Passive radiators

[Kernowdragon]

Hi all, excuse my ignorance but I'd appriciate your help. I'm looking to built a sub using a 15" driver I have, but I also have a pair of 8" PR's sitting around. The question is... given that the surface area of the driver is almost the same as 2x 8" PR's could I built the cabinet to make use of the PR's?

 

[Cal Weldon]

Hi KD,
Four 8" drivers = one 15"
You need a minimum of equal cone size for a PR and it's better to have 2x so...
For one 15" driver you would need eight 8" drivers and...
We haven't talked about mass, compliance or excursion yet.

 

[weltersys]

As a general rule, the passive radiator(s) should have at least double the displacement of air (Vd) as the active woofer(s) in the system.
Vd= Xmax times Sd (cone area). Passive radiators may be rated in Xmech rather than Xmax. When a passive radiator reaches it's limits, it will usually make some sort of a bad, loud noise ("whup, whup") unlike ports, which make less audible "chuffing" wind noise.

As you can see in the example below, 2x8" would have about 5 dB less output at 36 Hz than a single 15" with the same excursion.

 

[Kernowdragon]

Thanks Guys, that's a clear no then. So ideally I need to find a couple of 15" PR's then, looking at something along the lines of the Dayton Audio cube design.

 

[Cal Weldon]

Yes two 15" PR's are what you are looking for.

 

[weltersys]

Or one 15" PR with the displacement of two 15":
Dayton Audio's RSS390-PR 15" passive radiator has 60 mm peak to peak displacement, depending on the 15" Kernowdragon has, that may be plenty.

I had a Mackie HRS120 that used a 12" passive radiator with far less excursion potential than the RSS390-PR 15", and it was enough to make the drywall flap between 15-20 Hz ;^).

 

[schiirrn]

So the drywall acted as another passive radiator?

 

[weltersys]

That's one way of looking at it- when the room walls and floor move, it's another resonant system working.

 

[Cal Weldon]

All things equal, double the woofer will lead you to no harm.

 

[chris661]

All things equal, double the woofer will lead you to no harm.

Well, you specifically want double the displacement.

I'd generally aim to use an even number of PRs, though, and arrange them for force cancellation. The moving mass can be very large, so there's a very real risk of the cabinet going for a walk when large signals are applied.

Chris