article about double chamber speaker enclosure

[claudio]

Wcoil, how was the comparison between the system with Dip and the one without it?
Augspurger and Weems stated that a Dip is not very audible, compared to a Peak, and the entity of the dip is so small that doesn't bother at all: what did you ear experience state?



Regards

Claudio

PS

Kelticwizard don't you forget about me!!!

[kelticwizard]

Originally posted by claudio
Kelticwizard don't you forget about me!!!

Absolutely not!!

Believe it or not, I still have not found that article I held in my hand a few weeks ago.

But

I have some written material by Weems on the double chamber reflex that I Xeroxed years ago before I came across Weems' articles. That, I found. The article I held in my hands a few weeks ago, I still can't find.

It essentially says the same things that I have been saying all along, and what the article says.

Will post by tonight. Gotta run.

[Wcoil]

Claudio, to be honest dips are hard to hear and I can´t say that I clearly could hear it. The only thing I can say is that the system with equally large chambers seam to have better control over the bass driver and have a dry tunefull sound.

[kelticwizard]

This is excerpted from Great Sound Stereo Speaker Manual by David Weems. Available on Amazon and Barnes and Noble, among other places. Going into all aspects of speaker design in an extremely readable matter, Weems covers transmission lines and crossovers as well as the standard ported and sealed box.
http://www.amazon.com/exec/obidos/tg...glance&s=books

http://search.barnesandnoble.com/boo...sbn=0071348743


The fact that Weems switched drivers after building the cabinet, and decided to tune the box to a frequency slightly higher than optimum, changes nothing about the construction of the box. The change is from 43 Hz to 45 Hz-one fifteenth of an octave! The important fact is that the method is shown. The change from optimum frequency is not due to any difference from the simple ported box, as the excerpt makes clear. He could have tuned it to optimum if he wanted, with perfectly satisfactory results.

The Augspurger/Weems Method:

A) Design a normal ported box with the bass characteristics you want.
B) Whether by hand or by box program, design it to be tuned to your desired frequency by two vents, not one.
C) Build a third vent identical in dimensions to the other two.
D) Put a partition in the box which the total volume into 2 unequal volumes-the large volume twice as large as the small.
E) Put one port venting the large volume to the outside, one port venting the small volume to the outside, and one in the partition between them.

If you note the internal dimensions of the box, they are , 24 3/8" X 6 1/4" X 10", which equals 1523 cu. in. Those figures already deduct the the partition. The three identical tubes come to a total of 72 cu. in., so the total internal volume, minus bracing and space of speaker driver, is about 1451 cu in. Let's make our life easy and pronounce the driver volume and bracing as negligible, shall we?

I chose WinISD to model the box. To do so, you must first model a specific driver-any one will do. The box is tuned by a specific port in a specific volume-which driver is in there makes no difference. I pretended to model a Vifa 6 1/2" just to get the box tuning.

Win ISD shows a length very close to Weems' calculations-the two 7.6" ports tune the box to 46.5 Hz. (I'll explain the slight discrpancy later). Close enough to prove that what Weems is doing is use two ports to tune the box, then adding a third identical in dimension to the other two.

Although I have never built this sort of box, I have built virtually all of my ported and sealed boxes according to the principles I read in David Weems' "Designing, Building and Testing Loudspeakers", and he was always right on. I would find it hard to believe that following Weems' instructions on this box would lead to disaster, especially since David Weems has included this method in at least two different articles
AND a full size book he has published. George Augspurger himself is a very respected name in audio.

This does not mean that Wcoil's method does not work Wcoil's method might well work a little better than Augspurger/Weems'. But a "disaster" caused by following Weems' method would be hard to envision.

Following is the excerpt from Great Sound Stereo Speaker Manual.

[kelticwizard]

Here are the plans for the speaker in question. You may check the dimensions if you like. Again, this is from the Great Sound Stereo Speaker Manual by David Weems.

By the way, Claudio, you were correct in speculating that the partition vent would be located in the bigger enclosure, so as to affect the tuning less.

Opie: Sorry for not getting into which volume is tuned to what frequency, but as the excerpt shows, the way to approach it is to tune the box as a whole, make the extra port, and then go from there. I felt that getting into a "large chamber tuned to this frequency, small chamber tuned to that frequency" analysis would just get things hopelessly confused.

[kelticwizard]

As for the discrepancy in tuning from Weems' calculations and WinISD, it has to do with the fact that Weems likes to calculate the tuning for 2 ports by calculating the cross sectioanl area of the two ports added together, and then calculating the length for one port of that area.

However, WinISD and other programs have a correction factor that affects that calcualtion. The total cross-sectional area of a vent will calculate slightly differently if the area is spread between one port or two.

If you take Win ISD and input one 2.8" diameter vent instead of two 2" diameter vents, of 7.6" length, you will see that the box will be tuned to 45 Hz, just like Weems' calculations.

[kelticwizard]

Oops, forgot to mention: the reason the ports look odd is that there are 90 degree elbows at the end of them. They were too long to fit into the box.

[claudio]

Thanks Kelticwizard, nice job.
Next week I will receive the boxes and I will start the testing.
I think that in a couple of months I will be able to post some graph and impression on double chamber system.
Stay tuned!

Regards
Claudio

[claudio]

Hello,
I just ended a description of Augspurger-Weems Double Chamber Reflex with measurements.
You can find it in my home page in the Projects folder.Claudio Negro's Home Page

And since it is about time......Happy Christmas to everyone!

Claudio

[PTSOUNDLAB]

Hello Claudio,

very nice and interesting site !

Happy Christmas to everyone !

Pascal.