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Old 10th January 2012, 10:39 PM   #1
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Default Passive radiators

What types of non-linearities do PRs exhibit in typical system, and are they noticeable problems?

A driver which has significantly non-linear Cms/Kms exhibits HD; I do not see much data about PRs which have non-linear suspensions.
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Old 11th January 2012, 12:24 AM   #2
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I would think they would exhibit the same non-linearities as the woofer they were modeled after. Since you are you using a larger radiating area than the woofer(s) itself, it may be a non-issue. ie: 10" woofer, 12" drone, the drone will tend to move less and exhibit less non-pistonic action than the woofer if they are of similar ilk, which using similar units only makes sense when you are designing a PR system.
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Old 11th January 2012, 05:25 AM   #3
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Well, it would depend on the excursion of the PR, of course, which changes depending on power input, frequency, and box tune.

I doubt it matters whose PR you use as long as it's built well and has a linear suspension.

Perhaps the lesson here is to go supersize for PR diameter? Use an 18" for a 12" driver, 2x18" for a 15", 3x18" for an 18"? I've always felt there was a tradeoff between vent noise and PR HD.
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Old 11th January 2012, 07:29 PM   #4
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The nice thing about a PR system is that the drone is being propelled by cabinet pressure rather than by the point source of a voice coil so one would take that to mean it is about as pistonic a movement as you likely to get from a driver, no?
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Old 11th January 2012, 07:48 PM   #5
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I use an 18" PR with my 15" active isobaric woofers

No problem with distortion, the PR has a 2.9 inch peak-to-peak stroke and will never even get to 40% of that unless I want to wear the active cones like a rice hat.

If you're paranoid about PR stroke, just mount a piece of wood for bracing in front of the PR weight bolt--that bolt will smack the brace to warn you of excessive excursion.
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Old 11th January 2012, 08:10 PM   #6
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Quote:
Originally Posted by Cal Weldon View Post
The nice thing about a PR system is that the drone is being propelled by cabinet pressure rather than by the point source of a voice coil so one would take that to mean it is about as pistonic a movement as you likely to get from a driver, no?
Certainly. But, the only thing this would help (maybe) is cone breakup, which is not a phenomenon to worry about since subs operate where cones are rigid.

The interesting thing is that some ribbon drivers suffer from diaphragm breakup, though you would think that because the driving force is even, there's no force that could cause the diaphragm to bend.
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Old 11th January 2012, 08:11 PM   #7
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Originally Posted by 18Hurts View Post
I use an 18" PR with my 15" active isobaric woofers

No problem with distortion, the PR has a 2.9 inch peak-to-peak stroke and will never even get to 40% of that unless I want to wear the active cones like a rice hat.

If you're paranoid about PR stroke, just mount a piece of wood for bracing in front of the PR weight bolt--that bolt will smack the brace to warn you of excessive excursion.
Part of the problem is that we have no idea whether a PR's quoted excursion is "linear" or not; that is, when the PR is excursing that much, does it sound equivalent to a well-designed vented box with zero vent noise?
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Old 11th January 2012, 08:33 PM   #8
18Hurts is offline 18Hurts  United States
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Part of the problem is that we have no idea whether a PR's quoted excursion is "linear" or not; that is, when the PR is excursing that much, does it sound equivalent to a well-designed vented box with zero vent noise?
I just use the spec sheets that my PR is based from.

My 18" PR is based on the Exodus Audio Maelstrom X18

Xmax = 33mm
Xmech = 40mm

If it bothered me, I'd say I have 66mm of linear stroke to play with--it is rated higher than that as a PR in reality.

It would take a 15" sub with a Xmech of 36mm to equal my PR... no worries there!

Does it sound like a well designed ported box with zero vent noise? No, it does not. PRs tend to have their "own sound" since they don't pass anything but the tuned frequency they create and have a steeper roll off below the tuning frequency.

My PR is tuned to 21Hz so the sub has a distinctive sound quality to the thing. Reminds me of a sealed/ported mix in a way. One of the reasons for the very smooth sound is I tuned it with a SPL meter, test tones and weights in it's fixed location. Once I hit 20 to 40Hz +/- 2dB, I quit messing around with the weights.

The reason for the PR was to conserve space at such a low tuning and I've always liked the sound of them. No spider webs to clean out of the ports is a nice bonus.
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Old 11th January 2012, 09:05 PM   #9
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With a PR, the LESS of the Xmax used the better. Another issue is that they are mass loaded. As you go lower in freq, the mass gets WAY higher. This puts stresses on the suspension, and makes it more difficult to stop.

While it is certainly true that the air in the box is powering the PR, it is completely incorrect to think for a second that that air pressure is either linear or that the force or pressure is even across the surface of the PR. It's very not. The PR in essence integrates the nodes that appear over the surface of the PR turning them into more or less linear motion.

When it comes time to stop the passive, what stops it? (rhetorical question, of course)
It's going to be the air in the box and the suspension. EVEN assuming the driver came to a complete FROZEN stop, the PR simply is not going to do that, and with a whole lot of added mass (trying to go subterranean in frequency) this job becomes much harder.

In PR loaded enclosures, QL becomes an important factor that should not be overlooked.

The best idea is to use the largest PR you can manage, or get the largest surface area.
IF you use more than one PR, then a symmetrical mounting is best, so that the energy on each PR is about equal. Two on adjacent walls is probably not so great because they will "see" different pressure nodes in the box.

I like the 12" -->18" PR. But you have to take into account the decreased volume in the cabinet if you use the usual PR with a basket.

Mounting the PR facing up or down is going to mess with the suspension.

But as far as HD is concerned, the PR is acting like a high Q BP filter, so except to the extent that the cone (assuming a cone) has breakup modes at LF or there is higher freq energy from the main driver causing standing waves on the surface of the cone, it ought to be rather low in 2nd order and higher output.

Fwiw, the PRs I build for my Quadripole subwoofers can not have any cone break up whatsoever. They're not cones. Problem solved?

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Old 11th January 2012, 09:21 PM   #10
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How can there be "nodes" when the sound waves inside the box are of frequencies too low to do anything but pressurize the box? I mean, you even see the effect inside a room, let alone a box hundreds or thousands of times smaller.
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