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Old 16th January 2009, 03:14 AM   #1
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Default Mini Passive Radiators

Hello,
I'm currently making a set of speakers using Peerless 830887 midwoofers and 830880 passive radiators. They are 5.25".
I've heard that it's quite hard to design a good enclosure for PRs, especially seeing the weight discs are not adjustable on the 830880.
Could someone here with experience of knowledge of passive radiators be able to give me some tips to designing a good enclosure?
The Tymphany site has a page dedicated to the PR applications, but it's not very informative; just a bunch of specs.

I'm mostly wanting to know how PRs respond to enclosure size and if i still need to 'sound deaden' the enclosure. Also if there are other things i need to consider which i don't know of.

Any help would be appreciated.
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Old 16th January 2009, 03:45 AM   #2
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The PR is simply best thought of as a different way to do a ported enclosure.

If you tune the PR below the frequency that a good simulation program says that it wants to be, it will still work, but it will not give you a flat response. In general terms you can more easily make the PR tune below the normal box + driver rolloff/resonance point than a typical port. In some cases that is good.

The absorption of the walls of the box generally will not effect the LF response, so will not effect the PR operation. At some point if you absorb too much rear energy, or stuff the box (for example) which is a bad idea, the PR will not work properly. You want to set up the driver as you would if it didn't have a PR, in terms of the majority of the response - including whatever you'd normally do to the internal walls - except for reducing the intended Vb, and blocking the path to the PR...

Tuning the PR in the box depends almost entirely upon the mass of the PR. (of course the other parameters like compliance and Xmax count, but we're talking about tuning it in practice) You have to tune the PR in the box, you can't rely upon the supplied mass loading unless you are duplicating an alignment that is known to work with that mass loading, and do it rather precisely. The alignment consists of the driver's actual real world parameters and the real world actual volume of the enclosure (not the imagined volume), plus the PR's parameters.

So, you have to be prepared to find a way to add mass to the PR, if needed - and it usually is. As long as the mass doesn't move, and it is firmly bonded to the cone, you can add it any where that is balanced and easy to do. You can often just pry the dustcap up and then reglue... fyi.

The midwoofer, won't go low, unless it is a woofer, not a midwoofer. You can't get something from nothing. That means the Qt & VAS + box volume have to "work" at some low freq, if that is where the PR is tuned. (I haven't looked at ur drivers specs).

Then too there are "box losses" in a PR set up these count big time. Aka "Ql"...

At relatively high Fpr, the excursions are small, but as you get into the true bass freqs - below 40Hz. - the exursions get large quickly. In these situations using a PR that is the same size as the main driver is not a good idea. Usually one or two diameter sizes up is a good idea.
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