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An Easy Solution To Subwoofer Calibration

by December 31, 2005


"To shine or glow, to appear or expand suddenly." This is how Webster defines bloom. I think this definition will work just fine for audiophile use of the word as well. Bloom is a good subjective description of the way that musical performance fills a space, interacts with the volume of the room and provides the rich, emotive communication we, as audio and video enthusiasts, spend so much time and money pursuing.

The bloom and, dare I say it, the "palpable presence" of reproduced music is, very largely, affected by the capability of the components and speakers of the system. The room in which the music or soundtrack is played has a much larger effect. In fact, the room's effect is larger by an order of magnitude. And nowhere is that effect more obvious than in the very foundation of performance, the bass octaves.

The dimensions of the listening room or home theater play a fundamental role in the sound of the installed system. All rooms have an inherent decay time, as well as a series of resonances associated with their dimensions. In fact, audio engineers and designers must take into account dozens of levels of sonic reflection off each surface in order to adequately characterize the suitability of a space for musical performance. And things get really complicated if the space is anything other than a rectangle! What's a poor music fan to do?

One would think most home theater spaces or music rooms are sufficiently small that the rt60 (the time it takes for sound to decay 60dB) has a negligible effect on sound. That's not necessarily so. Rooms can "ring" for a substantial period; the room in the example below has an rt60 of nearly 0.8 second! Furthermore, a compact size can be the primary obstacle to good bass definition and flat response. Simply put, a small room can have a hard time holding and releasing a big sound wave!

bass_th.gif Bass is seldom as deep as most listeners think. The open E string of a properly tuned bass guitar is about 42Hz. Only movie soundtracks, pipe organs and some synthesized music have any appreciable or usable content below that frequency. Since almost any quality powered subwoofer has an advertised frequency response of 20Hz to 200Hz +/- 3dB, why is reproducing low bass so difficult? At first it appears that this is a problem easily solved by brute force; just get plenty of digital watts, combine with digital equalization and a long-throw stiff driver, and you'll get fine bottom end performance. If it were only so easy!

In reality, the effect of the room can easily cause peaks or dips in frequency response of more than 25dB in the bass spectrum. This is caused by constructive and destructive resonances, which are a function of the dimensions of the space. For example, a hypothetical room with dimensions of 10ft X 10ft X 10ft (a horrible room for music, by the way) will exhibit axial modes centered at 57Hz, 113Hz, 170Hz and 226Hz. This will cause a boost in maximum pressure zones (antinodes) of +20dB at 57Hz and -16dB at 113Hz in the same location as referenced to the expected anechoic output of the woofer! This is a 36dB swing in frequency response! If you are the unfortunate owner of a cubic room with these dimensions, you are probably doomed to loose, booming bass with no mid-bass power or definition.

Clearly room effects are extremely important to good performance. The good folks at RPG Inc. are not kidding when they say, "Even if the room dimensions are ideal according to some criteria, only proper positioning of the listener and loudspeakers can minimize low frequency acoustic distortion." Tuning your room can bring a more noticeable difference than doubling your speaker budget!

Henri Matisse once said; "What I dream of is an art of balance." While it may be impossible to change the dimensions of your theater or listening room it is not impossible, nor even all that difficult, to change the balance of its attributes. Realistic, high-impact audio performance can often be achieved by moving things around to make the best of what you have. And best of all, you already have the most complicated and expensive test gear ever created for such a project; your ears.

Choosing a starting point for an A/V "bass makeover" is dictated by the type of gear you have. If you are tuning your room for a two channel stereo rig, the process is significantly different than tuning a room for a full-on surround sound system with a powered subwoofer. Let's look at the steps for proper positioning and easy room treatment for a subwoofer-enabled multi-channel home theater system first. In my next installment we will explore easy setup tips for maximizing two-channel playback.

Chances are that the placement of your screen is somewhat inflexible. It's important to not come to this conclusion without real thought; a complete re-orientation of the room may be (and often is) the best route! Let's assume, however, that your rear projection or plasma set is in the only permissible location in the room. It is a safe bet that the best place for the subwoofer is probably not right next to the screen! So often many folks plop this box down somewhere in the front of the room near the LCR speakers, set the crossover to the prescribed 80Hz and then proceed to increase the gain until they "feel" the bass. There is a better way.

