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SUBWOOFER TUNING AND ROOM ACOUSTICS

George McKechnie

Surround sound technology has made the subwoofer an integral part of the home entertainment experience, for both music and home theater.  And although setting up the front, surround, and even rear speakers has become relatively straightforward (see Understanding Loudspeaker Differences; Matching Speakers to Your Room), selecting a subwoofer, positioning it, and tuning it to your room (so that it adds realism to important action scenes without overwhelming beautiful musical passages) can get complicated.  But once you understand the basic concepts, and follow the instructions below, you’ll be able to fine-tune your subwoofer to your room, so you can get the best sound possible from your equipment—short of calling in an acoustical expert.

ACOUSTICAL FACTS OF LIFE
Let’s start with some basic acoustical principles.  Five factors influence the performance of a subwoofer in a given room:

Room size.  Room size really does matter—but only up to a point.  If the largest dimension of a room (typically a diagonal line drawn through the middle of the room from a low corner to the opposite high corner) is smaller than 35-40 feet or so, the lowest frequencies of the audio spectrum won’t fit in the space as complete waveforms, because they are longer than the largest dimension in that room.  So the room folds them in half (or “doubles” them).

A 30 Hz note, for example, would get folded into a pair of 60 Hz waveforms, which together sound 3 db louder than the 30 Hz note the system was trying to reproduce.  The sonic result is booming in the mid-bass frequencies.  This effect is so common in everyday listening that some people accept it as normal, and don’t realize that it a distortion added by the room.

Room ratios.  A room’s dimensions govern how much doubling can occur; the ratio of room dimensions determines how close together these doubled notes will fall in the musical spectrum—that is, how much they will bunch up within a narrow frequency band to thicken the mid-base sound and catch your ear.   When rooms are built with parallel walls (as they usually are), each wall pair can create doubling, which will occur at frequencies associated with the distances between the walls (unless they are 40 feet apart).

The golden ratio for room dimensions is 1:1.6:2.5 (discovered by the ancient Greek entrepreneur Aristotle).  In other words, the ideal listening room would have a width that is 1.6 times the height, and a length that is 2.5 times the height.  This set of relationships minimizes the musical impact of room doubling by setting the room dimensions so that they are not close to being identical to or exact multiples of each other.

In practice, ideal room proportions can be obtained by multiplying each golden ratio component by a fixed length—say 15 feet.  In this example, the resulting room, measuring 15 x 24 x 37.5 feet, will minimize doubling at all frequencies.  It also provides a diagonal distance (about 47 feet) which is long enough to accommodate the lowest frequency in the audio spectrum without doubling.   At the other extreme, a room that is a perfect sphere—with all dimensions equal—will “ring” at a single frequency like a bell, sound nasty, and cause major headaches!

Unfortunately, few people have the luxury of specifying or adjusting room dimensions and ratios—especially after the house is built.  And the more that the height x width x length proportions deviate from the “golden ratio,” the more the doublings will accumulate at narrow mid-bass frequency bands, and the more the room itself will cause the sound pressure level (loudness) of the audio track to be uneven when played back.  That’s the theory.  In practice, the “random” placement of room contents—furniture, people, partial walls, etc—tend to reduce the impact of doubling of low frequencies by breaking them up.  For more on using furnishings to tune room acoustics, see Matching Speakers to Your Room.

Truth be told, very few rooms have dimensions that match the golden ratio, or even come close.  This is where acoustical calibration can be useful—especially at the mid and high frequencies.  See An Insider’s Guide to Home Theater.  Recently-developed systems (like Audyssey) that provide automated digital system tuning can be used in conjunction with a judicious balancing of the four factors below, to achieve satisfying frequency balance from all but the most difficult rooms.  But doing so does take effort and careful attention to detail.

Subwoofer location.  How the subwoofer couples to the room acoustically strongly influences how it sounds, and this depends in large part on the subwoofer location.  Placed on the floor in a corner, the net result may be booming low notes that seem much louder (and thicker in texture) than the music or voice dialog wafting above it.  But suspended from the ceiling like a chandelier, that same subwoofer might sound anemic, lacking enough impact to balance the total sound picture.

