October 30, 2013

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Subwoofer positioning & adjustment

To help make this article as helpful as possible, we called on two leading audio experts for help: Richard Hardesty and Robert Harley. We'll be quoting from Harley's book and Hardesty's "Audio Perfectionist" Journal throughout this article and otherwise shamelessly stealing their suggestions. Their contributions to making this a better article are simply enormous. (Thanks guys!)

Before we begin, let us give you a few warnings.

  1. The methods here will help get you the most accurate bass. If your goal is strictly maximum "slamitude," just stick your sub in the corner, turn up all the knobs and have fun.
  2. Finding the perfect settings and location takes time and effort. If you have low tolerance for moving speakers and twiddling with your audio gear, quit reading now. But realize that you're missing a few simple techniques by which you may be able to greatly improve the sound of your system.
  3. Let your ear be the final judge. We'll give you some techniques and advice, but the science of room acoustics is so complex it defies easy answers.

The Goal

"It’s relatively easy to put a subwoofer into your system and hear more bass. What's difficult is making the subwoofer's bass integrate with the sound of your main speakers. A well-integrated subwoofer produces a seamless sound, no boomy thump, and natural reproduction of music. A poorly integrated subwoofer will sound thick, heavy boomy, and unnatural, calling attention to the fact that you have smaller speakers reproducing the frequency spectrum from the midrange up, and the big subwoofer putting out low bass." 


Room acoustics have an enormous impact on the sound of speakers, including subwoofers. A phenomenon called "standing waves" makes bass response uneven from place to place in your room. To experience this for yourself, put on a CD with a strong, consistent bass line. Notice how the volume of the bass goes up and down as you move around the room. Stand in one place and then squat down - you will probably notice that the bass changes in the vertical plane as well!

Because the subwoofer’s location affects how standing waves are created, the first step to getting accurate bass response is finding the right spots for your subwoofer and your listening position. We'll share a few guidelines that may be helpful, but in the long run nothing beats trial, error and your own two ears.

Stick It In The Corner

This is the advice most often given and it certainly will yield loud bass. But corner placement may make the woofer sound as if it were only producing one musical note, giving a boomy quality to music. If lack of bass volume is your biggest subwoofer problem, this may be the answer for you.

Avoid Sitting Up Against the Wall

Bass waves build up and "hang out" at room boundaries (walls). Your system will sound thick and heavy when your listening chair is up against a wall. If you must sit against the wall because of furniture layout, turn down the volume of the subwoofer a bit to compensate.

Avoid Symmetrical Placement

"Avoid putting the subwoofer the same distance from two walls. For example, if you have a 20' [51 cm] wide room, don't put the subwoofer 10' [25 cm] from each wall. Similarly, don't put the subwoofer near a corner and equidistant from the side and rear walls. Instead, stagger the distances to each wall."

Put The Subwoofer As Close To The Main Speakers As Possible

Even though bass sounds are non-localisable, cabinet resonance and other factors conspire to make this less true in practice than in theory. It'll be much easier to get seamless blending between sub and main speakers if they are on the same side of the room. If possible, put the subwoofer behind the plane of the main speakers. At very least, keep the subwoofer in the front half of the room.

Here's An Old Trick

Put your woofer in the same spot as your listening position. It's best to raise the subwoofer off the ground to seated ear height (use a sturdy, non-resonant platform). "Play a piece of music with an ascending and descending bass line such as a 'walking' bass in straight-ahead jazz. Crawl around the floor on your hands and knees until you find the spot where the bass sounds smoothest, and where each bass note has about the same volume and clarity. Avoid positions where some notes 'hang' longer and/or sound slower or thicker than others. When you've determined where the bass sounds best, put the subwoofer there."

Use Two Subwoofers

Using two asymmetrically placed subs will minimise the effects of standing waves in your room, yielding smoother bass response as well as better dynamic range. If using two subwoofers, you must use the identical model of subwoofer. If two different models are used, even from the same manufacturer, at some points they will help each other and at others they will fight each other causing an uneven response.