An Easy Way To Solve A Complex Problem

[jaco] Place your subwoofer where your primary seating area is. Connect it to your system and set the crossover and volume to a good starting point; we'll fine-tune that later. Use program material that offers a combination of sustained low tones and fast, powerful midbass. A good choice for this is the work of Les Claypool of Primus, especially that band's seminal "Pork Soda" and the cut "My Name is Mud." If you prefer jazz, look to the work of Jaco Pastorius. If concert music is your thing, you may want to play one of the bigger works of Wagner. They key is to have a musical selection (or movie soundtrack selection if you'd prefer) that has lots of dynamic energy in the mid and lower bass from 150Hz to about 40Hz.


Turn the system on and bring the volume to your most typical listening level. Now get down on your hands and knees and crawl around the room slowly, listening for a change in bass quality, quantity and definition. Take your time. You want to find the spot in the room that provides the best integration of bass and upper ranges and the smoothest response. It is important to know that the best spot sometimes isn't along a wall or in the front of the room. Make sure you listen to regions throughout the room - and really listen for the integration and quality of the bass detail. In my own music room installation, for example, the subwoofer sounds best when placed as a coffee table between and in front of the primary listening seats some eight feet off the front wall!


Once you've determined the best potential location or two for subwoofer placement, change the layout. Put the subwoofer in that spot and go back to your listening position. Using the same tracks at the same volume, listen again. What changed? What needs to change? If the bass is a bit thin, perhaps move the subwoofer closer to the walls to reinforce the bottom end. If there is still a "hooty" or ringing quality, check to see if the distance from the subwoofer to the various wall surfaces is identical or multiples of a single measurement. Ideally, you want the woofer positioned from the nearest three room boundaries at distances that are as different as possible.


By now you should have found the location where the bass is the smoothest and most detailed in the room. It's time to integrate that bass into the sound field. There are various control schemes, test tones and equalizers available on most modern surround sound receivers. Using these built-in tones and an inexpensive sound level meter can give you a real advantage in setup. If available, follow the advice in your A/V receiver's owner's manual or subwoofer owner's manual for precise tuning tips.


Here is what I do to "dial in" my own rig. Select an NPR FM Station or quality digital video news feed featuring a deep male voice. Try to select a program source where there is as little dynamic contrast as possible - in other words the voice changes pitch (no monotones here) but doesn't span a very broad range from soft to loud. A great source program for this exercise is the speaking voice of Garrison Keillor from "A Prairie Home Companion". With the system playing at your average playback level, decrease the volume of the subwoofer all the way to the minimum. Now slowly increase the gain until the voice has power and loses any nasality. If you begin to think it sounds "chesty," as though the speaker were in a barrel, bring the gain back down. With a male speaking voice, you should not hear the subwoofer as a discrete sound source at all. Set up like this, the system will have maximum transparency and the most natural frequency response in your room.


One additional control is available, the phase control. Now some of you have probably twiddled this control back and forth, heard very little if any difference and so left it at the center detent. That's one technique. Here's a better way. Select a mono musical source, preferably a musical piece originally recorded monophonically and properly mastered on CD or DVD. One of my favorite pieces for this is the reissue of John Coltrane's "Soultrane" on DCC Classics, especially the track "Russian Lullaby."


Now, playing the soundtrack through the Dolby Pro-Logic settings, you should get output only from the center speaker and the subwoofer. Listen carefully to the interplay between the bass guitar and bass drum or piano left cable.jpg hand. The impact of the drum is much higher in frequency than the deep, resounding skin sound. What you want to accomplish is to have the initial impact of Arthur Taylor's drum kit precede the deeper drum skin sound and the note from the bass guitar (by the way, it doesn't hurt if that note is lovingly teased from the instrument by Paul Chambers!). This establishes the correct phase relationship between your center speaker (and by extension your left and right main speakers) and the powered subwoofer.


By now your system should be sounding pretty good. Measure everything and get quality interconnects of the right length to settle this into a permanent installation and start thinking about how to treat the wall for the best performance from your surround monitors. That too will be part of the next installment. Till then, happy listening!



Many thanks to Impact Acoustics for this informative article.



About the author:
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A sales and marketing professional, Joe holds degrees in Electrical Engineering and in Applied Business. He has been honored several times within the consumer electronics industry, being selected to serve as a judge for the prestigious Consumer Electronics Association "Mark of Excellence Awards" and having served on the Board of Directors of the Satellite Broadcasting and Communications Association.

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