[rick57]

I've got a pair of nice looking professionally made speaker cabinets without backs (for a modest price), internally about 550 H * 320 W * 300 D (22" * 13" * 12").

I have some 1960s Goodmans Axiom 201 full-rangers (same ones Thorsten used in an OB) and am thinking of mounting them in the cabinets, to form a "U-baffle with a top".
This is intended to be an easy 1st experiment with what's on hand, rather than a best effort or primary speakers.   

I've read Linkwitz and John K a number of times, but don't understand it all 100%.

I understand that if I did as intended, I'd get resonant peaks based on the internal cabinet dimensions and distances front-back. I hope they could be largely tamed by 2-3 notch filters.

I believe that varying the amount of stuffing will allow some tuning of the low end. The Axiom drivers won't have much Xmax, so I intend to use sub below some Hz, to be learnt.

Is my understanding close/ realistic expectations/ other problems?

Responses greatly appreciated

Richard

[ttan98]

I have some experience with mounting drivers on box without backing, it is some times refer to as U-frame.

I experienced resonance frequencies at the bass ends(between 100-300Hz) and I used 4order LR(HP x-over at 100hz) to reduce these resonances, cannot totally eliminate them all together. The 1st one is the dorminate one, the resonance freq. depends on the dimensions of your box. I suggest you measure the resonance freq using measuring instruments rather than predict using  any formulae. Also I use some dampers(stuffings) in the enclosure reduce the resonances.

I am fortunate that I have measuring instruments that I can use to measure them accurately. I will use notch filter to eliminate it, the 1st dorminant one.


If you don't like to deal with resonances, then use a Y-frame(I heard it has no resonance), I think that way I suspect the bass end will drop off faster. The advantage of U-frame is that it improves the bass response. I get good bass response from my woofer(starts to roll off starting at 50Hz) eventhough they are quite cheap.

I hope this is of some help.

[rick57]

Hi,

Thanks for your response.
I expected to have to measure, but hadn't got to thinking how . .

I have a yet unused ECM8000 mike ~ what is the best software/ way to measure (resonance) frequencies?

Thank you

[ttan98]

ECM8000 has a very flat response.

I used a sine wave generator derived from s/w or from ext. signal generator, placed the mick close, less than 1/2 meter to avoid bounce from floor. With a fixed  output level, measure the o/p from speakers via the ECM8000. I sweep from say 25Hz to 300Hz.

Before you make any measurements ensure the following:
1. the enclosure is well stuffed.
2. the woofer is rigidly mounted on the enclosure.
3. burn in the woofer for at least 10-20 hrs(more better).

When you sweep the freq., you will observe res. peaks from the response.(I use HP filter the resonance peaks were not so prominent but still there).

hope this helps.

[rick57]

Thanks, TT

Some things I hadn't thought of yet . . 

Cheers

[zobsky]

Ideally, you would want to consider non-parallel wings (possibly of slightly different dimensions).

Side to side bracing may aid vibration control,  and a moderate amount of stuffing should help  with any resonance issues, as well as damping the sound bouncing off the rear walls of the U-frame.

[scorpion]

Why put them in a box, anyway. They'll be excellent in a flat baffle OB ! 
Use the cabinets for some other projekt.

/Erling

[ttan98]

Why put them in a box, anyway. They'll be excellent in a flat baffle OB ! 
Use the cabinets for some other projekt.

/Erling

Unless the Qts is high enough possibly >= 0.7(I maybe wrong here, my guess) the bass response from flat panel open baffle is NOT good enough, mounted on baffle width slightly wider than driver diameter. The bass will improve as the width increases using the same driver, even then the Qts must be quite high, say >=0.7.

My woofer has Qts=0.55(claimed by manufacturer), I measured the bass response, it was quite disappointing on flat open baffle(the width is not very wide owing to WAF). AFter I mounted it on U-frame (with the same width as flat open baffle), the bass response improved dramatically. I bought these woofer very cheap, for experimentation, once I understand the design of speakers much better I may switch over to high quality speakers drivers to get the performance that reflects the quality of the drivers, that is when my money is well spent.

Thanks for your help over the last few posts, also I just bought a DCX2496, this will improve my system even further.

[rick57]

Thanks all for your further idaes

> Use the cabinets for some other projekt

That may well be the best idea

I might put the Axiom 201s in U-frames . .

zobsky,
What are the benefits of non-parallel, and slightly different dimensions ?

These drivers are pre Thiele Small, so Qts is not not known, except that (having a few drivers) I just ordered Woofer Tester 3 (from Parts Express) . .

Cheers