One Note Of Caution

Most subwoofers are magnetically shielded but not as "tightly" shielded as, say, a centre channel speaker because of their more powerful magnets. This may damage your TV if placed too close to the set (though plasma, DLP, LCD and front projectors are not affected). Select an unused video input on your TV to bring up a single colour screen. If you see any colour distortion anywhere on the screen, an unshielded speaker is too close to the set and should be moved away from the TV until the colour distortion disappears.

Subwoofer Adjustments

Position and adjustments are interactive so every time you move your subwoofer, you need to readjust the sub volume, low pass filter and phase (polarity). When properly adjusted, you won't "hear" the subwoofer at all. It will sound like your main speakers are making all the sound, except with a whole lot more bass than they could muster all on their own.

Here's how to get it right:

Setting Level

"Get a suitable test CD with sine wave signals or warble tones covering the range of 20Hz to 200Hz or so. Using a sound level meter, match the output at the listening position at 50Hz and 150Hz by adjusting the volume control of the subwoofer. Make sure the volume control on your preamp remains at the exact same volume."

If you don't have a metre or test discs, use a CD with vocals and a consistent acoustic bass line and set level to the point where the upper and lower ranges of the bass sound well balanced. The bass should not be so loud compared to the rest of the system that it sounds boomy or muddy. On the other hand, the subwoofer should not be so low that the system sounds thin.

Setting Low Pass Filter (Crossover)

If you are using the LFE input on your sub, the crossover is controlled by the receiver and not the subwoofer. The LFE input is bypassed on the sub but now you will set the "crossover" adjustment via the speaker setup menu on the receiver. If your main speakers are full size with good bass response, set the low pass filter to 60Hz - 80Hz to start. If your main speakers are bookshelf designs or in-wall, set the low pass filter in the 80Hz range. If you have compact satellites, set the low pass filter to 120Hz to start. Put on a test CD or DVD with test tones. "With a fixed input level, carefully measure the output level at the listening position for each interval between 30 and 200Hz and write it down or make a line plot on a sheet of graph paper. Listen while you measure. You hear differently than the sound level meter does. Trust your ears, not the meter." Raise or lower the low-pass filter setting on the subwoofer to achieve the smoothest response. Turn the low pass filter down if there's an excess of output around the crossover point, turn it up if there's a response dip.

Do not be alarmed if you have response variations of several dB from test tone to test tone. You are seeing normal variations in response caused by the speaker's interaction with the room. Absolute flat response is a worthy but somewhat unrealistic expectation in most systems and rooms. Further, be sure the test tone you are using is either a warble, or filtered pink noise. Straight test tones are not suitable for acoustic response measurements but are useful in detecting rattles and buzzes.

If you don't have a meter and test disc, use your ear to make this setting. Put on a CD with male voice and a consistent bass line. Adjust the low-pass filter until the male voice sounds "full" and natural but not thick and heavy.

Setting Phase (Polarity)

"Using a [test] signal at the nominal crossover frequency [you set in the step above], set the phase of the subwoofer(s) to deliver the highest output at the listening position." It helps to have a friend on hand to change the polarity setting on the sub while you measure and listen. When setting the phase setting by ear, play some music (not a movie) that has a walking but repetitive bass line. Country music, Latin and certain dance tracks work well for this. Focus closely on the region of sound below the male voice. This is where the subwoofer transitions to the main speakers. Whichever setting sounds "faster" or "fuller" in that region of sound is the correct setting. In some situations, you may not hear any difference at all, particularly with compact satellite speakers. In this case, simply leave the phase switch to "0" or "normal".

Do It All Again

All these adjustments are interactive. Once you set polarity, go back and re-adjust level and low-pass filter settings to get the smoothest response.

The final proof of optimal performance is in the listening. Put on a CD with an acoustic bass. The bass instruments should appear to be coming from the main speakers, not the sub. The sub should be audibly "invisible."

What About Movies?

If you listen to movies with your new set-up, you may find that the bass is a little "light" and doesn't deliver the impact you expect. No problem - just turn up the subwoofer volume until you are happy. Mark the volume settings that are best for music and movies and readjust as you switch sources. If that's too inconvenient, choose a mid-point level that gives you the best balance of music and movie performance. But keep in mind that filmmakers use bass effects as "punctuation." Bass shouldn't be a continuous drone.