Seldom will a single subwoofer position deliver perfect sound to all the seating positions in a room.  That’s why better results can usually be achieved by using two smaller subwoofers (rather than a single large one), as long as they are placed asymmetrically in the room—one in a corner and one along a wall, for example.  With two subwoofers, it is possible to significantly reduce the variation in sound pressure level from position to position.  And with each subwoofer doing half the work, the distortion produced will be reduced by at least half, yielding a more detailed sound.  Placed symmetrically, they will simply double the problem.

Subwoofer size.  Not all subwoofers are created equal.  A 15 inch subwoofer which specs out flat to below 20 Hz can sound great in a very large space.  But it will most likely be oppressive in a small room.  On the other hand, a small 8 inch subwoofer (rated flat only to 40 Hz), can sound very good in a small room), but wimpy in a large room.  Why?  The large unit will create too much mid-bass acoustical doubling in a small room, whereas the small one can take advantage of the doubling to fill out the sound.  In a large room, the 8 inch unit will have to work too hard to put out the needed sound pressure level, and produce too much distortion in the process—while the large subwoofer will create satisfying bass support, without concern about doubling effects.

Listening position.  Even if your room is well designed, with an appropriately-sized subwoofer, variations in performance will occur as a function of where you sit.  Low frequencies may sound too loud in some seating positions and not loud enough in others. Careful subwoofer placement and tuning can optimize the bass response for a favored seating position, but create unsatisfying sound in other positions.  Solution: Add a second sub in a location that is placed asymmetrically in the room relative to the first sub.  Then optimize the second sub to the second listening position.

Until relatively recently, acoustics has been a somewhat inexact science.  It took the acoustical experts several decades to get the sound right at Lincoln Center in NYC.  Today, however, powerful computer models are available that can predict the acoustical response to a room.  But the process is complex, and involves considerable time and expense.  So it’s beyond the reach of the average homeowner.

Recently-developed consumer-grade calibration technology—which is now built into some premium-grade surround processors and receivers—can make adjusting a surround sound system to your room’s acoustics relatively easy.  But it cannot in itself adjust away all the subwoofer problems.  In fact, that may be why you’re reading this Insight report.

4_Acoustics_and_subwoofer_tuning-02A PRACTICAL APPROACH   An alternative and perhaps more satisfying way to tune your woofer is by using your ears and your mind; in the process, you’ll become a more skilled listener.  Tuning by ear is done by following an iterative process: listening and evaluating, then adjusting the woofer controls or position in the room, then listening and evaluating again, and so on until you are satisfied with the results.  The goal is to balance the lowest frequencies coming from the subwoofer with the upper bass frequencies produced by the main speakers, in order to minimize (or eliminate) differences in perceived loudness and textures—across the specific listening positions that matter in your room.

The least favorable subwoofer placement can produce areas of bass emphasis of up to +8db, and bass dropout of as much as –8db throughout the room.  Please understand that we’re talking here about a room phenomenon, not a subwoofer defect.  (Many subwoofers measure + or – 1 to 2 db in an anechoic test chamber).  Careful subwoofer adjustment and placement in your room can significantly reduce the amplitude of the peaks and troughs created by the room dimensions.

For more precise bass response, consider adding a second subwoofer.  This can significantly improve the situation.  Just remember to place the second unit in a position that is asymmetrical to the first one.  Implemented carefully, the two-sub approach can further even out the bass response, allowing you to achieve perhaps as little variation as plus or minus 2 db—a  very good result, as the ear isn’t as sensitive to amplitude differences at the low frequencies (where subwoofers operate) as it is at, say, Middle C (256 Hz).

Tuning Procedure

1 – Place the subwoofer in the general location where you think you want it to live.  A corner will give more emphasis to the upper mid bass (40-80 hertz), so start with this placement if your primary speakers are small bookshelf models with limited output below 80 Hertz.  If you are using large floor-standing speakers for left and right sound, start with the sub along a side wall for less mid-bass emphasis.  In either case, start with the sub about ½ to 1 inch out from the wall.

2 – Select a musical passage that contains at least one minute of uniform, detailed, mid and low bass material.  For example, there’s a section in the third movement of Beethoven’s Fifth Symphony where the string bass section saws away consistently for quite a while.  Or perhaps a selection from Dark Side of the Moon is more to your taste.  In any case, you’ll want a section of solid and sustained low-frequency music that travels up and down through the subwoofer crossover point (of say, 80 Hz), at more or less equal volume, with lots of detail—preferably from acoustic (not amplified) instruments.

