Is this right - "critical Q sub specs" ?

[RobWells]

I've just been re-reading the 'critical Q subwoofer' project here :

http://members.ozemail.com.au/~joeras/sub_index.htm

And thought the box size looked bigger than what I've modelled myself.

I read the faq's here :

http://members.ozemail.com.au/~joera..._q_sub_faq.htm

And someone else has asked him about the vol :

"I have read with interest your sub-woofer design web pages, but have not been able to follow how you decided on a V_b of 42L. I have used both WinISD and Unibox to determine Vb for a Qtc of 0.5 and both programmes suggest around 20L would be required. This is of course for the Peerless 830500 speaker in a sealed box using manufacturer’s T-S parameters or your measured values. Can you throw any light on this matter for me?"

The reply :

I am familiar with WinISD and entered the Basic Fs, Qt and Vas. With this 'quick and dirty' method got the value of 0.46 - not bad. so you must have entered the wrong values. May I make it very clear that the 0.5 Q result came NOT from using computer modelling, but actually building the box and then measuring it. There is no substitute for the REAL world and the simulated one.


I entered the basic ts parameters and agree with the ~20L for the 0.5Q.

What I did notice is that the car version models to a 42L box.

I e-mailed him but no reply...

I know he states that he built and measured the box, but he does state clearly that the 'wrong' specs were entered.

Am I doing something wrong ?

Cheers,

Rob

[Ron E]

For his given parameters of

Fs=18
Qts=0.23
Vas=124L

This gives a Q of 0.5 with an undamped box at
Vb=Vas/((0.5/0.23)^2-1)=33.3 liters

with a stuffed box, the volume can be reduced, but the F3 will go up.
You could get a Vb of 21.4Liters, but the Fc would be about 47Hz and the F3 would be almost 67Hz.

Anyway, most of the guys' semiliterate arguments about Q vs distortion performance are specious. A higher Q woofer will often have much better overall performance than a low Q one. Basically the guy makes a lot of claims and offers no evidence, and when questioned he submits a non-answer. He doesn't say what level he measured distortion at that I could see, and he presents calculations on his website that are wrong and when questioned he says they were measurements and not calculations.

I really enjoy all the websites out there where people tout the advantages of a "critically damped" sub and then say that it is music only and "not for home theater". If your sub cannot handle both it is poorly designed.

His subwoofer will work fine for many purposes, but please don't get the idea that it is some holy grail that was attained through inspiration and long arduous effort.

[Dave Jones]

I was sort of dazzled by the idea a few months ago. But then I realized that once you add equalization to flatten the response curve, the Q changes dramatically in any case. Linkwitz has a design using the same driver in a closed box with EQ. I would suggest checking that one out. I don't think there's much doubt that Dr. Linkwitz knows his stuff.

I wound up using the 10" version with a passive radiator, the way it was designed to be used. Since then, Peerless has come out with "XXLS" versions designed for sealed boxes and standard port tubes.

[RobWells]

Thanks for the replies.

I was planning using 4 per side in push-push for stereo subs, if the box size was small enough (and I get a room big enough to put them in ) Thats why I noticed the difference between my calcs and his. His calcs I'd need 180L, my calcs I'd need 100L . A big difference!


The specs for the xls from the peerless site are these :

fs 18.1
Qms 3.7
Qes 0.21
Qts 0.20
Vas 139.2L


I also thought the distortion measurements were 'interesting' - I'd have thought a good way would be to put the sub in a corner, mic at listening. Play a test tone, measure spl at listening , measure the distortion. Put second sub in same place, repeat. At least this would give a fair comparison.

Cheers,

Rob