[rubjel]

Hi, like the title says I want to make U-frames for my two PD2450 24" drivers.
I have done alot of reading on the websites of Martin J King, John Kreskovsky and Linkwitz among others.
The question of depth/length of the u-frame still remains. I know the dipole peak is the limiting factor, however I run an active setup so I will be able to EQ this peak out. 4th order LR at approximately 150hz.
If the dipole peak has been removed from the equation, how deep can I make the u-frame without any issues? I was planning on 16" depth/length, and possibly some damping to obtain cardioid behavior.
I suppose too much length would introduce some "box sound", but at what point would this be noticeable?

Is there any correlation between cross sectional area and length? The inside square cross sectional area of my u-frames will be 23" x 23".

Thank you in advance.

Best Regards,

[fdandrews]

I had the same question a short while ago about my U-frames using a 21" woofer from MCM. MJK suggested a U-frame depth of between 5.5 and 6.5 inches, not counting the baffle. See the thread: "How do I determine the optimum U-frame depth", not too far down the page. My U-frames were originally 18" deep (I don't remember how I came up with that depth), with stuffing in the back,  but after I cut them down and took out the stuffing, they sounded much better. Waaaay better. My U-frames are 24" square outside dimension, with 3/4" ply sides and a 3/4" baffle. 22.5" inside dimension. Very close to what you are considering.  Based upon MJKs advice, I think 16" is too deep, and I am not sure stuffing the back actually affects very low frequencies. Maybe read that thread and try a baffle depth based upon his advice.

[matevana]

Since Rubjel plans to use a steep LR filter to address the dipole peak, he might otherwise benefit from a deeper frame, similar to having a wider and taller baffle but with a less imposing look. But if I remember correctly, the Precision Devices 24" driver doesn't have as low of an Fs you might think, (low 30's?) so I'm not sure how much would be gained. Perhaps if the driver's Fs was in the teens, the decision would be more clear. If this is correct, I think I would opt for a more shallow cabinet with less filtering.

[rubjel]

Thank you both for your responses, very much appreciated!

fdandrews, its very interesting to read about your experience with baffle depth. Did you cut them down from 18" to approx 6"? In what way did they sound better? Was it better because the dipole peak was moved higher in frequency, or the general character of the sound was better?

I suppose the deeper the baffle, the more congested and box-like the sound will be.. I certainly don't want to loose that open well defined bass character I'm getting now without any baffle at all. Also, did the low bass suffer noticeably with the shorter depth?

Best Regards,