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INSIGHTS ABOUT SUBWOOFERS
by Steve
Deckert Over
the past 30 years in this country, there has been a trend in
audio to go for convenience first and high fidelity second.
This has resulted in the DOWN SIZING of speaker cabinets to
increase the marketability. While this generated a garden of
cute little speakers (little being the key word) it has done
little for efficiency or fidelity. What suffers the most when
you down size a speaker-- bass response. After a period of time
the market got tired of clinging to the “loudness control” on
their receivers and demanded some real bass. And so was born
the sub woofer, a cabinet designed to play only low bass. HOW DOES ONE WORK? A
sub woofer works by typically playing frequencies between 20
cycles and 100 cycles. The “bookshelf” or “small tower” speakers
in this country are commonly only going down to between 40 and
60 cycles. That leaves at minimum an entire octave of music
missing from your listening experience. To fill in the octave
of missing information a sub woofer is used. WHY ONLY ONE? A common question, why only one sub woofer cabinet, when we all know there are two channels? Well, it is an issue of localization. The pure objective of a sub woofer is to make your present speakers sound like they have bass, NOT to sit over in the corner and draw attention to itself as it booms away. So, you would think that you would need two, and would almost have to set your speakers on top of them, right? Wrong! Different frequencies have different distances between wave fronts. For example, a 1000 Hz tone generates sequential waves spaced around 11 inches apart. The higher the frequency, the shorter the distance gets. The human ear was designed and shaped the way it is, to triangulate these frequencies and tell the brain where the source is located. That way you know which direction to run when you hear a noise in the middle of the night. If the noise you heard were lower than 100 Hz, you would really panic, because not only would you feel the noise, but it would come from all around you. You ears would not be able to locate the source because you don’t have a fat enough head! The lower frequency wave fronts are spaced farther apart than the distance between your ears, so instead of reflecting off your head and ears, the wave front simply passes through your head. The kind of Low frequencies that you are looking for out of a sub are in the neighborhood of 40 feet long. So, a sub placed anywhere in the listening room will create bass that is impossible to localize. (Boy there are about a million possible exceptions to that statement, but lets just go with it for now.) You
can start to see why only one sub is needed to create the illusion
of bass coming from both speakers. Because of the large size
and length of low frequency wave fronts, most recordings have
a natural side effect of being recorded with monaural bass anyway.
TWO WAYS PEOPLE USE SUBS This is the most important thing to understand, and the reason I wrote this paper, so if nothing else, please try to grasp this. The high fidelity application of a sub woofer is very different from the common application of a sub woofer. In high fidelity playback systems the desired goal is to reproduce all frequencies at the same pressure, i.e. flat response. If a pair of high fidelity speakers have a response that rolls off at 40 cycles then the application of the sub woofer would be to start at 40 cycles and play on down to 20 cycles. It would be exactly matched in volume with the high fidelity speakers. The result of this would be flat response, in so much as you would never be able to “HEAR” the sub woofer. In fact, since you high fidelity speakers go down to 40 cycles, and 90% of all recorded music lies in the band between 40 and 20,000 cycles-- your basic impression of the sub woofer would be that it doesn't seem to be doing anything. The common application of a sub woofer is far from high fidelity because of two things: The user turns up the volume to a level greater than his speakers, and turns up the crossover frequency to 90, 120, even 150 cycles. He does this because he is waiting to hear the sub work, and that is what it takes to get it to draw attention to itself. In this application, the response of the speakers are altered by overlapping them with a thick veil of bass. The
common application being the default application of most buyers
of sub woofers has create from a high fidelity point of view,
a ridiculous trend for DOWN SIZING sub woofers as well! Just
as down sizing normal speakers killed the low bass, down sizing
sub woofers has done exactly the same thing, kill the low bass.
You will find that the majority of smaller sub woofers only
go down to 30 or 40 cycles. That rather dictates which application
you’ll be applying doesn’t it? PLACEMENT The most asked question about sub woofers, no doubt, is where to put the thing. In the high fidelity application, between your two speakers. In the common application, in a corner of the room wherever it’s convenient. Difference between the two... possibly a lot; possibly none. It depends on the quality of the sub woofer and the odds that where you placed it will remain in phase with your speakers. To have the luxury of placing a sub anywhere in the room you want, and create no fidelity issues, is to have an active crossover with sophisticated time alignment adjustments to make sure that the overlapped wave fronts between your speakers and the sub woofer are in perfect time (Phase). Unless the sub is located between your speakers, the chances for phasing problems are different for each frequency. This is where lots of experimentation comes in. In a larger listening room, when the sub is placed on the rear wall without any kind of electronics to correct phase, you will find that reversing the phase of the sub by swapping the positive and negative on the woofer terminals will create a closer phase alignment with your speakers and more bass. Adding more than one sub woofer to your system, and locating the two on different walls can create a phasing disaster and you will more than likely have less bass than you did with just one sub woofer.
Decware is a trademark of High Fidelity Engineering
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