Vented alignment to maximise transient response - diyAudio
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Old 30th November 2004, 09:41 AM   #1
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Default Vented alignment to maximise transient response

Iím about to build my first speakers, vented two-ways using the Usher 8945 6.5 inch:
Fo = 35 Hz, VAS = 34 litres,
Qms = 1.96, Qes = .37, Qts = .31

I thought Iíd use Unibox to come up with an alignment that gives best bass extension, ie tune on the lower end of the scale.

But some comments here that (not surprisingly) thereís a trade off - you lose efficiency and/ or power handling.
With the smallish driver, intended for a vented box, Iíll go vented, but Iíd like to maximise transient response. In a sealed box, IIRC this is Q = 0.66.

While you rarely hear of Q in connection with vented boxes (though IIRC Brian Steele has a spreadsheet that calculates it for vented also Ė donít know if itís meaningful) Ė
* How do you know when transient response is low/ high/ maximised/ *by how much*?
* What is the equivalent vented design basis or method, to a sealed alignment with a Q of about 0.65-0.7?
* Can you tell Unibox or other to design to an alignment (eg QB3 or B2)?
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Old 30th November 2004, 09:58 AM   #2
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The transient optimum Q for a sealed box is 0.5.

For vented box I think you need the BB4 alignment to achieve best transient response.
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Old 30th November 2004, 10:53 AM   #3
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richie00boy Iíve re-examined Brian Steeleís spreadsheet (from http://www.diysubwoofers.org/prt/), and he has a ďSBB4Ē alignment. Is that the same thing?

This spreadsheet for this driver gives
- three vented options with F3 from 53 Ė 61 Hz
- two sealed options, F3: 85 or 88 Hz.

SBB4 for this driver has a volume of 15.3 litres, Fb 38 Hz, F3 61 Hz: the highest vented F3. I guess thatís consistent with higher transient response, but here thereís no measure of it, to judge the trade-off .

If transient response is increased by reducing volume, there is only so far one would want to go. Though loss of F3 of 8 Hz is not huge.

Interpolating his formulas, to create a new alignment, if volume is decreased to only 10 litres (less than a third of VAS), Brianís formula of Qb yields 0.68 Ė very close to my target. But I havenít yet been able to interpolate his formula to derive F3.

Comments?
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Old 30th November 2004, 01:31 PM   #4
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The SBB4 alignment is an extention of the original BB4 alignment to cover drivers whose Qts falls outside of the optimum band.

I haven't looked at the DIY Subwoofers page, so I can't tell you what's going on there, but I guess that the spreadsheet just gives 3 different vented alignments, of which one is a (S)BB4.

Transient response is optimised at a certain volume. Going either side of that will worsen it as it becomes either overdamped or underdamped.

Download a copy of WinISD to have a play. If you find it too daunting having to try and come up with the driver parameters, here is a very simple closed box design programme I wrote.
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Old 30th November 2004, 03:27 PM   #5
Pan is offline Pan  Sweden
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Q lower than 0.5 gives even better transient response.

/Peter
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Old 30th November 2004, 06:04 PM   #6
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If you download winISD pro you can watch the efficiency and power handling decline, and the delay increase as you lower the tuning. The low bass excursion can also add a great deal of distortion when the tuning goes too far south, but you can't see that.

That driver is not really optimised for a sealed enclosure with low bass output. It would work well in a sealed enclosure tuned to 80 or 100 Hz with a bass woofer or sub below it as part of a 3 way/2 way w/sub. If you model in winISD pro you will get much better response curves with a ~ .4 Qts, for example, with good excursion(Xmax) capabilities. Naturally, you will sacrifice efficiency over a vented enclosure, but the roll off is more gradual.

http://www.linearteam.dk/default.aspx?pageid=winisdpro

Follow the guidelines in the help files.

Tim
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Old 30th November 2004, 07:14 PM   #7
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Quote:
Originally posted by Pan
Q lower than 0.5 gives even better transient response.
I suggest you check that out again If you go below Q 0.5 the response to an impulse becomes too damped. That means it's like dragging your hand through thick treacle -- it can't respond properly (fast enough) according to what your arm is trying to do.
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Old 30th November 2004, 09:12 PM   #8
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If you want good bass response and good transient response just put the Usher 8945 in a small closed enclosure so that the Qts is between 0,7 and 0,9 and then add a capacitor with a very large value in series with the speaker. The capacitor will be between 600 and 1000uF, just try it out and see what gives the best results.

The capacitor will act like a 6db subsonic filter, but it will also add 3 to 4 db in the low bass register (40-60hz) and it will dampen the high bass (around 100hz).
So what you get is an extended flat bassresponse with a transientresponse known from closed enclosures.

The only drawback is that the driver needs a high Xmax. The Usher does have a high Xmax (for it's size) so it won't give any problems.

I have been experimenting with this a lot lately and I must say that the results are great. My next project will have two 7" drivers using this type of system.
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Old 30th November 2004, 09:30 PM   #9
Pan is offline Pan  Sweden
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Do you mind explain more what you mean?

Dynaudio D260 and T330D have Qts below 0.5 and they seem to do just fine on impulses.

The 8554 and XLS units used by SL have low Q and do not suffer from bad impulse response either.

I have used drivers as bass or midbass in open baffle with Q 0.35-0.4 and they have had dynamics and impulse capacity that goes a long way as well.

Transient response is mostly two things;

Rise time which is set by BW and
Decay/overshoot which is set by Q of resonances.

The wider the BW and the lower the Q, the better transient/impulse response. Or am I missing something?

/Peter
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Old 30th November 2004, 09:33 PM   #10
Pan is offline Pan  Sweden
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Bombardon,

that solution (while Iīm sure can turn out nice) will have a relatively high Q with a 3rd order HP function, making it worse than a 2nd order closed box in transient response... it will be closer to a tightly tuned bass reflex box that can also be close to 3rd order in a great part of the roll off.

/Peter
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