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Old 11th December 2006, 11:35 AM   #1
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Default Transient Response: Sealed v Dipole

I've accumulated a few different drivers and some leave, that I hope to take soon to make some progress on a backlog of projects. I was going to build a horn midbass (about 100 - 500 Hz), but belatedly realize I don't have the time for such complexity, for quite some time . .

Above all I'm looking for whatever gives the tightest, best transient midbass response . Also likely to move to a tube amp, so speaker damping becomes more critical. I'll develop the rest of the system to be coherent from that starting base.

With sealed aligmnents you can handily predict or "choose" step response: varying box volume varies the damping 'spring'. If using a driver in a sealed box, I'd aim for a Qtc of about 0.55.

Using the same driver in a dipole (I'm not buying any more drivers!), the operating passband moves up a little, which needs to be covered by the sub's range going higher, depending on baffle size; room modes are minimized, etc.

Widely acknowledged differences aside, how does the transient response of dipoles compare with sealed boxes?

Can either the Edge or MJK's worksheet calculate how step response relates to driver parameters or baffle size? Or over a certain range, is system Q in a dipole = driver Qts??

Thanks,
Richard
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Old 11th December 2006, 03:26 PM   #2
MBK is offline MBK  Singapore
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Technically, the transient response depends on the damping of the final rolloff characteristic and its filter order, and this means if your driver rolloff is equalized (dipole) or achieved by box design (sealed) to give the same number and order, and cutoff frequency, the transient response should be identical. Subjective differences will exist, however.

But if you use a sub anyway below the dipole, your transient response and filter order of the midbass will simply reflect whatever crossover you chose for it. What the driver does outside its passband will not affect your Q anymore: your Q will be your crossover Q.
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Old 11th December 2006, 08:02 PM   #3
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Quote:
Originally posted by MBK
But if you use a sub anyway below the dipole, your transient response and filter order of the midbass will simply reflect whatever crossover you chose for it. What the driver does outside its passband will not affect your Q anymore: your Q will be your crossover Q.
It's a different forum, but it's like an echo!

The above is true unless your crossover overlaps the driver / box rolloff region. That's the big gotcha, Rick, as has been mentioned to you elsewhere. The filters will add and your crossover to the sub will not sum properly unless you take this into consideration.
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Old 12th December 2006, 03:51 AM   #4
MBK is offline MBK  Singapore
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I second that - and you may of course use the driver's rolloff, the dipole rolloff, or both, as a part of your crossover design. This implies measurement .
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Old 12th December 2006, 05:26 AM   #5
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So if I understand this correctly, the step response around the crossover is determined by the Q of the crossover slope.


If the dipole’s baffle is big enough, and response goes low enough – allowing a crossover at eg 80 Hz, and the resonance of the dipole baffle is in its passband but far enough away (eg 340 Hz), the Q of the XO is not relevant, so the Q then is . . the Qts of the driver?

For a flat baffle for midbass, is this the relationship between baffle size and resonance:

resonance corresponds to the shortest wavelength around the baffle, eg a 1 m (c 3.3 feet) wide baffle, ie 1 m from the front of the driver to the back of the driver, 1 m wavelength ie the resonance would be at 343 Hz?

Thanks
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