I'm still learning-- passive radiator question

[brad347]

Hey. This is probably a stupid question, but here goes (be kind).

I understand the basic principle of the passive radiator, or at least I feel like I do. But there's one thing I don't quite get.

With the excursion of the driver, the passive radiator would move in the opposite direction, correct? When the driver goes 'out,' the PR goes 'in,' right? I assume that, in addition to adjusting for cabinet volume, the PR will also produce some sound as it moves in inverse proportion to the driver.

So effectively, wouldn't they be acting in opposite polarity (i.e. 'out of phase') with respect to one another? How does this keep from harming the overall sound due to destructive interference?

Is this why some PRs are mounted on the back of the cabinet?

Maybe someone can clue me in. Thanks!!

[Hoerninger]

How does this keep from harming the overall sound due to destructive interference?

In the working area it is not destructive.
It is not the passive radiator alone, it performs with the box volume as a resonant circuit which must be tuned correctly.

Hope this helps.
___________
Peter

[jcrobso]

The passive radiator is a variation of the bass reflex, for lack of a better term.
Yes it is a sealed box, but the mass of the radiator and the volume of the box form a tuned circuit. At the band pass of the circuit there will be 180deg phase inversion and the radiator will move in phase with the woofer cone.
This the same principal as a bass reflex port.
Above the band pass tuning the radiator mostly stops moving.
I have not listened to a system with a PR in about 35 years, so I don't remember exactly how they sounded. But over the years T&S box tunings have taken over the market.
NOTE: the box must be very air tight!

[grumpy]

... and below band, the PR does as the OP initially posited, causing the roll-off to
be steeper than for a sealed box (yellow=sealed, tuned port=cyan, PR=pink):

[brad347]

At the band pass of the circuit there will be 180deg phase inversion and the radiator will move in phase with the woofer cone.

OK, is there any way to explain exactly why this happens?

Maybe I just need to do some more reading on bass reflex designs.

[grumpy]

http://www.phys.unsw.edu.au/jw/Helmholtz.html

http://en.wikipedia.org/wiki/Passive_radiator_(speaker)

http://en.wikipedia.org/wiki/Reflex_port

the speaker is driving a mass/spring system, and at resonance (freq where the
port/box or PR/box would "ring" if hit with an impulse), the mass will vibrate
in sympathy (in phase) with the exciting force.

[brad347]

thanks for the links, will read.