Low Qe or High Qe?

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Raoul

Member
2003-02-03 8:39 am
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Hi Lars,

Dickason said of driver selection in the Loudspeaker Cookbook:

"In terms of driver Qts, closed-box loudspeakers generally require woofers with a fairly high Qts of greater than 0.3..."

(of vented system) "...only Qts between 0.2-0.5 generally provide satisfactory responses."

Not sure if that helps, but there you go.
 
Qts<0.3 hornloading

0.3<Qts<0.4 basreflex

0.4<Qts<0.55 closed box

Qts>0.55 infinite baffle/dipole

It isn't stricktly like that. Things overlap and depend on other T/S-parameters as well (like Fs). Fs/Qts gives Efficiency Bandwith Product (EBP) which is also used as a rule of thumb.

Qts combined with Xmax gives an good idea what kind of quality (sub)speaker you're dealing with, high Qts , low Xmax being the worse.

Simulating will sometimes show supprising facts, so don't go totally out on rules-of-thumb anyway.

Mvg Johan
 
Thanks Guys! I did some simulations and it seems if i take a Qts 0.7 and put in a too small closed box, it will peak severely at 100-200 Hz. But if i take a low Qts (0.4) it will only peak 1-2 dB. But the low Qts unit will also give a slightly higher spl than the high Qts unit. Is this in compliance with reality?

The project in question is a flat panel subwoofer with 64 drivers 6 inch each. Each in closed boxes of 4 liters. The size is 2 x 2 meters but only 6 inches in depth. (A wall of bass!!)

The simulator program (Waldo) shows 105,6 dB / 1 Watt (100 Hz up) and -18 dB at 35 Hz. I think the large baffle will give some help on the -18 dB. (???)
So i'm going to compensate the -18 dB (or whatever) with a gainer stage, and some power. The total loadability of the 64 units is 6 kW so with the right amplifier it can be made linear to the 35 Hz, and with 40W per driver a spl of 139 dB should be possible. (Total 2.5 kW). (log(2500) x 10 + 105.6) = 139 dB
 
But the low Qts unit will also give a slightly higher spl than the high Qts unit. Is this in compliance with reality?

Yes, a lower Qts indicates a more efficient speaker. But also the lower the Qts becomes, the less it's suited for low-frequency reproduction.

I don't know what on earth you would need a SPL of 139 dB for, except bassheavy parties starting at 200 people outside or so. So what is the intended use?

Mvg Johan
 
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