QB5 sub, too good to be true? - diyAudio
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Old 19th September 2008, 09:16 AM   #1
Pashley is offline Pashley  Canada
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Default QB5 sub, too good to be true?

I've been looking at an article on qb5 alignments, http://sound.westhost.com/qb5align.htm (excellent site, IMHO)

I'm wondering about applying that to a subwoofer. The basic idea is that you tune the box so there's a hump down low in frequency response, them use eq to smooth it down to the
response you actually want. All the eq is cut so you are reducing some risks which are increased by schemes that use boost (Linkwitz transform & most other bass eq); exceeding either the amp's ability to deliver power or the speaker's excursion limits. Sounds useful.

Looking at tables in the article, it appears there are a couple of "magic" values of Qt where F3<=Fs, around Qt=.32 in Group 1 and Qt=.445 in Group 2.

Parts Express have woofer a selection guide:

Click on any parameter heading to sort by it -- most people would likely look at "sealed F3" and "sealed volume" or the corresponding "vented" numbers.

I sorted on Qts and looked for .32 or so with a low Fs. Lots of possibilities, including some smaller drivers that might work for folk who need a compact sub. I'm not much worried about size, so the one I like is a Pioneer 15 inch

Qts=.32 so F3<=Fs=24 Hz, efficiency is pretty good at 96 Db 1W/1m, and it is only $50.

Box size comes out at around 8 cubic feet, not ideal but not horrible for a 15" woofer. A 2-foot cube or a more-or-less reasonable length of sonotube.

So this isn't a compact sub and any vented vented system gives somewhat poorer transient response than a correctly designed sealed box. I can live with those, unless the transient response really sucks.

Other than that, it looks amazingly good. 24 Hz F3 and 96 Db efficiency with a $50 woofer and dead simple eq. Wow!

What am I missing here? Are there problems I have not noticed?

Has anyone tried something like this? How did it work?
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Old 20th September 2008, 06:34 PM   #2
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I read that a few months ago, it's a neat trick. The author makes a good point, that it's ideal for midranges, where the steep rolloff will make it easier to integrate with a sub.
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Old 2nd December 2014, 02:28 PM   #3
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I am looking into QB5 alignments for a threeway big experimental Onken style cabinet. I am looking for the upper and lower limits of the test enclosure volume. There is some information on QB5 here. Satellites and Subwoofers

I can simulate QB5 in Aj-horn with a series capacitor. But Aj-horn does not simulate the group delay of the QB5 alignments. Are there simulators that will show the group delay of QB5? So i can get a visual on what i am doing? Would be nice to tune the delay peak out of the problem freq. range / tune for lowest group delay.

Winisd on my computer is stuck on the driver editor, i cannot fill in a name for the driver... If WinIsd would work properly, would Winisd do the trick? Or is there another way to simulate this? Hornresp? Maybe someone with a bit more math and Akabak knowledge can help?

Last edited by Phenoholic Anonymus; 2nd December 2014 at 02:33 PM.
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Old 2nd December 2014, 11:03 PM   #4
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I usually use the Excel spreadsheet from here: The Subwoofer DIY Page - Ported Systems
It has several filter options which you can use to implement the EQ.
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Old 3rd December 2014, 05:07 AM   #5
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I just tried Winisd beta on an older computer, nope dead in the water... Even tried the Alpha version still left on the old computer no result . Thanks Don for the excel diysub.org suggestion, much appreciated!

Will try the alignment by trail and error, hope to make some sawdust today.

Bernd Timmermans from Hobby Hifi published formulas for QB5 vented alignment. His approach is the other way around compared to the soundwesthost article. Driver provides the Q peak, not the enclosure or an opamp / circuit. These formulas work for drivers with Qts between 0.5 and 0.7 and stiff suspension.

Cabinet volume Vas * 3
Fb = 0.6 *fs
Fb = -3dB

The series capacitor is heavily influenced by the inductance of the voice coil. The value must be found emperical. This is a ballpark starting point for the value.

C = _______
Fb * Z

Z is driver Re, value of the capacitor (C) is in uF. The formula for the portlenght (L) in mm. d = diameter port in mm.

2.340 * d *d
L = ________- 0,8 d
Fb * Fb * Vb

Apologies for the strange Fb * Fb notation, read to the power of two. Layout limitations.

Sadly he got an enormous amount of Flak for this idea and the alignment from the online German diy community. His findings where the same as Robert C.White's in the Soundwest hosts articles: improved vented speaker performance, lower group delay, extended headroom, lower excursion belowe Fb.

Simulations in AJ-horn suggest a large bandwidth of possible QB5 allignments.

Last edited by Phenoholic Anonymus; 3rd December 2014 at 05:14 AM.
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Old 3rd December 2014, 05:38 AM   #6
freddi is offline freddi  United States
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3rd order sealed boxes can be practical - I've not looked at the vented case - an unfavorable interaction is possible if cap value - which AJ are you using? if its just a cap in series then I think hornresp will show some stuff.
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Old 3rd December 2014, 05:48 AM   #7
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AJ-horn 4 & 6, v.6 on the old computer.

In theory you add an order to the alignment with QB5 compared with normal vented enclosures, so group delay goes up, in practice you lower the group delay peak in frequency. Would love to play around with different alignments and see what happens with group delay.

I am big fan of 3rd order closed box subs.

This is the alignment range for a Fane 12-200, Qts 0.56, vas 73,26 liter, fs 62.85 Hz cabinet volume 140 liter 3.600 uF cap (red) upto 200 liters (black) 1.300 uF cap.

Click the image to open in full size.

Last edited by Phenoholic Anonymus; 3rd December 2014 at 06:00 AM.
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Old 3rd December 2014, 05:53 AM   #8
freddi is offline freddi  United States
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(I've got 600uF poly caps for closed box) - seems like actively eq-ed 6th order reflex can sound good if tuned low enough - I've used the old EV Interface eq box which can be rescaled for other fb.

Click the image to open in full size.
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Old 4th December 2014, 07:15 AM   #9
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I just spend lots of time wading through a gravejard of broken speakersoftware...It's broken or it's impossible to use a highpass capacitor. Tried and downloaded 4-6 programms. Frustrating.

Nice caps Freddi oil?
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Old 4th December 2014, 01:49 PM   #10
tb46 is offline tb46  United States
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Hi Phenoholic Anonymus,

Try the Filter Wizard in Hornresp, it works and it's easy.

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