Sub Enclosure help!

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HI All,

I have a 15" Pyramid Subwoofer that I want to build an enclosure for today. The sub will be used for music and HT, so I'm looking to build a sealed enclosure. Here are specs on the box:

Fs- 21Hz
Qt- .37
vas- 18.53 cubic feet

I ran this through a box volume calculator on the web and entered a Qtc of .8, and got a volume of 5.042 and an f3 of 40.74Hz. Am I entering these values correctly? The box says "Qt", and the web-calculator said "Qts", but I assumed they were the same. Anyway, once I entered the box volume into "Golden ratio" calculator, I got box dimensions of :

12.35" depth
33.33" Height
20.58" Width

This is a pretty oddly shaped box for a 15" speaker, methinks. I just want to know if these numbers are correct/if the box will sound good. Any confirmations/suggestions would be _great_. Thanks!

diyAudio Moderator Emeritus
Joined 2001
I ran your numbers through WinISD and they checked, although I really did not have to. I knew the numbers would work out exactly as you stated, and they did.

A sealed box that is smaller than it's Vas is called an "acoustic suspension", or "air suspension", enclosure. The classic way these things are built is to take a speaker that is about .4 Qts. and put it in a box that is 1/3 the Vas. You should end up with an Fc, (closed box resonance) about double the Fs, and a Qtc, (closed box Q) around 0.8.

As you can see, your speaker comes quite close to the "classic acoustic suspension" design. People have building boxes very similar to yours for 40 years or more, and getting similar results.

The internal dimensions you quote come to an internal volume of about 4.88 cu ft-close enough to 5.0. The dimensions fit the Golden Ratio where each dimension is 1.6 the size of the next smallest dimension.

However, there are other Golden Ratios that were mentioned on this forum, and I'll see if I can give you the thread. This might give you alternatives you find appealing.

Found it! Here is the discussion on the Golden Ratio you should read:

In sealed systems, a final Q between .7 and 1.0 is considered most desirable. That is what you have, so it should sound good.

Good luck! :)
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