Some Theory Underlying Sub Design
June 1, 2005
As you go down the frequency scale, there comes a point — more accurately, a range — where sound starts to change from something your ears perceive to something your body feels. That's where you realize that the idea of sound being air in motion isn't an abstraction, that it's real enough to test the construction of your house if you get it loud enough. The frequency range where this realization is strongest, the region below 150 Hz, where "audible" really starts to give up and "feelable" eventually rules, is the realm of the subwoofer.
Before Home Theater arrived with its crashing-bus, jet-plane-groundshake, volcanic eruption, stampeding buffaloes, and sounds-like-the-end-of-the-world effects, the place of subwoofers wasn't major in audio — and certainly not in video. There were some full-range speakers that went down low enough to produce the air movement of a 32-foot-long organ pipe and the subharmonics of a big bass drum, and a few people really valued this ability. But the amount of musical material in which it came into play was vanishingly small. And there was certainly nothing in Chevy Chase videos, or even in something with sweep like "Out of Africa," that contained any subsonic, body-affecting frequencies. Only the most avant-garde audiophiles — and some car-oriented teenagers interested in stressing their bodies while sitting still — had heard of, let alone owned, subwoofers.
But then came Dolby Pro-Logic and Arnold Schwarzenegger epics, and "subwoofers" began to turn up everywhere. The quotes are around the term in this case because a large (maybe dominant) percentage of today's subs, as displayed in your nearest appliance-heaven, aren't even capable woofers, let alone subsonic frequency producers. Even if you have little or no experience with audio-video equipment, you may know these so-called subs are as realistic as instant coffee, and couldn't produce even decent mid-bass performance. But we're spelling it out in the hope that it might help prevent anyone reading this from mistaking whatever ersatz stuff they've heard for the real thing on display at audio-video specialists carrying our and other fine subwoofers.
What is the real thing, and what are the real benefits of a good subwoofer? Well, one more rant about what it isn't and we'll get to the positive reality.
The rant is this: From what you may have heard issung from inside the car idling next to you at the stoplight, and/or from the sounds coming occasionally from your next-door-neighbor's place, you may identify the sound of subwoofers as ear-bending distortion being produced by some kind of jukebox you're supposed to live inside — where bass pounds away relentlessly, threatening your kidneys and leaving you no place to hide. Nope, that's not what subs are for. Most of the time, the bass you're hearing in those cases, which is usually so close to your ultimate tolerance level that you wonder what's happening to the guy who is actually in the same space with it, isn't sub-bass or even particularly low bass. With the exception of rare subsonic purveyors like Ace of Base, what you're usually hearing from the next car, apartment, or house is relatively undemanding mid-bass (let's say 100 Hz) being played VERY, VERY, VERY LOUD!!!!! While some people absorbing this kind of bass unfortunately have plonked down the cost of a real sub without ever using its actual capabilities, most of the people playing this thumping, never-ending, totally unmusical mid-bass are just using BIG speakers that don't have to be particularly good, just nice and boomy — and capable of pushing Rap to the threshold of pain in a pretty confined space.
Now, Onward and Downward to Real Thing
It's not that good subwoofers can't also make obnoxious bass sound offensive. Indeed they can. But the purpose of low bass in good sound reproduction is to give you the foundation of music and of Home Theater material — the range where your body is left with the conviction of being part of and affected by real, lifelike events. The reason they put the really big pipes on the organs in some churches is to leave you with the gut feeling that there's quite a bit going on under the surface of life — to let you know that "moving" can be a verb, not just an adjective for movie or music critics. And the reason they put some of those bass happenings in Home Theater movies is to convince you that the critics who said "moving" weren't kidding.
In music, really low bass comes from the organ, the bass drum (the big guy, not to be confused with the tympani), and sometimes the thundersheet/gong variety of percussives. There are also some recordings, from Ace of Base's to Enya's, where it's done electronically. Either way, recordings with bass and sub-bass content are rare (way under one per cent), but when the material is there, the effect is visceral — a lot different from the uncomfortable ear-pressure that pounding mid-bass generates. Much of the time, though, the presence of a good subwoofer, as we'll detail more in just a moment, gives you a welcome added sense of weight in orchestral music, which many people find worth the price of admission even without the deepest bass effects.
