limit of their bass reproduction.
A dedicated LFE or ‘.1’ channel delivers bass-only information below 120Hz, and is separate from the speaker crossovers. If you’re using a subwoofer, the LFE channel will be automatically sent to it by the AV processor, amplifier or receiver.
There are two main ways of connecting a subwoofer to your system: a low-level connection, also known an LFE, .1, or line-level connection; and a high-level connection, also know as a speaker level connection.
A low-level connection is usually identified as the LFE input on the subwoofer, and simply requires a single RCA coaxial cable to connect it to the subwoofer output on your AV processor, amplifier or receiver. With this type of connection all the low-frequency effects from a movie soundtrack are directed to the subwoofer, along with any frequencies below the chosen crossover point.
A high-level connection looks like the speaker terminals on the back of an amplifier, although some sub manufacturers may use a Neutrik Speakon connector instead. Whether using speaker cable or a dedicated Neutrik Speakon cables, you connect the sub to the amplifier's left and right channel speaker terminals. This ensures the sub receives exactly the same signal as the main speakers, allowing it to retrain the more musical characteristics of the overall bass signal.
This is the dark art of subwoofery, and the area where there’s often a great deal of confusion. The most common approach is to place your subwoofer at the front of the room, allowing it to blend better with the main left, right and centre speakers. Bass is omni-directional, and you shouldn’t be able to pin-point a sub’s location, but if you can try moving it along the front wall until it sounds like the bass is coming from all around you.
A sub can sound more or less localised in a specific position due to room modes. Every room has a standing wave pattern, and this creates modes (peaks and nulls) as the standing waves bounce off the walls like water sloshing around in a bath. When positioning a sub it’s best to avoid peaks that are the cause of localisation effects, and nulls where you won’t experience much bass at all.
One simple solution to this problem is to try placing your subwoofer in one of the corners. Corner placement excites all possible room modes, resulting in a denser standing wave pattern in the room. This reduces (but doesn’t eliminate) the potential to encounter peaks or nulls in the room.
While nulls or ‘dead spots’ should be avoided at all costs, peaks or ‘boomy spots’ can be tamed using equalisation (EQ). These days most AV processors, amplifiers and receivers include some form of room correction that can be very effective at equalising the subwoofer channel and smoothing out the bass response of your system.
Unfortunately, despite all your efforts to optimise the placement of your subwoofer, the fact is that a single unit will not have an optimal frequency response at all listening positions in the room. The use of dual subwoofers greatly increases the modal density in the room, resulting in a smoother frequency response at more listening positions, and less potential for obvious peaks or nulls. This approach also makes the use of any room correction more effective. So if you have the budget and the space, why not consider multiple subs?
Subwoofers are a great addition to any sound system, providing high-quality bass sounds. When choosing a subwoofer, it's important to consider the driver size, enclosure design, power handling, crossover, connections, and placement. By selecting the right subwoofer for your needs and following these simple guidelines, you can enjoy the best possible bass performance.