Vas, cms, etc... for Dipoles

[Raka]

Is there any clearly unsuitable TS figure that invalidates a driver for openbaffle use?
I mean, could be considered as suitable to move in free air a driver optimized for small BR boxes? What if the suspensions needs the compliance of some enclosed air?

[anatech]

Hi Raka,
For open air use, I'd be looking at Qts values over 0.6 and maybe higher. Anything lower than that really needs an enclosure. A Qts of 0.707 would be optimally damped, higher values start bringing a "hump" in the response at resonance. You want to stay below 1.0 I would think.

-Chris

[Raka]

You are talking about equalization and frecuency response, but I mean the other parameters and performance of the suspension.

[anatech]

Hi Raka,
This is all interactive unless you are applying equalization.

I may be misunderstanding your question. A stiff suspension will make for a low compliance and a high Qts. This is what I had in mind.

-Chris

[Raka]

And what happens to a woofer with Qts higher than 5, but an electrical Q of 0.35?

[anatech]

Hi Raka,
Do you mean Qms = 5? Qts is the total electrical / mechanical compostite. There is only so large a magnet that can be used to damp the cone, so at some point the mechanical Q will be so large that the total Q becomes over 1. At that point you are talking about electronic EQ and possibly motional feedback systems.

-Chris

[Raka]

yes, sorry, I meant Qms = 5

[anatech]

Hi Raka,
Okay, (0.35*5.00)/(0.35+5.00) is about 0.327. So Qts is 0.327. This speaker might be happy in a sealed box depending on it's Fs, or a ported box. In open air it would be overdamped I would think. I'd have to go back through my notes from years ago to see if those are reasonable figures or not for a speaker.

A speaker with a Qts of 0.383 is crying for a classic B4 ported alignment. Just as a point of reference.

-Chris

[kelticwizard]

Well, a woofer with a Qts = 0.33 will be 9.6 dB down at Fs.

On the other hand, if Fs =20 Hz, and you were expecting the woofer to go down to only 50 Hz, that would seem to be okay. If I were going to go open baffle, I would purposely look for woofers with large Vas, since they would be more efficient for the same Qts and Fs than drivers with smaller Vas. And open baffle does not care about a driver's Vas.

Finally, I would point out that some years ago, Carver came out with an open baffle subwoofer for his ribbon speaker design. He purposely made the Qts of the drivers very, very high-like 2.0 or thereabouts-so that the peak in output would compensate for natural rolloff when the low bass notes come around from the back and cancel the front wave. I imagine he made the Vas very large as well, though I have no information on that.

[Raka]

Originally posted by kelticwizard
And open baffle does not care about a driver's Vas.

That's the point I'm trying to investigate, but I'm not sure that an open baffle considers irrelevant the Vas data. For your information, my dipoles are almost finished and correctly equalized, I'm in the phase of listening to music, and get familiar with the things that do well (a lot) and the points to improve (not much )