Why can't a subwoofer be as small as a tweeter?

It's air... You have to move air. The smaller the woofer, the more it has to move. So much that it's not feasible. A tweeter has almost no excursion...

It isn't impossible, you just can't hear the resulting tone. You can't even attempt it with a modern speaker because the internal crossover will roll off frequencies below 2-4 KHz.

Consider a pair of headphones. Their drivers are about 1.5", somewhat larger than a typical tweeter. From a distance of about a half-inch, they produce the entire frequency spectrum. Start to move the driver away from your ears and you'll hear the low end disappear almost immediately. It's still there; the driver just isn't powerful enough to make it audible.

This guy is completely wrong. This is a very common thing that people think, it also yields statements like "you can't ever get low bass out of headphones because the drivers are so small" or that there is some kind of relationship between the diameter of the driver and the largest wavelength (lowest frequency) it can reproduce. This is all bullox.

It's not, however, an irrational conclusion to make if you don't know what you're talking about, because we all know that bass drivers are large and high-frequency drivers are small.

If you think about the energy carried by waves, a very high-frequency wave is very high in energy, and it only takes relatively little amplitude to carry a lot of energy. Low frequency waves are not very energetic, and require very high amplitude to have the same energy.

In order to be loud, you need a LOT of bass amplitude, and relatively little high-frequency amplitude. So high-frequency drivers don't need to displace a whole lot of air and create a lot of pressure, so they can be small. Additionally, high frequencies obviously require very fast motion, which requires a driver with very very little mass. Making something very large and more massive like a big woofer move that fast back and forth is very difficult to do.

Conversely, to make bass loud, you need to move a LOT of air. You don't NEED to have a very large driver to do this. If you don't need very much bass, you don't need to move much air. This would be the case in headphones for instance because it's basically right at your ear. But just because you need to move a lot of air doesn't mean the driver has to be huge. You could just have a small driver that has a lot of excursion. But this is difficult logistically to do, hence generally you'll find larger woofers.

You absolutely can produce a 20hz tone with a tweeter. You can produce that tone with many headphones which have drivers comparable in size to most 1" tweeters, without any problem. But you'll never hear it our of a tweeter (or likely even out of headphones) because the amplitude will be so small because the tweeter is just not designed to have the excursion to move that much air. Maybe if you had a small 1" piston tweeter of some kind that moved a foot back and forth at 20hz you might hear it, but that's a silly thing to build when you can just use a larger woofer.

You can also see this in what good bass drivers look like. If you ever look at big massive 15" drivers, they usually don't have big huge surrounds to facilitate many inches of excursion. Instead they're big enough to push a lot of air around without moving as much. If you look at powerful subwoofers built around very small drivers (Sunfire comes to mind), you'll notice that those much smaller woofers will generally have big fat surrounds on the drivers because those woofers are designed to have inches of excursion, and move a LOT to push just as much air around. Different design challenges, different goals, different kinds of design tradeoffs. But you can see in the comparison how there is an inverse relationship between the size of the driver and how much excursion it requires to produce a lot of bass.

The other aspect of course, is that a lot of bass is also felt physically on your body. So while you can hear bass on headphones, the experience is very different because you're missing the thumping in your chest and whatnot that you also FEEL physically when listening to speakers in a room with a lot of bass.



So to answer your question, you are correct in your thinking. There is no theoretical reason why you couldn't make a very small subwoofer driver. But there are a zillion practical reasons why doing so is just pretty silly when you need to move that much air. You could dig a huge pit mine with a teaspoon if you wanted to, but who would want to do that? But it doesn't mean you couldn't or that there is some physical law preventing you from digging a giant hole with a teaspoon.

What you reported hearing is an urban legend I've heard many times. It kind of makes sense, until you actually think about the physics of it. Kind of like people who say that you can't have low bass in small rooms because the room isn't big enough to "support" or "contain" bass frequencies that long. They take a rudimentary level of understanding and totally misunderstand the what and the why.

