Originally Posted by jpanzar
Hello, Forum Readers.
This question is purely one of scientific curiosity. Can anyone tell me WHY a subwoofer must have a large diameter? Why can't it have the same diameter as a tweeter? It might be harder to hear if it was as small as a tweeter, but a guy at a sound shop told me that it is IMPOSSIBLE to produce a 20 hertz tone with a tweeter. WHY NOT? Don't you just send a 20 hertz signal to the tweeter, its speaker cone moves in and out 20 times per second, and you create a 20 hertz sound wave? Why is it impossible?
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.
I like to understand how things work, but this question has left me baffled. I've researched the web, to no avail. If you can answer this question I will be very grateful.
Thanks for reading.
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?"