In Home Theater, it's all effects like the ones we've mentioned, sometimes as large as life and sometimes a whole lot larger. (Since most of us have no standard for what the helicopter banging against the sides of the tunnel in "Mission Impossible" is supposed to sound like, who can even tell the difference a lot of the time between "realistic" and beyond? And who cares? It's moving, all right, even if it just makes you jerk back in your seat! That's Show Biz, and the emotional, adrenaline-rousing effect of Home Theater is definitely the reason why many people want it )
If you take a good look at a really big organ pipe, the 16-to-32-foot variety, you can not only see why the amount of air in those things might move you, especially at loud volumes, but why it takes a lot for a loudspeaker to reproduce the sound that something so big produces in real life. And needless to say, self-destructing buses and menacing Terminator events require just as much air (or more) to be stirred up. Here is how our subsonic speakers do it:
The Whys and Wherefores of Our Subs
A subwoofer is a specialized, low-frequency-only speaker that handles frequencies from 150 Hz or so downward, usually allowing you to choose whether you want to cross over to it at its upper 150-Hz limit or somewhere below (down to 50 Hz or so.) Most good, powered subwoofers come both with a continuously variable choice of crossover points from 50 to 150 Hz and with a volume control to adjust the sub's output to match the levels of your main speakers. Manipulating these controls together gives you a very wide range of choice and control over the amount and nature of the lows you experience.
There are three good reasons to add a good subwoofer to an ambitious audio/video system:
— With no other job than to reproduce very low frequencies, a sub can be designed specifically to do so with greater impact than you normally would get from the low-end performance of a full-range system. The lower you want to go, and especially as you approach the truly subsonic 20-30 Hz range, the greater the sub's advantage over the woofer of a full-range speaker system.
— When the subwoofer is powered by its own internal amplifier (as all ours now are, after experimentation with both active and passive models), you can best deal with the biggest requirement of sub use — which is not only to deliver deep low-frequency and subsonic fundamentals, but to do so at realistic power levels. People who want a sub do not usually play music or Home Theater material at unobtrusive, "background" volume levels. They want to be moved at least a bit, and the amount of air required to move them is considerable. While some high-powered amplifiers have no problem driving a passive sub, most of the amplifiers that most people use in music and Home Theater systems don't have the low-end punch a passive sub would require for high-volume sound. And, just as important in some situations, some amplifiers sound much happier over the rest of the frequency range when they are not dealing with high-volume bass and sub-bass material. It's possible to relieve them of low-frequency responsibilities when there are low-level "line" inputs on a sub that allow you to bypass your main power amplifier for lows. (You connect a preamp output directly to the sub, which then feeds the signal for the rest of the range back to your power amplifier.) This is possible, of course, only when you have a separate amp and preamp, or when an integrated amplifier or receiver has a special, volume-controlled pair of line outputs. (The line outputs for tape recording on an amp, preamp, or receiver won't do, since they supply a constant output you can't vary.)
— The final advantage, which is more meaningful on music than with Home Theater material, is that a good sub can be connected to route only the frequencies above its crossover point to a systems's main speakers. Whether that's done via the line input arrangement just mentioned or by direct speaker connections that run speaker cables to the sub first and then on from it to the main speakers, your speakers won't have to handle lows at all. Many people report notably smoother sound from their main speakers when the lows go only to a sub. The reason for this is probably lower intermodulation distortion in the midrange of a speaker when its cone is not called on to make long excursions for bass. (There are people who feel the improvement is also, or mainly, due to an absence of "doppler distortion," which is a speaker's version of the frequency shift you experience as the source of a sound — such as the siren of a police car or ambulance — shifts. But the theoretical physicist among speaker designers insist that you would get an audible "doppler effect" only if fairly high frequencies were being produced by a speaker making tremendous excursions — beyond those normally required even for very-high-volume lows.) Whatever the reason, many of us experience a welcome further lift in main speaker performance when a sub is in use: an increased sense of ease and transparency. It's doubtful whether the improvement has any meaning in Home Theater, but if you like to listen to complex orchestral music at lifelike volume levels, you may well feel the improvement has great value.
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