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Old 8th February 2005, 09:37 PM   #21
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The air inside the box is like a spring. Think of a weight hanging on the end of a "slinky" toy spring. If you shake the top, the bottom does not follow exactly, and they way it moves changes with the frequency you shake it.
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Old 9th February 2005, 04:14 PM   #22
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The driver produces heat but that has nothing to do with why polyfill or wool works.

Both ported enclosures and PR enclosures don't want or need "stuffing". Wall absorption of higher freqs, sure.

The port and the PR are very similar - they are both "transformers", impedance transformers in fact.

The air in the port is not "light" at all, in fact it is pretty darn stiff at resonance, which is why it works.

The "backwave" of the speaker isn't what drives the port or the PR at least not the way you'd think. It's a "phase inverter" set up. That means that when the driver moves in, the PR or Port also moves IN!

Within the linear range of the PR, neither the PR nor the Port is particularly able to "move" more air. In fact in some cases the PR is more effective since it starts with a larger surface area, meaning the velocity of the air being moved can be lower - closer to a 1:1 ratio (perfect match).
(ie. better radiation impedance)

Also the PR has no problem with "chuffing" or port noise - it has other problems.

At resonance both the Port case and the PR case have the strong short motion of the driver being converted to weaker long motion... the driver cone "stands still." It's a phase inverting "lever" in concept...
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Old 9th February 2005, 05:47 PM   #23
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Default more questions about passives

Hi ! does anybody know if there is something to gain whith two instead of one passive radiator ,if the size is relative to the driver. say you have a 10" peerless xls and a 12" passive or a 10"perless and two 10"passives? most bang for the buck but sound quality is the most importent.
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Old 9th February 2005, 08:20 PM   #24
Svante is offline Svante  Sweden
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OK, guys, here is a plot of a typical PR system.

The black curve is the response. It is very similar to a bass-reflex system down to some 30 Hz in this case.

The red curves are the amplitude of the driver (solid) and the PR (dashed).
-The resonance between the mass of the PR and the compliance of the air in the box occurs at about 37 Hz. This resonance is equivalent to the helmholtz resonance of a BR box. At this frequency, the cone is almost at standstill.
-At ~13 Hz, the free air resonance ot the PR can be seen (ie the resonance between the mass and the suspension of the PR). At this frequency, the radiation from the PR and the driver is equal, but of opposite phase. This causes a dip in the response. This dip is not present in a BR system.
-It be seen that the PR moves considerably more than the driver at the PR resonance.
-It can aslo be seen that the driver moves more than the PR at higher frequencies.
-Finally, it can also be seen that the PR moves about the same as the driver at low frequencies, but in opposite phase. This last one is the only that we have any intuition for (unfortunately, since this is different from the others, and not very important).

Click the image to open in full size.

HTH
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Old 9th February 2005, 10:42 PM   #25
ume is offline ume  Sweden
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Do the grafs explain my question ? maybe im stupid ,but please try again...10" activ whith one 12" passive or 10" active whith two 10" ?do you really need two passivs?for better sound qualaty?
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Old 9th February 2005, 10:50 PM   #26
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Quote:
Originally posted by ume
Do the grafs explain my question ? maybe im stupid ,but please try again...10" activ whith one 12" passive or 10" active whith two 10" ?do you really need two passivs?for better sound qualaty?
I think that post was unrelated to your question...

Generally speaking, rule of thumb, if you use a 10" active driver, you want to go TWO "driver sizes" up for the PR. That means a 15" PR.

If you want to use two PRs (and the mounting relationship between them does matter) then you can figure out Sd for the 15" case and see how the pair of 10" drivers compare since you need at least the same surface area.

As far as sound quality, it's difficult to predict that. But generally speaking, the larger the PR, the less it has to move for the same output which usually equates to greater linearity, or "lower distortion". In practice, that may not be meaningful unless you're pushing your system up toward its max power handling & max SPL.

If ur choice is between one 12" PR and two 10" PRs the pair wins given similar Xmax ratings.

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Old 10th February 2005, 05:26 PM   #27
Svante is offline Svante  Sweden
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Quote:
Originally posted by ume
Do the grafs explain my question ? maybe im stupid ,but please try again...10" activ whith one 12" passive or 10" active whith two 10" ?do you really need two passivs?for better sound qualaty?
No, it was in response to the original question.

Quote:
Originally posted by Fazorcat
I have a question about passive radiators. I heard that in a box that has one driver and one passive radiator the PASSIVE Radiator should move more than the driver. Is that correct?

I just tryed to make one of these with an 8" driver and a 10" PR and the driver seems to be moving more than the PR. I feel like that is wrong. Is it?
It can clearly be seen in the graph that the PR moves more than the driver at certain frequencies.
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Old 12th February 2005, 03:27 PM   #28
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Quote:
Originally posted by Svante


No, it was in response to the original question.



It can clearly be seen in the graph that the PR moves more than the driver at certain frequencies.
(not sure what is what on the top graph... and I'm not sure where the Xmax graph is for the PR... you might get a more classical looking curve if you "tuned" the PR up at least an octave from where it appears to be at 13 Hz... I'm not seeing the PR's Xmax peak at its resonant frequency? )


Again, the *displacement* is greater at maximum, but the Xmax depends on the Sd of the PR. Given a large enough PR or enough PR surface area spread across multiple PRs, the Xmax may or may not be greater than the active driver...

Thus, the rule of thumb to use a PR two speaker sizes up from the active driver...

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Old 12th February 2005, 05:17 PM   #29
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The graph shows displacement (red curves) and response. The frequency that corresponds to the "tuning frequency" (let's call it fh) of a BR box is about 37 Hz, which is pretty normal, I think.

The displacement of the PR does not, typically, have a maximum at fh, even though the sound from the PR has. If you tilt the displacement curves by + 12 dB/octave, you will get the more commonly shown sound pressure curves, and the knee in the PR curve at ~37 Hz will become a peak.

Note: the displacement should be read on the "mm" axis, the "dB" scale is out of sync with that.
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Old 13th February 2005, 12:42 AM   #30
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Quote:
Originally posted by bear
... you might get a more classical looking curve if you "tuned" the PR up at least an octave from where it appears to be at 13 Hz... I'm not seeing the PR's Xmax peak at its resonant frequency?
Bear, you'd best not answer any more questions about box theory until you study a bit. Stick to them silver cables and such you are so famous for.
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