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Old 18th January 2006, 12:24 AM   #1
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Default request for an explanation of "Q"

i read that the speakers Q is its damping factor, or something like that....and that low Q speakers (<.3) should be in ported boxes, and that high Q (>.7) should be in sealed enclosures...

i know when one is designing a box, he can alter the Q he wants the system to be, i hear .707 is a popular Q because it is in the middle....

since the designer gets to essentially "choose" which Q value he wants, is this the new overall damping of the system or just the damping of the enclosure? does a high Q enclosure damp the speaker more than a low Q enclosure does?
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Old 18th January 2006, 12:47 AM   #2
SY is offline SY  United States
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There's a pile of different parameters called Q, and they all have to be kept straight. Sorry!

Basically, Q is the inverse of damping. You can get a rigorous definition of the meaning of Q in general from any physics text that covers vibrations and waves. The definitions of Qes, Qms, Qts, Qtc, Qb, Ql, and the various other members of the Q family specific to loudspeakers can be gotten out of a speaker design text; if you don't have a copy of Dickerson's Loudspeaker Design Cookbook, run right out and get it.
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Old 18th January 2006, 12:51 AM   #3
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That's Vance D-i-c-k-a-s-o-n. Sorry about the censor.
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Old 18th January 2006, 12:53 AM   #4
poobah is offline poobah  United States
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Q is about losses in a resonant system. The higher the Q, the lower the losses, and the higher the resonance.

Tap a good wine glass on the rim... it sings a note, one note, and keeps singing for awhile. This high Q. It has low losses, the energy just keeps "ringing" until all the energy in the glass radiates out as sound. Now this wouldn't make a good speaker because anytime the music hit that note... the speaker would wig out. In this case you want a speaker box to combine with the speaker to lower the Q.

Now tap a plastic cup up on the rim... thunk. This is low Q system, the energy you added was rapidly dissipated (lost) in the material itself. Now this could make a good speaker, but you certainly would not have to go crazy damping it with an exclosure.

Now you have a real-life-physics-based-"feel" for Q. In many things, high Q is an indicator of quality... not so simple with speakers... Q would have more to do with "flavor" or "type". Don't equate low Q with low quality... speakers are inherently low efficiency devices... 3% give or take a few.

The speaker savant's will come along now and confuse the bujeesus out of you; and save you from ramblings such as this.

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Old 18th January 2006, 01:08 AM   #5
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Quote:
Originally posted by poobah
Don't equate low Q with low quality
No, never would. High Q is often the result of a small motor.
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Old 18th January 2006, 03:17 AM   #6
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good replies...thanks to all!
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Old 18th January 2006, 03:27 AM   #7
poobah is offline poobah  United States
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Good question,

You are most welcome!
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Old 18th January 2006, 04:30 AM   #8
zenon is offline zenon  Canada
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I just googled this. it has an interesting table summarizing the popular alignments.
http://www.winebase.com.au/audio/avsbdm.htm
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Old 18th January 2006, 10:20 PM   #9
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The reason a vented box needs a lower Q (higher damping) than sealed is because the latter's motor only has to control the driver mass, while in a vented design it's the driver mass plus the additional mass of air in the vent (or PR).
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Old 19th January 2006, 02:34 AM   #10
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And the higher the acceleration of the cone, the greater the seeming 'mass' of the air (air in the box and in the port) that has been set in motion by the driver. Thus the importanceof the low Q driver when designing ported systems. That is a 'general' rule though, there is no reason, other than it being a general truism. I've had plenty of results that end up showing another result, as have others, I'm sure.

Now, once that has been understood, it should be noted that the resonance (q) of a port/box (both are designed as a mated set, ie the port resonance is a part of the box - as a 'design') should not be too close to the resonance ( Hz) (q) of the driver. If both are the same, then.. the woofer my end up exciting itself to death. PSB did this a while back, with the 500 series of speakers. And ended up replacing many a driver. ie, thousands. Ouch. sorry to use you as an example, Paul.

Oh yeah. The fact that air can be compressed and act like a 'spring', is what allows it to be analysed with the idea of it (in a sealed box - a ported box is similar but a bit different --in analysis), as a specific volume, in a box, to have a 'Q'. No spring---no 'Q'.
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