QB3 alignment table confusion

[johngalt47]

I am confused about the results I am getting when computing vented box volume using alignment tables.

In two different instances, the first with a TangBand W4-616S and the second with a Vifa P13WH-00-08, the box volume is way too small (it seems to me).

I'm using the QB3 / QL15 table from the 6th edition of the Loudspeaker Design Cookbook.

The Vifa Vb comes out to about 4.5 L or 273 cu. inches which seems way too small.

 

[richie00boy]

It would help if you posted the T-S parameters you were using. If VAS is very small and Qts is low you will get an impossibly small box.

 

[johngalt47]

Vas = 10 L

Qts = 0.33

fs = 60hz

 

[kelticwizard]

Sounds about right. You might want to increase the box volume 10 - 15% if it bothers you.

Incidentally, WinISD, which operates on the same Thiele-Small data as the Dickason chart, gives a recommendation of 5.2 L tuned to 73 Hz. Pretty close.

Basically, the ideal volume for a woofer with a Qts of 0.38 or 0.4 will be Fs, and the box will be tuned to Fs as well. the F3 = Fs.

When Qts = 0.32, the box volume is Vas/2, approsimately. So you are about that range.

Qts= smaller than 0.38, your box volume will be smaller, perhaps much smaller, than Vas.

These things do not have to be exact, and as long as you are within a reasonable range, everything should work about reasonably close.

 

[Ron E]

Incidentally, WinISD, which operates on the same Thiele-Small data as the Dickason chart, gives a recommendation of 5.2 L tuned to 73 Hz.

WinISD beta does not use the same data as Dickason's charts. Dickason solves the QB3 equations and the figures he gives are exact, as far as the simple model he uses is concerned. WinISD uses approximate curve fits from a 1981 JAES article. The equation WinISD uses for suggested boxes is:
Vb=20*Vas*Qts^3.3

Using QB3 Ql=15 equations I get
Vb=4.469
Fb=69.889

Using Ql=10 I get
Vb=4.538
Fb=70.854

The biggest disservice Dickason did for DIYers is make alignments seem more important than they are. When a woofer has a low Qts, it is permissible to use a box much larger than given by the formula or alignment charts. It should sound good unless the response has a peak in it, so feel free to make the box bigger and tune a bit lower than recommended if necessary. Mind the Xmax of the woofer because it is easy to extend the response too low and you can have a bottoming problem.

The other thing to keep in mind is the effect of all of the series resistance (from coils and speaker cables and amplifier output impedance) on Qes and Qts. This raises Qes and Qts and necessitates a slightly larger box than T/S specs alone would dictate.

I think Vb=12 and Fb=50 would be OK for this woofer - but plot it out and see how it looks to you.....

 

[B4]

I put the vifa into LEAP quick cab. and I got

QB3-
Vb=5 Ltr.
Fpf=70 Hz

You have to remembrer that a driver like the vifa with a Qts close to 4 all the alignments will look very similar. Why not go for an Extended Base Shelf at 7 Ltr's and Fb of 67 Hz? The response is almost the same.

 

[johngalt47]

Sounds good to me. How do you know which alignment to use?

 

[johngalt47]

WinISD does not like the values I entered for the TangBand W4-616S. I'm not set up to measure drivers just yet and if the values supplied by the manufacturer are not acceptable what should I do?

 

[B4]

Sorry I made an error. For a Q of 7 it should be tuned to 60 Hz for an Ext. Base response and 6.4 Liters. The volume does not take into consideration any internal volume such as port, drivers, or bracing. No fill was used for size.

I just used the data already in the database. The most accurate method would be to measure all parameters for yourself. I never trust the manuf. specs but Vifa is very good at posting good parameters. All these box sizing programs just approx. the values. Testing is the best way to go and use fill or blocking to fine tune to your taste.