passive radiator q's

[MossBassist]

say, this is probably common, but it's interesting enough to be worth asking. a passive radiator works/vibrates only at a specific frequency due to inaudible vibration from the speaker, right? well what if you simply had another passive radiator tuned to a frequency lower than that of the initial radiator? would it still pick up vibration from the loudspeaker and reproduce the sound even though there was another radiator at a higher frequency? also, i'd assume the mass on each PR would be halved. this seems like it could be potentially useful in smaller speakers (3 1/2" speakers @ 60hz lowest, 1st PR @ 40hz, 2nd PR @ 20hz) also, couldn't this make the system more accurate? due to the fact that the PR's would be at a lower mass, they would have less inertia(return to 'home' position faster) and therefore rumble or sound from excess inertia in larger PR's would be partially eliminated? all very experimental, but before i start on my own i'd like to see what you guys have to say.

[benb]

Firstly, this thread probably belongs in one of the speaker forums, maybe multi-way, though no doubt you could use a PR with a full-range driver as well.

Read up on bass reflex speakers. A passive radiator is pretty much an equivalent to the port in a bass reflex design. Some such designs do sometimes have multiple ports, but all the ones I've seen are tuned to the same frequency. I don't recall ever seeing more than one passive radiator on a speaker, and I don't know if would or could be any advantage of two PR's or ports tuned to different frequencies vs. one.

[RJM1]

No, the mass of the passive radiator has to go up to lower the resonance.
You might want to look here.
The Subwoofer DIY Page - Passive Radiator Systems

[picowallspeaker]

The use of a little FR and a PR reminded of this
Computer Speakers - what you can get for 25 USD or 15GBP
Which is a commercial desktop speaker set reviewed by Sreten
The main features are the all-in-one construction benefiting from industrial production ,well engineered .

[MossBassist]

Originally Posted by RJM1

No, the mass of the passive radiator has to go up to lower the resonance.
You might want to look here.
The Subwoofer DIY Page - Passive Radiator Systems

hey, yeah i thought about this today. thanks for clearing it up.

@benb: yeah, i though of a passive radiator as a component of a speaker system.

but yeah, does anyone acutally know if two passive radiators at different frequencies work. i mean, if not, i'm ready to try to figure out on my own, but yeah, just to make things easier or avoid mistakes.

[sreten]

Originally Posted by MossBassist

but yeah, does anyone acutally know if two passive radiators at different frequencies work.

Hi,

Yes I do and they don't if tuned to significantly different frequencies.
Like twin different ports, below the resonance of the short one, that
port prevents the cabinet pressurising so the long one does not work.

For similar sized PR's to the driver it needs x2 excursion or 2 of them.

Forget about 20Hz for a 3.5" driver, 40Hz is only doable IMO with
something like an expanding TQWTL, e.g. see the "Metronome".

With a small box and PR's the best approach is active bass boost
and high pass filtering via a peaking 2nd order amplifier filter, this
can be built into the feedback loop of the amplifier, as can BSC
and any overall driver EQ.

rgds, sreten.

Note : to tune a PR an octave lower (!?) you need to quadruple mass.