How to choose the perfect subwoofer
Most people equate subwoofers with the thump-thump they hear from other cars at stoplights. Volume at the expense of quality. There is no sugarcoating the message: it’s called bad bass. In the world of the dedicated audiophile, this is what gave the subwoofer (and with it bass) a bad rep. But the idea with any speaker system is to accurately recreate all frequencies in the audio spectrum equally, including bass. What you want from a quality subwoofer, placed correctly in a room, is a deep realistic sound, not the thump-thump. So let’s be clear that a good subwoofer doesn’t make a boom sound, but just stronger (and lower) bass than is possible with normal speakers.
The fans of thunder and rumble who want to add more bass to their experience can certainly turn it up, but the key is that you don’t have to. So let’s have a chat about what makes a good subwoofer.
A little word on subwoofers and what you need to know
Every discussion about subwoofers is going to revolve around two specs: driver size and power.
We’ll get to power in a moment. First, let’s talk drivers.
Generally speaking, subwoofers are boxes that can pose as a side table as far as size goes. Inside is a fairly large driver (the part that makes the noise) and usually the amp that powers it.
Unlike most speakers, subwoofers have built-in amps that are designed to work with the driver of this specific subwoofer, making them a team designed in heaven. Also, the built-in amplifier frees up the massive amount of power sucked up by bass from your AV receiver and so you have a strong case for the addition of a subwoofer to your AV system.
Back to subwoofer drivers. In order to perform its best, a driver should have some space in the cabinet behind it. So unavoidably, in a good subwoofer a 12-inch driver will have a decent size box, while an 8-inch driver could have a smaller box.
The problem is that lower bass waves are really long, so the driver has to work hard to produce those waves in volumes you can hear. One way to do this is with a bigger driver – 12-13 inch drivers like the SVS PB-13 is not an uncommon size — but you can find even bigger ones like the Klipsch Reference R-115SW or the Velodyne EQ-Max 15 and DD15. Velodyne even has a 18 inch subwoofer in the DD18.
The other way to generate the volume you need from a subwoofer is with lots and lots of power. As the driver and cabinet size goes down, the power needs to go way up to compensate. So, in theory, a 8-inch sub in a small box could sound similar to a 12-inch sub in a big box if it had significantly more power. Power in a subwoofer can range from 100 Watt to a whopping few 1000 Watt.
A third way to increase volume in a subwoofer is the use of multiple drivers: Some subwoofers feature passive radiators. These are additional unpowered woofers within the enclosure that move in tandem with the powered driver. (The Definitive Technology website has a little educational video that explains the principle with a spring. You can find it here toward the bottom of the page)
Several manufacturers use passive radiators to provide deep, powerful bass from a surprisingly compact enclosure like for instance the Definitive Technology SuperCube Series.
And then there is the topic of sealed vs. ported subwoofers. We would like to refer you to one of our earlier blogs on the topic that you can find here.
But with all the info on watt ratings, driver sizes and smart technologies, these figures in themselves are not a complete picture of a subwoofer’s performance. They are just some easily findable spec. The problem is, specs alone can’t distinguish a quality subwoofer over a bad one. They may lead you in the right direction, but a well-designed 10-inch, 100-watt sub might sound great, and a poorly designed 12-inch, 500-watt sub might sound terrible. This is where reviews and demo’s come into play, and are worth seeking out.
My mum used to say you should buy shoes from a shoe maker, not a dress maker. She had a point. If you’re looking for the best bass for money, consider a premium subwoofer from a specialty manufacturer like SVS or Velodyne. It needs to be said: In a home theatre room, with a giant TV or projector and a stout set of speakers, an excellent subwoofer will blow your mind.
Dual subwoofer magic
We are always preaching that two subwoofers (some say even four, but we won’t push it that far!) in a home theater system is better for a variety of reasons.
Let’s just be clear about this one first: We don’t necessarily want to add a second subwoofer for more bass. It can be a reason, but most of the time it is not. The main reason is better bass.
Adding a second subwoofer automatically reduces the first sub’s workload, which effectively lowers the distortion for a given playback volume of bass.
Because low-frequency bass waves are so long, they interact with the room itself quite significantly. This really is room acoustics at its worst (or best?).
In the past few weeks we have touched on the issue of standing waves and nulls on a few occasions.
Basically, a bass wave reflect against the walls in your room, they return toward the subwoofer and can overlap with the original sound wave. In some spots, the waves will cancel each other out, creating a ‘null’.
Even with careful placement, a single subwoofer will not have an optimal frequency response at all listening positions in the room. Just moving even a few feet away from the ‘sweet spot’ will often result in notably worse bass sound quality.
Adding a second subwoofer fills in those gaps where bass response is weak. This helps make every seat in your home theater a good one, with plenty of tight, deep bass.
We hear your sigh and we understand that, for domestic reasons, the multiple subwoofer route is frequently less than ideal, but we really want to tell you it sounds spectacular. If you can swing it, you should.
This is probably a good time to point out that when choosing dual subwoofers for your system, it’s a good idea to choose identical subs for optimal cancellation of room resonances.
Subwoofer placement
It will come as no surprise that subwoofer placement is crucial, and a little time spent finding the right position can reap huge benefits in bass sound quality.
One of the benefits of low-frequency sounds is that they are less directional than higher frequencies (they are omni-directional). That means you don’t have to put it near your front speakers. But if that’s where you want it, then we recommend choosing a front-firing subwoofer to push the bass directly towards you. Corner or side-wall placement? Consider a down-firing subwoofer like the SVS PC2000 or the more basic Audioengine S8 Powered Subwoofer. Both have the driver on the bottom, which delivers bass more evenly throughout the room.
As far as subwoofer placement is concerned, we could spend an entire blog (or two) on this topic alone. It is definitely something you want to pay attention to, but it is just too much to fit it all in one blog.
The annoying bit about subwoofers placement: unless you’ve got a firm handle on the science of acoustics and a good understanding of how your AV equipment works, placing and setting up a subwoofer for optimal performance can be a sweaty case of trial and error.
Some subwoofers come with built-in EQ; We’ve mentioned the MiniDSP Umik-1 that teams up extremely well with the Room EQ wizard a few times in the past and also Anthem’s ARC room correction system can all be helpful in figuring out the best subwoofer placement for your specific situation.
We like the SVS Merlin tool that helps you select the subwoofer that matches all brands of speakers (not just SVS speakers!) and gives guidelines on optimal crossover. None of it is foolproof or perfect, but it all helps.
To finish it off, we also would like to refer you to the following two Audioholics blogs on subwoofer placement basics and dual subwoofer placement and set-up here and here.
The other thing to check out is the ‘subwoofer crawl’? Look at this helpful Audioholics YouTube video here.
It may take a few passes to get things just right. You might even upset a few neighbors or roommates along the way (although we recommend that you don’t:-)). But that glorious moment when your subwoofer is delivering the kind of bass that makes your spine tingle is worth any associated work and grief.
And if all else fails, just give us a ring and we’ll come to the rescue.
Have fun!
Sources: Cnet, Crutchfield, Audioholics, SVS, Definitive Technology, Velodyne, AVSforum