So bravo to you for being a rare American with actual scientific curiosity and doing a common-sense check of "wait, does that actually make sense?"

For a 1.5 inch tweeter to have the same extension and SPL as an average 15 inch with a 1cm excursion, at 100dB, you'd need an xmax of more than a meter.

What you could do though, is get like 100 tweeters, or more precisely 1.5 inch woofers, and then you could potentially match something like an average 15 inch woofer with an excursion of like 1cm... Of course, it would make little sense to do so...

Calculator, woofer size vs excursion & frequency/SPL. http://www.baudline.com/erik/bass/xmaxer.html

Nothing wrong with the fact that any size driver as the ability to produce a 20Hz tone but the very important fact Is that you will never hear it and that is is that really matters in the end.

The distortion levels from a 1" piston tweeter would also be much higher then a large woofer who has to move a lot less distance to product higher SPL.

The real goal in audio is to have clean SPL and the less distance any mechanical part has to travel the better the driver will be because of less overall distortion.

Bigger the woofer the better!!! This includes woofers in main speakers!


I will disagree the best subwoofers have large surrounds on them. I have 14 different subwoofer drivers now and they have massive surrounds. Look at the LMS5400 18" woofers surround, that is massive surround "Patent-pending XBR tall profile surround for 3" peak-to-peak travel"

http://www.parts-express.com/pe/show...number=293-666

You simply can not have great Xmax without massive surrounds since they have to extend a long ways for the movement of the driver!!

Great comments from some of you, especially Wiggles.

There are some new drivers that we're evaluating at Triad that are 1" in diameter, and are -3 dB at 100 Hz. We're thinking of using many of them in a soundbar application. The problem is the driver's sensitivity is shockingly low, and because of limited excursion from a 1" driver, it won't move much air. But if you used 16 or 24 of them in an array, you could build a pretty cool speaker for a flat panel video display. Still, you'd need hundreds of them to attain a pants-flappage rating.

From Hoffman's Iron Law: Of these three variables, small cabinet size, deep bass and high sensitivity, you can pick any two.

Of course, the use of multiple drivers implies a concomitantly larger cabinet size.

. . . although by doing so, one or even two of the cabinet dimensions may be reduced, in order to provide a 'better' room geometry fit (and/or 'appearance'!)

That offers some options (within limits) but the volume must remain the same, of course.

Isn't energy just related to the frequency just as in Planck's equation: E=hf where h is Planck's constant and f is the frequency?

That's for electromagnetic waves.

That equation, e=hf, is for electromagnetic waves, but energy (e) has a specific definition and I'm just wondering if what Chris is thinking of might be properly described as some other term. What do you think or better yet, what's the definition for energy with respect to sound waves?

If I'm not mistaken, Acoustic Energy is a function of Sound Pressure: This Wikipedia entry suggests that the product of sound frequency and particle displacement is the number that drives up sound pressure. For any given [specified] sound pressure, a lower frequency will require a larger particle displacement, and thus a bigger speaker with more mechanical (and/or electrical) power transduction capability.

Well, I recall that with sound, and I guess you could say for ocean waves too, the energy of the wave is proportional to the square of the amplitude which I think is kind of what you're getting at SoundChex.
So, if we take two tones, one at 20Hz and one at 10kHz, of equal amplitude, will they have nearly the same energy or will one have significantly more?

Is it not possible to use a 10" (nominal) diameter woofer and simply adhere the perimeter of the plate to the perimeter of the speaker diaphragm?

Agreed... at least to establish the concept.

You could also cut the cone off and extend the tube from the voice-coil. You'd have to find a substitute for the surround though to hold the coil in the gap.

Something like this might make it easier:
http://www.amazon.com/MB-Quart-Refer.../dp/B00375LCDO

It's funny to think how long the tweeter-sized element's travel range would have to be to generate enough sound pressure at low frequencies