3 – If the woofer has a phase control, set it to zero degrees for now.

4 – If the main speakers have decent bass (down to 40 Hz or so), set the roll-off frequency on the sub to 40-50 Hz.  If they’re bookshelves, set the crossover to 60-80 Hz. With very good floor-standing speakers, you may want the sub to cover only the bottom octave of sound (20-40 Hz), so start with the sub crossover at 40 Hz.

5 – Play your test material at a comfortable listening volume, then adjust the “level” (or “volume”) control on the subwoofer (not the surround processor or receiver) up or down to a point where you just can’t identify the location of the music coming out of the woofer when you are sitting where you like to listen to music or watch a movie.  Please note that the sound pressure level for low frequencies will vary throughout the room, so you’ll want a friend to adjust the level control at the woofer while you listen from your favorite position.  It’s two-person job.

6 – Once the level control is adjusted to the point where you can’t quite hear where the low notes are coming from, have your friend gradually adjust the crossover frequency control on the woofer up and down, as you continue to listen and evaluate, and repeat the frequency adjustment—a little at a time, then listen, then adjust again, then listen)  What you’re listening for now is smoothness and detail in the low frequencies; thus the need for a one minute or so of detailed bass frequency music, so there’s time for you to really listen into the texture of the music.

If the frequency control needs to be adjusted up, your assistant may also have to re-adjust the level control—because you may again be able to hear mid-bass music coming out of the sub, and need to turn the level down to the point where you once again can’t hear it.  This is the tricky part.  Also, if the woofer is positioned far away from the main speakers, you might want to try adjusting the phase control to 180 degrees, then listen to hear if this helps with smoothness and detail.  In my experience, it seldom does.

7 – If you can’t achieve the low-frequency detail and smoothness you want in the place you want (your sweet spot) via the adjustments detailed in step 6 above, then gradually move the sub out from the wall (or corner), about ½ inch at a time, then listen again to see if this helps.  If you like to obsess, you might try going back to step 6 after you move the sub, and play more with all three variables again—level, frequency, then distance from the wall—until you’re satisfied that you’ve optimized it all.  But don’t push it—listening fatigue can leave you in a state of confusion and distort your judgment.

8 – Now play some other music selections you like, and relax.  If something sticks out in the bass frequency range, or seems missing, make a note, and continue listening to other recordings—as few recordings themselves are perfect.  If a pattern emerges over a series of your favorite recordings, then maybe your initial test passage wasn’t ideal.

If this is the case, find a second test passage, and return to step 6.  But keep in mind that while the soundtracks of video source materials (DVD and Blu-ray discs) are engineered to rigid recording standards, audio content (CDs and LPs) are not.  They vary according to the creativity and balance preferences of the particular sound engineer who mixed them—which might not be ideal for your set-up.

9 – Once you’re satisfied with the blend and detail of the bass frequencies, sit back in your seat and enjoy the music or video.  Don’t expect total perfection, as some room effects are hard to control fully, and source material varies in reproduction quality and tonal balance from disc to disc.

10 – The above process is a good strategy for fine-tuning the subwoofer in the room—to a single listening position.  Chances are, the bass response is not as good in other locations in the room—perhaps an important location where a loved one likes to sit?  What to do?  Add a second subwoofer.

11 – To tune a second subwoofer to the room, don’t place it symmetrically in the room relative to the position of the first sub.  For example, if the first unit is in or near a corner, place the second subwoofer along a wall—preferably not close to the first unit.  Placed symmetrically in the room relative to the first woofer, it will double your trouble!

12 – Before you plug in the second woofer, listen some more to the woofer you just tuned from other seating positions in the room, to find the worst position for low-frequency sound (too loud or too soft).  Use this listening position to optimize the performance of the second woofer.  Or, if there is just the two of you, tune the second sub to the other favorite seat.

You’ve worked very hard.  Let’s hope you’re system rewards you with very satisfying bass performance, which integrates perfectly with the main speakers in the room.  Now relax and enjoy your system.