Isobaric theory

[Tommythecat 0]

So there isn't a topic on isobaric mounting, I figured I might as well create an advanced discussion on it to answer any questions. We might have an idea that isobaric mounting allows us to use smaller enclosures, but why?

The important parameters are:

Re

Le

Bl

Mms

Cms / Kms

Rms

Sd

from these we can characterize a driver/system using Vas, Fs, various Q's and so forth.

When we mount two woofers isobarically the Electrical parameters Re, Le, and Bl will change - but it's like wiring a dual coil driver, we dont really change much here. The Mechanical parameters Mms, Cms, Rms, and Vas will change while Fs and Sd will not change.

Since Mms and Kms have both doubled, and therefore Cms would be halved, Fs will not change as:

Fs = 1 / 2*pi*sqrt(Mms*Cms)

and in our case our new Fs = 1 / 2*pi*sqrt(2*Mms*(1/2)*Cms) = the same thing

Now when Cms is halved our Vas will change we get:

Vas = Sd^2 * Cms * rho * c^2

(rho is the air density constant and c is the speed of sound - assumed to be at a relative temperature and elevation)

and in our case Vas = Sd^2 * (1/2)*Cms * rho * c^2 => (1/2)*Vas

The only thing left that changes, Rms, only affects Qms so we can just ignore it really (because Qes will also be changed accordingly).

So what does this mean? When building a box for an isobaric pair you can do it in two ways:

1) Build half the box, obtain the same transfer function. This used to be useful because in the early days of car audio the woofers had very large Vas's and required large boxes - now you could cut that in half. These days car audio woofers have very small Vas's.

2) Build the same box you would have for 1 woofer. Now you've got double the box size allowing you to tune a bit lower(although you still have to deal with port length in the same size box) and get a lot of low-end output.

Other benefits/costs can be brought up like Power Handling (increases) or Monetary cost (price goes up).

Edited July 9, 2007 by Tommythecat

[DevilDriver 28]

Well posted and, dare I say, rather timely as well.

[///M5 2,833]

Nice. The whole half the box can be real helpful when going old skool.