Boss your bass

I probably get more questions about integrating subwoofers into a home-theatre system than any other. It can be confusing. It doesn't have to be.

One of the biggest causes of this confusion was intended to be its solution - that's the "subwoofer output" of the typical A/V receiver. It makes sense: you see an output labelled Subwoofer or LFE (low-frequency effects), so obviously you're supposed to connect a subwoofer to it. Well, not necessarily.

The problem is bass management or, more precisely, the lack of a consistent standard for it. Almost all powered subwoofers have their own adjustable low-pass filters, which means you can set a frequency below which they will function. Most subwoofers allow you to choose a point between 60Hz and 150Hz.

One problem is that many A/V receivers also have low-pass filters, and sometimes those filters are set higher than the ones in the subwoofers themselves. Some A/V receivers don't allow you to set an LFE filter below 100Hz.

Another problem is that some A/V receivers and processors don't output any bass signal in two-channel mode. Then your subwoofer won't work, for the simple reason that it isn't getting a signal. That's okay if you don't want to use your subwoofer when you're listening to music, but chances are you won't find that too pleasing a prospect.

What to do?

First, check the rear panel of your A/V receiver or processor. If it has an unfiltered output, use it, although you'll still need to check the manual to confirm how it handles stereo bass. If you're dealing with a higher-end product such as a separate preamp-processor, the chances are better that it will have flexible bass management options.

If you have a processor that directs low bass to the left and right speakers in two-channel mode, or if you have an unfiltered output, speaker setup is relatively logical: just connect the output to the sub. But if your processor isn't so cooperative, don't despair - there are ways to work around it.

Tricking your kit

Almost all subwoofers have two sets of inputs: a Low Output (line-level) set and a High Output set. If your receiver doesn't give you the bass-control options you need, you'll have to trick it, and the easiest way to do that is to use the speaker-output option. That means you connect the subwoofer to the front left (FL) and front right (FR) speaker outputs of your amplifier, then connect the speakers to your sub's high-level (speaker) outputs. Then you trick your processor by misinforming it when you run its speaker setup function.

Most processors have a setup program called Speaker Setup or Speaker Selection, usually accessed through the remote control. You then follow a step-by-step procedure on your video display. You're generally asked if your speakers are large or small. Which you choose tells the processor how to portion out the bass signals. If you select large, low frequencies are sent to the speakers. If you select small, the processor generally sends the speakers only the sound above a specific frequency (this can be arbitrarily quite high). Most processors also ask if you have a subwoofer - once again, sometimes it's best to fib a bit.

If you've connected your subwoofer to the FL and FR speaker outputs, you should tell your processor that the FL and FR speakers are large, and that you do not have a subwoofer. The processor will then direct all bass frequencies to those speakers. This will give you the maximum grunt, and your subwoofer will be doing its job.

If your receiver has only a filtered line-level output, however, things won't always be so clear cut.

Here's the problem: if you tell your processor that your FL and FR speakers are large, it will send all the bass in those channels to those speakers and none to the subwoofer. You don't want this - even large speakers with built-in subwoofers benefit from the help of an external sub. First, you need to determine if your sub has an unfiltered input (a few do). If it does, use it.

What you'll probably have to do, however, is use your receiver's filtered output and your subwoofer's filtered input. First, turn the sub's variable low-pass filter as high as it will go - you want to make it as transparent as possible. Next, tell your processor that your FL and FR speakers are small. Your processor will now send all of the bass signals for those speakers to your sub. You should probably also tell your processor that your centre-channel speaker is small, too; the pre-pro will then send all of the centre's bass to the sub as well.

Surround effects speakers are not typically intended to reproduce deep bass, so they tend to be small, and should be designated as small when you set up your processor.

Are we sitting comfortably?

You're probably feeling pretty comfortable by now, so naturally there's a complication: THX. If your receiver or processor is THX Certified, it is required (by the folks at THX) to use a very tightly specified type of bass management. In THX mode, your receiver or processor will employ active low-pass filtration on all of your speakers, shunting all the sound above 80Hz to the speakers and everything below 80Hz to your sub.

If your entire system is THX Certified, this can work extremely well. By the same token, THX is great if you play only movies on your system. However, if you have full-range loudspeakers up front (FL and FR or all three front channels) or you listen to a lot of music with your multichannel system, you may not wish to use THX's filtration system.

The answer's simple: you don't have to use THX processing. Simply choose your Dolby or DTS option. (THX is an extension of Dolby Digital processing that works within DD's specs; it's not a separate decoding scheme.)

Okay, you've got your processor properly set up and your subwoofer connected to it. Now where the heck are you going to put that sub?

Subwoofer placement

Everybody knows that bass is non-directional, so you can just put it anywhere, can't you? That would be nice but, in a word, no.

In the sense that deep bass radiates equally in all directions (to the sides and rear of the speaker just as freely as in the direction that the driver is facing), bass is omnidirectional. But we're not dealing with a theoretical driver in a theoretically perfect room when we attempt to place a subwoofer for optimal sound; we're confronting a real speaker in a real room. And everybody knows that reality bites.

Most subwoofers these days are fairly well engineered, it's the room that creates the problems. Well, it's really the interaction of the room with the bass tones reproduced by the subwoofer, so it's pretty near impossible to separate the two.

