High Efficiency Speaker Asylum

Vas question

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3LockBox

How is it that a woofer, say a 12" woofer, with an FS of 21hz, a Qts of .25, and a Vas of 10.21^3', can have an optimized sealed cab of only 1.65^3', but a ported cab (tuned to FS), has to be nearly twice as big. I would have thought it would be the other way around.

Also, if the Vas requirement of two speakers is additive, does that hold true for other parameters, such as the Q factors? (I'm referring to designs employing more than one woofer in a cabinet).

Follow Ups:

RE: Vas question - djk 00:00:29 06/28/07 (1)
"Also, if the Vas requirement of two speakers is additive, does that hold true for other parameters, such as the Q factors? (I'm referring to designs employing more than one woofer in a cabinet)."

The Vas is additive, the Q remains essentially the same.

"I would have thought it would be the other way around."

Since the vented box goes lower than the sealed box it has to be bigger, the driver is the same efficiency in the mid-band, so Hoffman's Law is in order.

A good forced alignment for that driver would be 2.65 cu ft net, tuned to 26hz. A two-pole high-pass filter with a Q=2 at 26hz ahead of the power amplifier will make the system flat to 26hz. The system will be roughly 1% efficient, or about 92dB/W/1M above 40hz. If you have a good solid room, the room gain may allow for a lower Q in the filter, or no filter at all (but still recommended for driver protection).
- Addendum - djk 00:03:54 06/28/07 (0)
  Addendum:
  
  A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
  A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
  A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).
  
  The alignment given previously is a quasi 6th order.
Wouldn't this be another application of Hoffman's Iron Law? - hollowboy 17:39:00 06/27/07 (0)
That driver in a teeny sealed box will have it's Fc pushed way up: if the Qts is tripled to get a Qtc of .75, the Fs is also tripled to get a Fc of 63Hz.

The ported cab will have more bass. It would probably be shelved down a little compared to the midbass, but extension to 25Hz "beats" extension to 63Hz. Since the driver is the same, the efficiency is the same ...which means you have to "pay for" that extra low end extension with extra enclosure size.
what's the Qtc of that "optimized" sealed box - mhardy6647 16:01:20 06/27/07 (1)
A too-small sealed box will give big boom (Q > 1)... it was a common trick in de old days.

all the best,
mrh
- note that I didn't answer your question :-) - mhardy6647 16:03:02 06/27/07 (0)
  because (of course)... I don't know the answer!
  
  all the best,
  mrh

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