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Fazorcat 5th February 2005 10:04 PM

Passive Radiator
 
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?

Cal Weldon 6th February 2005 04:37 PM

I think that would be better put that the passive radiator should be able to move more air than the woofer. The 8 inch moves a certain amount of air. The PR being that much larger only has to move about 1/2 - 2/3 as far to have moved the same amount of air.

To check the sealing of your cabinet, push the radiator all the way in. As you do this, the woofer will move out. Hold the PR in place and watch to see if your woofer returns to it's postion or stays in the out position. If it stays ( for 10 seconds or so), your cabinet is sealed. If it slowly returns to center (less than 5 seconds), then your cabinet is leaking. That is if your drivers are fully sealed and are suited for a PR system.

Cal

John_E_Janowitz 7th February 2005 06:29 AM

To do a PR system correctly, your PR's need to be able to displace 2x-4x the volume that the active driver does. A good general rule is to use a pair of 15" PR's for a single 12" woofer, pair of 12" PR's for a single 10", etc.

Now in your design, the PR essentially works like a port. At and around Fb, the tuning frequency of the box, the PR will do most of the work and the woofer will barely move. At 1/3 octave above Fb, the woofer will be at it's peak in excursion.

A PR system is the most critcal of any to have well sealed. If it is not well sealed you kill your bottom impedance peak, and the added output from the PR's is greatly decreased.

For some general PR info you can look at the FAQ I put up a long time ago: http://www.stryke.com/prfaq.htm

John

BillFitzmaurice 7th February 2005 12:56 PM

The only time that you'd notice the PR moving further than the driver cone is at the system resonant frequency, and you'd have to be sending it a sine wave signal at that frequency. Throughout the rest of the pass band the PR and cone will displace about the same amount of air, and since your PR is larger than your cone it has to move less than the cone does to do so.

Fazorcat 7th February 2005 05:44 PM

Ok, Thanks for the Info. You guys ROCK!

Cal Weldon 7th February 2005 09:38 PM

Quote:

Originally posted by John_E_Janowitz
To do a PR system correctly, your PR's need to be able to displace 2x-4x the volume that the active driver does. A good general rule is to use a pair of 15" PR's for a single 12" woofer, pair of 12" PR's for a single 10", etc.
The rule of thumb that I was brought up on was that for one 12" active use two 12" drones or one 15" The potential displacement was to be approx double what the active driver was. You have stated 2 - 4 times the displacement but given only examples of 4X.

Cal

simon5 7th February 2005 09:56 PM

I would like to know why we must have 2 times the displacement?

If you got like 1 times and a quarter, why it's not enough? You will never use all that excursion anyway?

xstephanx 7th February 2005 10:02 PM

becuase a PR is more efficent than a woofer( I believe, but I'm probably wrong). and will displace mroe than the woofer at tuning. assuming they have the same amount of displacement, if the woofers excursion is reduced by 50% at the Fb, then that means that you can only use half that woofers output at Fb, which is slightly limiting IMO. Plus you want the PRs movement to be as linear as possable.

Cal Weldon 7th February 2005 11:29 PM

Quote:

Originally posted by simon5
I would like to know why we must have 2 times the displacement?
In days of old we didn't. The first passive system I owned was a JBL LE8T with an identical, purpose made drone. Perhaps as we went along, the need for a larger area was realized. Perhaps it will continue. Perhaps John was right a couple of posts back when he gave the example of close to four times the area. ie: one 12 and two 15's

Cal

simon5 7th February 2005 11:36 PM

I tested it in WinISD, and for an unknown reason, John is right.

I guess there's something behind that, probably not totally connected to displacement, but when you give more displacement, the frequency response curve looks much much better.

I used an Ascendant Audio Avalanche 18", to get a good FR curve I used three 18" PRs with 38 mm Xmax.


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