In The Zone
By GLENN DAVIS DOCTOR G
There are a number of misunderstandings about room acoustics. They resurface regularly, are promulgated by the current crop of gurus, and lead to inevitable wasted money or unsatisfactory results.
I would like to shed some light on one of the more enduring confusions that I have been hearing for quite some time. It’s particularly relevant to the small and large studios as well as other listening environments. And it takes the form of the statement something like, “you can’t get bass in a small room”
The current pundits making this assertion will produce some formulas showing what the supposed lower limit of bass response is going to be for a given maximum room dimension. Or they will repeat the conventional wisdom that is necessary to have “a room with a physical, long dimension of half the wavelength of the lowest frequency your monitor can reproduce,” with the implication that you cannot get (good? any?) bass response below this limit.
Unfortunately, the gurus sometimes seem to be out of touch with reality, especially what might be called technical reality, or engineering as we sometimes call it. Fortunately, the situation is far more optimistic than these predictions would lead you to believe. So before you go looking for more real estate, read on and take cheer – you can get good bass in smaller rooms, and in fact you are already doing it in some cases, as I will now discuss.
There’s one type of “small room” that nearly all of us are quite familiar with. It has a sound system in it, frequently with very powerful low frequency response. It goes past my window, and yours, every day. It’s called an automobile. My rattling glass panes will testify to the bass response of these “rooms” as will your ears if you are unlucky enough to be caught in one of them. The longest dimension of most of them is six for seven feet; let’s call it eight feet to include your favorite vintage gas guzzler, or limousine, if business is really going well.
Now, if The Guru’s Rule is true, there should be (no? poor?) bass below 70 Hz, assuming that the speed of sound is about 1130 feet per second. Manifestly, there is lots of sound output below 70 Hz in many car systems. Enough in fact to make you reach for your ear plugs.
Next, let us consider another, even smaller enclosure that by definition has a woofer in it – the loudspeaker enclosure that houses most of your direct radiator systems. Therefore, if we put our head, or better a microphone, inside the box we should hear (no? little?) bass within this small “room” Ever try it? I can assure you it’s not quiet in there at any frequency.
Finally, just to hammer home the point, there’s another extremely small enclosure that produces good bass on occasions; that’s the little space defined by headphones and your external ear canal. The longest dimension here is 2 inches. This should, according to the conventional wisdom, lead to the conclusion that we cannot hear (any? much? good?) bass below 3400 Hz from headphones!
OK, these are enough common sense examples that we all know about. They should give the lie to this very common fallacy that you cannot have extended low frequency response within a small space. No advanced technical knowledge required – you have all heard at least two of these examples yourself.
Is that it? Room size has no effect on low frequencies? Not at all. But reality is a little more complex, as I will explain.
What can be correctly predicted by the half wavelength formula (especially if you use the correct speed of sound) is the frequency of the lowest room mode, or resonance, for a rectangular room. Rooms, all of them, what ever their shape, will have these natural resonances, dependent upon their dimensions and shape. It’s a function of enclosing air with reflecting surfaces, no more, no less. Below this limit, a loudspeaker will simply “pressurize the room” rather than excite its resonant modes. But it won’t suddenly fall silent. It will simply have to survive without the “modal support” that most rooms provide, however unevenly.
Nailing another popular fallacy; creating irregular shapes, for rooms or loudspeaker enclosures, doesn’t get rid of these modes, or even markedly change the number of them. All it does is move around (unpredictably, unless you go to a great deal of very complex computer analysis) the frequencies or locations at which the modes occur. They’re still there, except we’ll have a harder time predicting how they’ll be distributed along the frequency spectrum and throughout the space.
Notice that I didn’t say, in the last sentence above, “along the low frequency spectrum” That’s because these room modes occur from the lower limit set more or less by the longest room dimensions, right on up to ultrasonic bat territory, if you have a loudspeaker that can generate them.
However, what happens is that there are more of them as the frequency goes up. The wavelength gets shorter and shorter and hence there are more “wavelength multiples” within each room dimension. As they say in the high brow journals, the modal density increases as the frequency increases. At low frequencies there are fewer modes with larger gaps between them.
This leads to what we have all experienced: If you slowly sweep a sine wave through the low frequency part of the spectrum, even a perfect loudspeaker has a ragged seeming response, because the room modes reinforce at some frequencies and cancel at others. And this is the crux of the issue about room dimensions and bass response. Most loudspeaker designers are assuming, correctly, that these modes will be there to boost the low end output of their products, even if the boost is irregular. There’s nothing wrong with this – rooms generally do have modes. Yes, there are ways around this, but that’s another, lengthy matter, and doesn’t lead to any immediately easy solutions.
However, if we have a loudspeaker that can produce frequencies below the lowest mode of the room it’s in, we’ll certainly hear them. The problem for the loudspeaker is that it’s now operating in what is called “the pressure zone” and doesn’t get any (erratic) help from the room resonances to support/amplify/distort it’s output. This puts a big load on the woofer, which is having to work increasingly harder anyway as the frequency drops. Many loudspeakers cannot keep up with the demands, and this leads to the erroneous conclusion that the room is the problem.
Most “reference” loudspeakers are not designed to produce anything below 40 or 50 Hz anyway, wherever they’re placed, no matter what is said, or advertised about the speaker’s credentials. Using the half wavelength formula, a rectangular room with a 14-foot length would produce a reinforcing mode at 40 Hz and make most listeners hear “great bass”
In fact, the smoothest bass response will be found below the lowest room resonance, in the pressure zone, if the loudspeaker can handle it. The amplitude will be controlled only by the woofer output and the room absorption, which can be, but is not always controlled.
To summarize: The size of the room/enclosure/whatever only determines where the lowest resonance will occur. The room resonances will sporadically reinforce (and equally sporadically cancel) the bass output above this frequency. Below this frequency, in the pressure zone, the woofer is largely on it’s own and often falls short, as would be clear in an anechoic/outdoor environment.
There’s a great deal more to room design at low frequencies than simply ignoring its dimensions, but you don’t have to be automatically deprived of good bass anywhere – not if your loudspeaker can survive the pressure zone!
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