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Elbert 24th March 2008 08:01 PM

Very large enclosure issue...

Now, I have a pair of old SEAS 13" woofers in good condition which i intend to use for a 3-way active design.

Now, I have a calculation program (CAAD 3, an old freeware application)
which I have been using just for playing around a little.

Now here's the issue; According to the calculations, the ideal box size is around 170 L, giving a -3dB of around 35 Hz.

In terms of low frequency, this is where I would be happy to end up, so that's OK.

I am however a bit puzzled about the cabinet size, as the original SEAS datasheet quotes: "closed box 50-100 L" and "Bass reflex 70-100 L"

170 L is obviously a lot more than the guidelines provided by SEAS.

I have tried tweaking around with the program, and for a vented box with a volume of 100 L, I can obtain a -3dB of 42 Hz. with a linear response.

Ok, but not great for a 13" driver with a Fs of 27 Hz....

My first assumption is that a 170 L box with a -3dB of 35 Hz is a theoretically extreme application of the driver not considered a "real world" application by SEAS.

My second thought is that the driver could well be capable of delivering very low frequency response in a 100 L vented box, but that the software I'm using is somehow not quite appropriate.

Any views and comments from those of you with practical experience with using box calculation software would be greatly appreciated.

Best regards,

PeteMcK 24th March 2008 09:50 PM

Hi Elbert,

Unfortunately, with a Vas of 270 L, your software seems to be giving you the right answers....
But, because you're going active, you could consider using them in a sealed enclosure with a Linkwitz Transform circuit:

jnb 24th March 2008 10:35 PM

In order to achieve acoustic suspension, your box should be less than a third of Vas. In an ordinary design, this rule of thumb sets your box at 100L or less.

Your Qt in the closed box ought to be higher than 0.5. This also suggests a box of this size.

Elbert 24th March 2008 10:38 PM

Mass loading..
Hi Pete!

Thanks for taking the time to reply!

First of all, good to know that my software is probably adequate, that gives me some reassurance regarding my baseline here....

I've always been a bit skeptical about equalizing in this ways due to the sacrifice in maximum SPL level, but the reasoning on power distribution VS frequency certainly has something going for it, so it might be an option after all.... or I could simply build a 170 L vented enclosure! :)

But then I tried something interesting, in my driver file, I increased the moving mass from 45 to 80 g, and this yielded an interresting result.

I could now get away with a vented enclosure of only 102 L still obtaining a -3dB frequency of 33 Hz. The only evident tradeoff was an overall loss in efficiency of 2dB, leaving em with 91 dB/w/m which is still fine by me!

I guess the higher frequencies might suffer from this, but as I'm aiming for a pretty low x-over here anyway (typically 150-200 Hz), this shouldn't be a problem.

A 100 L box would be preferable over one of 170 L..

Are there any other issoes or problems with mass loading a driver I need to be aware of?

Elbert 24th March 2008 10:45 PM


That's a very interesting thing you are pointing out there... from my novice point of view, what you are saying makes it sound like my design software is after all failing to take something in to account here...?

jnb 24th March 2008 11:16 PM

I wouldn't say that, I'd simply say it's giving you the freedom to design as you wish.

SY 24th March 2008 11:26 PM

Looking at the spec sheet, the driver has a high sensitivity. That means high BL, that means well-damped. Well-damped means that the f3 is going to be significantly higher than fs for any sort of flat alignment. It's really more of a midbass driver, which is consistent with the voice coil height/air gap spec (not super impressive Xmax).

I'd use a small enclosure and accept the high f3 OR a very large one to overdamp and use boundary reinforcement to bring up the low end.

Your software is telling you the truth.

Elbert 24th March 2008 11:33 PM

Ouch.. and I thought I had a good sub woofer candidate driver here....

Well, F3 of 35 Hz is still ok, and I guess a pair of hefty active 3 way speakers of 170 L would look quite cool, sort of a pair of Cervin Vega's, only with Hi-Fi sound! :)

Svante 24th March 2008 11:58 PM


I suppose that the software you use does not give you actual response curves? If so, there is a risk that you stare too much at the -3 dB point that the software so happlily spits out.

There are other free softwares that will give you a response curve instead, and in this case I think that I would go for a BR of ~90-120 litres tuned to 25-30 Hz.

This will give a slightly sloping response towards lower frequencies and this will fit nicely with the room gain.

You could try Basta! in my signature if you like.

badman 25th March 2008 12:05 AM


Originally posted by jnb
In order to achieve acoustic suspension, your box should be less than a third of Vas. In an ordinary design, this rule of thumb sets your box at 100L or less.

Your Qt in the closed box ought to be higher than 0.5. This also suggests a box of this size.

Curious, I hadn't heard this rule of thumb. Care to elaborate on the 'why'?

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