Why aren't there more very high excursion PR's?

bcodemz

Member
2014-02-21 1:19 am
I'm looking to make a mini sub, and I'm wishing there are 5'' passive radiators that have 10-15mm of excursion. Instead, the Peerless and Dayton passive radiators have a rather pathetic 6-8mm of excursion. 10-15mm of excursion is clearly doable. The 3.5'' Dayton ND90 has 12.5mm of excursion. The TB W5-1138SM has 9.25mm of xmax, and probably at least 15mm of xmech.

To any driver designers out there, is there a reason why there aren't that many very high excursion passive radiators? With a passive radiator, there isn't a need to worry about distortion, so wouldn't all the PR need are a very wide surround and the proper suspension to accommodate the excursion?
 
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bcodemz

Member
2014-02-21 1:19 am
You could, but why not just have higher excursion PR's?

With high excursion woofers, you need 3 to 4 normal PR's of the same size for each woofer. By doubling the excursion of a PR, you reduce the number of PR's needed by half, which saves a lot of space. Also, the amount of mass needed would be greatly reduced, which is not insignificant when you have 3 or 4 PR's in a tiny box (not unrealistic since that's the whole point of using PR's), and the mass requirements become very high.
 
At resonance almost all the output comes from the PR, meaning it's swinging pretty well while the active driver is almost completely braked. Because PR -3dB output is around 6 dB up from what would be sealed driver rolloff, the PR does have to have Way more displacement capability. You can get it either in diameter increase or excursion increase. In the old days when 6mm was a lot of excursion for anything, the PR was often chosen one or two sizes larger. Now, with crazy high excursion from woofer drivers, you still have the same choices. Either you employ the same increased excursion handling techniques to the PR, or you use more, bigger conventionally designed passives. One problem with extreme excursion in a PR is it has to handle a lot more static radial load than a driver, from the tuning mass. A suspension that has the same maximum radial load capacity but many times more excursion capacity is going to be taxing on materials, probably leading to things like multiple spiders and probably added resistive loss. All in all, unless you have a serious box surface area problem, you're better off with more, big PRs. I've seen 3 18" PRs used with one extreme excursion 18" driver.
 
How do they get a passive radiator in a sub to be in phase with the main speaker at any more than a very narrow band of frequencies? Surely they must leech energy from the main speaker which would mean it makes less SPL.,

Pretty much the same way bass reflex works, you are just swapping the tuned air slug in the port tube with a tuned passive radiator.

Being tuned correctly both resonate in sympathy with the active driver if excited by it via the air enclosed in the box.
 
Hi,

Perhaps all your assumptions are not right and there is more to it.
The ND90 has 4mm linear excursion and parameters not suited to PR's.

You can be sure generally manufacturers know what they are doing.

rgds, sreten.

The OP knows this, what they are saying is why can't a large excursion passive radiator exist if a driver that has 12.5mm of safe movement is available (even if the motors xmax craps out after 4). Ie take the ND90 remove the motor and tune the spiders stiffness for a reasonable value of Qm. Certainly this would be a very inexpensive passive radiator with very nice excursion capabilities.

Given that most 3" drivers are likely to only have 3-4mm of actual xmax it would therefore only require one ND90 PR per driver instead of the usual 2 PRs.
 

bcodemz

Member
2014-02-21 1:19 am
At resonance almost all the output comes from the PR, meaning it's swinging pretty well while the active driver is almost completely braked. Because PR -3dB output is around 6 dB up from what would be sealed driver rolloff, the PR does have to have Way more displacement capability. You can get it either in diameter increase or excursion increase. In the old days when 6mm was a lot of excursion for anything, the PR was often chosen one or two sizes larger. Now, with crazy high excursion from woofer drivers, you still have the same choices. Either you employ the same increased excursion handling techniques to the PR, or you use more, bigger conventionally designed passives. One problem with extreme excursion in a PR is it has to handle a lot more static radial load than a driver, from the tuning mass. A suspension that has the same maximum radial load capacity but many times more excursion capacity is going to be taxing on materials, probably leading to things like multiple spiders and probably added resistive loss. All in all, unless you have a serious box surface area problem, you're better off with more, big PRs. I've seen 3 18" PRs used with one extreme excursion 18" driver.

Could you expand on this a little bit, especially the static radial load part? This is great information.
 

Erknie

Member
2016-02-25 11:27 pm
I'm looking to make a mini sub, and I'm wishing there are 5'' passive radiators that have 10-15mm of excursion. Instead, the Peerless and Dayton passive radiators have a rather pathetic 6-8mm of excursion. 10-15mm of excursion is clearly doable. The 3.5'' Dayton ND90 has 12.5mm of excursion. The TB W5-1138SM has 9.25mm of xmax, and probably at least 15mm of xmech.

To any driver designers out there, is there a reason why there aren't that many very high excursion passive radiators? With a passive radiator, there isn't a need to worry about distortion, so wouldn't all the PR need are a very wide surround and the proper suspension to accommodate the excursion?

The passive radiator thread says you are supposed to use a PR that is twice the surface area of the bass speaker, at the same xmax, if the speaker has a higher xmax then you need an even larger PR.

http://www.diyaudio.com/forums/full-range/262762-selecting-passive-radiators.html
As someone mentioned the well known rule of thumb is that if the PR is of the same nominal diameter as the driven driver (e.g. woofer) then you should use two of them. This should really be restated in terms of total displacement, Sd*Xmax. In total the PRs should have at least TWICE the total displacement as the woofer. There really isn't any reason why one must use a PR with the same diameter as the woofer.
If you want to keep the box small then don't use a PR.
 
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