An Easy Solution to Subwoofer Calibration

A lot have been said about subwoofer's placement. Many thought a sub should be omni-directional and placement should be a no-issue. However, it is not the case.

Here, we explore about it. Below is an useful article by Joe Cornwall extracted from audioholics.com for your reading.

"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 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."

........to be continue...