Having seen that some people are still struggling with the fact that not every subwoofer on the market needs to go into a certain type of enclosure, I figured that I’d take some time and briefly go over WHY every subwoofer does not need to go into a certain enclosure. I'll try to write this in as simple a method as possible without confusing anyone with jargon that isn't necessary. I’ll divide this into five parts: sealed, ported, infinite baffle, transmission line and horns. Let’s start with sealed enclosures, shall we?
What is a sealed enclosure? It’s a closed space behind the woofer which acts as an airspring. As the speaker moves throughout space, the air behind the cone becomes pressurized. This same pressure is what keeps the speaker linear and controlled. The sealed enclosure will typically have a greater transient response and lower group delay than a ported enclosure, but that is merely a rule of thumb and I won’t say that it’s always true. Now, what should you look for in a sealed enclosure? Well, many systems in the world have what is called a Quality Factor, or Q factor for short. This Q factor determines a lot of things: how long a bell will ring after being struck, how long it takes for a charge inside an inductor or capacitor to become neutral and for us, how well a speaker can be controlled by itself and in an enclosure. Using the sealed enclosure as an example, the smaller the box, typically the higher the Q.
Why? A higher Q (>.5) means that the system is called UNDERDAMPED. What is under damped? That means that it takes a lot longer for something to return to its baseline condition. Hey, that’s kind of hard to grasp, how about an example? Sure thing! Think about the bell I mentioned earlier: if you strike a bell and touch it, what happens? It stops ringing, obviously. Now, let’s assume that the bell is flawless, is radiating in free space with no air or heat or anything to stop it. This bell would therefore continue to ring forever, which means that it has an infinite Q. The higher the Q, the “freer” that something is to oscillations above its equilibrium state. For a circuit, that means that it takes a long time after an impulse is sent to it for it to return to equilibrium, for a speaker, it means the same thing! If you take a speaker with a high Q (called Qts which is a combination of the mechanical and electrical Q factors which I’ll talk about later) and play a tone into it, the more ability that it has to oscillate freely. Since for us that means that the speaker will become uncontrolled easily, you need the control of the sealed enclosure for it to work best.
The sealed enclosure itself ALSO has a Q which you need to look at. As I said, the smaller the enclosure, the higher the Q. Why? Because as the airspace drops, the speaker must pressurize a smaller amount of air which thusly means the pressure in the box rises. That means that it takes longer for the speaker to return to equilibrium as it has to dissipate that pressure for longer. In the end, that corresponds to a very peaky response because the sound will go from baseline to a gain over baseline, to under baseline to over baseline over and over until the system reaches equilibrium. It’s that simple! The higher the system Q, the more tendency for the sound to be peaky or boomy at certain frequencies near the roll off point since the frequency response is not flat at all frequencies.
If you make a larger enclosure, then the frequency response becomes flatter to a point where the speaker will actually end up rolling off TOO early. This is just as bad as a peaky response because it can cause what many of you know as bottoming out. What happens is that because there is not enough pressure in the box to keep that high Qts cone under control, then the speaker behaves like it’s just sitting outside an enclosure or in what’s called infinite baffle (I’ll tackle that later). This is BAD for speakers not made for IB applications because it leads to over excursion and subsequently mechanical damage to the speaker. While many speaker companies use various methods to stop bottoming out from happening (special tricks to the ends of the voice coil such that as the speaker is going to clear the magnetic gap which causes excursion, the motor essentially has nothing left to charge up and the cone can’t go any further), if you push it hard enough, it will happen!
So, how do you tell how large to make the enclosure? Well my friends, you need a few parameters: the Fs of the speaker, the Qts, the Vas, Xmax, Sd and Vd. If you don't fully understand any of those, just ask, because they aren't as simple as you might think (Xmax is not just simply how far the cone can move linearly, it's helpful to know why that's its linear limit ). ONce you ahve those, you're going to want to either use a design program or if you're like me and trust tried and true mathematics, you'll use a sealed enclosure design table. These are are tables with numerous values for the desired SYSTEM Q or Qtc that you want. Some useful Qtc values are:
Butterworth: 1/sqrt(2) = .707 (flat magnitude response)
Bessel: 1/sqrt(3) = .577 (flat group delay)
Chebyshev: > .707 (maximum power handling and efficiency, bad transient response)
Transient perfect: .5 (critically damped, right on the limit of usefulness for high Q drivers)
So let's say that you want a Butterworth Q of .707 which has a flat magnitude response. You go to the table, then along the left side of the table, you'll find numerous Qts values for the speaker. Once you find the Qts for your speaker, it's time to choose a value called "alpha". All that alpha is is (Qtc^2)/Qts - 1. Take your alpha value and now you're good to go! THe Vb, or net air space, is the Vas of the speaker divided by alpha, or Vas/alpha. That's it! While there are numerous other things needed to design specific parts to your sealed enclosure, knowing how to pick the correct volume is probably the most important. That's all for sealed, and if you want some more specifics, then let me know!