So what's the problem with rooms? It's those annoying parallel walls, mostly.

Four walls ...

When a sound is reflected back and forth between two parallel surfaces (say, a room's side walls), it creates room modes or eigentones, which are sound waves that interfere with one another by producing a series of peaks and valleys in the sound-pressure levels (SPLs). Confused yet?

This is an oversimplified version of complex acoustic concepts. If you want more detail than I go into here, just Google "standing waves", "room modes" or "eigentones" and knock yourself out.

Basically, the room has alternating areas of sound-pressure peaks (emphasis) and nulls (drop outs). These are called "peaks" and "nodes". It's hard to explain, but easy to hear - just put your subwoofer down at any place in your room, play a track with a consistent bass beat and walk around the room. You'll hear the bass get stronger and weaker based not on the music but on your location within the room.

Smoothing out the room's reaction to the subwoofer is the tricky part, but it's possible as long as you don't just put the subwoofer down in the first place it fits.

First, let's get rid of a few pieces of "common knowledge" that are far more common than knowledgable. We've already discarded the "put it anywhere" school of thought, but there are two more pieces of "good" advice we need to get rid of: "put it in a corner" and "park it in front of a wall".

You do get loud bass in room corners because of boundary reinforcement, but it's not a subtle effect and it doesn't deliver uniform bass boost. You get flatulent one-note bass that does nothing to enhance either music or films. Placing a subwoofer in front of a wall does the same thing as sticking it in a corner, only slightly less so.

If you don't want your subwoofer in the corner and you don't want it in front of a wall, then you should place it halfway between the corners, say, in the middle of the room, right? Unfortunately, no. The key to minimising standing waves is staying away from symmetry. (It's also important if you wish to preserve domestic tranquillity - very few significant others think highly of the "subwoofer in the middle of the room" school of decorating.)

So what do you do?

Try this!

First, try to place your subwoofer as close as possible to your main speakers. The sub's sound will integrate more closely with the speakers' sound that way. If that's a problem for any reason, consider placing the sub close to you (this trick works especially well in really big rooms and really small rooms).

Actually, if you want to draw a map of the best places in the room for your subwoofer, consider putting it in your listening position. Since bass radiation is non-directional, if you play your strong bass-beat track with the sub in your chair and then walk through the room, you can make note of the spots where it sounds particularly good. These spots are places you can position your subwoofer.

Once you have a home for your subwoofer, you're going to need to adjust it to blend with your loudspeakers. If you have an SPL meter, you can use a test disc with test tones to roughly adjust the levels and low-pass filter. If you don't have a meter, you can do a pretty good job by ear, and final adjustment is always done by ear, even if you do have an SPL meter.

What are you measuring and listening for? Even response. If you're using an SPL meter, you want all the tones to measure more or less the same (you'll never get ruler-flat response). If you're doing it by ear, you want the tones to sound the same - again roughly, since some tones will be emphasised and some subdued.

The first adjustment you need to make is to set the subwoofer's low-pass filter. You'll set this based on the type of speakers you have. If your main loudspeakers are "large", you'll only want to reproduce the lowest bass through your sub, so you should set the crossover somewhere between 60Hz and 80Hz. If your speakers are "medium" or "mid", you'll want to cross over somewhat higher: between 80Hz and 100Hz. If your speakers are designated as "small", set the crossover as high as it will go (usually between 120Hz and 150Hz).

Now, listen to your bass tones. These will usually be a series of tones going up in pitch by fixed amounts from 20Hz to 200Hz; listen to these, and, if you have a meter, measure them. Try the series again after you have raised or lowered the crossover point and continue until you are satisfied that you are getting a reasonably smooth response from the bottom to the top of the range.

You'll never get a perfectly smooth response, but you want it to be fairly uniform, especially right at the crossover point.

Paradoxically, you'll have it adjusted properly when you can't hear when the subwoofer takes over. A very good final test is to play some male speech - a radio newscast is perfect. Ideally, it will not sound too chesty.

I can hear it now

Most subwoofers allow you to adjust polarity. Get a friend to help with this one. Listen to your steady bass-beat disc again and have your friend adjust the polarity switch. It's set properly when you hear the strongest bass.

Here's the tricky part: tuning in the polarity might change things you've already adjusted, so go back and listen to the bass-tone sequence again and make sure everything is still relatively smooth. Sometimes it won't be and you'll need to go through it all again. But don't worry, you're almost there.

After you're satisfied that you've got the bass tones as smoothly integrated as you can, it's time to listen to some music. But don't just listen to bass-heavy music; listen to some vocals and solo instruments, as well. On most material, you shouldn't even be aware of anything coming from the subwoofer at all; the sound should seem to come from your main speakers.

But turn the sub off completely and you should hear the sound get smaller and less full. When you can't wait to turn the subwoofer back on, even on a solo acoustic guitar recording, you've got it set up right. Well, for music, at least ...

Some listeners may still want a tad more slam for home-theatre use. In that case, almost all subwoofers have volume controls, and canny listeners even mark two spots on the volume control: one for music, one for movies. As for myself, I just set it for optimum music performance, which is what I'm pickiest about, and leave it there.

But I'm weird that way.

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