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.
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.
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.
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.
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.
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.
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.
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?
_-_-bear
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? _-_-bear
It doesn't matter what the Xlin of the driver is (why does the Xmech matter?). What does matter is what the sim says about the excursion of the PR at expected power inputs.
Hi,
Talking about sims, one of the nice things about Jeff Bagby's "Woofer Box Model and Circuit Designer" is that you can just change the "No of PR's", and see the results, e.g.: in the Excursion graph. This software also gives you access to digital filters. It's my first choice for a passive radiator system.
Regards,
Talking about sims, one of the nice things about Jeff Bagby's "Woofer Box Model and Circuit Designer" is that you can just change the "No of PR's", and see the results, e.g.: in the Excursion graph. This software also gives you access to digital filters. It's my first choice for a passive radiator system.
Regards,
Xmech matters of the active driver since if you ever hit it, it moves the PR even farther. I feel better knowing that if my family gets a little too wild and hits Xmech on the active speaker, it won't damage the PR.
The Xmech on my 15" iso woofers is around 10 to 12mm, my 18" PR has an Xmech of 80mm--no worries! Sure, the active woofers can get blown but then I have an excuse to get higher Xmax woofers the next time. A LAB15 should not overdrive the Mal-X PR so I have a built-in upgrade path.
Is it possible for your PRs to hit Xmech in the system that it's in? Poorly designed, if it is...
The lowest I've ever ran my PR under test tone conditions was 10Hz
The active speaker was protesting, my house was protesting but the Maelstrom X18 laughed at it all. I'm sure I could get more movement out of it with a LAB15 at full power but even then it won't get near the limits of the thing. I seem to recall that Exodus Audio listed the Xmax at 37mm one-way.
There are sites on the web that give general guidelines for PRs, these are what I went by although my Vd of the PR is 8 times higher than the active woofer--nice to have extra I guess.
In theory, the Vd (volume displaced by PR) should be at least 2 times the Vd of the driver it is being used with. Yet in practice, a good rule of thumb to go by is to have anywhere from 3-4 times as much displacement in the PR. This is to ensure the longevity of the PR's by preventing excessive continual over-excursion.
Advantages
Simplicity of tuning. By merely adding and removing small amounts of mass, the tuning frequency of the enclosure may be changed up or down by as much as 15 Hz or as little as 0.1 Hz. Precision tuning is very possible. Ability to tune small enclosures to very low frequencies without the loss of volume due to internal ports taking up enormous amounts of precious space. No port noise or any kind of air-turbulence of air speed levels to worry about. Pipe resonances and port standing waves are non-existent because there are no ports or vents in this system. Better driver stability below fB due to increased damping on the driver below fB. This is because of specific compliance characteristics of the PR which help to keep the driver under better control at subsonic frequencies.
The active speaker was protesting, my house was protesting but the Maelstrom X18 laughed at it all. I'm sure I could get more movement out of it with a LAB15 at full power but even then it won't get near the limits of the thing. I seem to recall that Exodus Audio listed the Xmax at 37mm one-way.
There are sites on the web that give general guidelines for PRs, these are what I went by although my Vd of the PR is 8 times higher than the active woofer--nice to have extra I guess.
In theory, the Vd (volume displaced by PR) should be at least 2 times the Vd of the driver it is being used with. Yet in practice, a good rule of thumb to go by is to have anywhere from 3-4 times as much displacement in the PR. This is to ensure the longevity of the PR's by preventing excessive continual over-excursion.
Advantages
Simplicity of tuning. By merely adding and removing small amounts of mass, the tuning frequency of the enclosure may be changed up or down by as much as 15 Hz or as little as 0.1 Hz. Precision tuning is very possible. Ability to tune small enclosures to very low frequencies without the loss of volume due to internal ports taking up enormous amounts of precious space. No port noise or any kind of air-turbulence of air speed levels to worry about. Pipe resonances and port standing waves are non-existent because there are no ports or vents in this system. Better driver stability below fB due to increased damping on the driver below fB. This is because of specific compliance characteristics of the PR which help to keep the driver under better control at subsonic frequencies.
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