Aren't passive radiators out of phase?

The dual 18" PR's in my HT SW really only kick in at very low frequency i.e. sub 30 HZ. Off the top of my head they each have a mass of around 1500 to 1800 grams. This is about 3.5 pounds. At these low frequencies the 15 inch driver's main job is to drive the PR's, not to produce direct radiated sound.

My 18" music SW is a 7.5 cu foot sealed box which I augment with a BASSIS. This avoids any potential problem with ports or PR's.

Mish, your HT sub sounds like a different yet probably more "technically correct" approach to using passive's than how most other designs go about it. What brand and model of sub is this ? Such a design doesn't ring any bells to me.

As to your "music" sub, have you ever thought about adding another driver in a push-pull or isobarik / compound arrangement ? Either of these can increase apparent box volume, lower FS, increase transient capabilities, lower distortion, etc... Only drawbacks to such designs is the lower impedance.

Is your music sub with the 18" a single or dual VC ( voice coil ) ? Sean
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PS... Sorry for getting off topic but it's not often that you get someone with BIG subs like that on a thread like this. Especially of such dis-similar designs : )

Thanks to all for the enlightening info.

NO! PRs are IN PHASE!

The passive radiator is a "phase inverter" just like a port or vented box is. There is phase shift across the working spectrum of a woofer, and a similar phase shift across the working (high Q) spectrum of the PR. However, at the primary frequency of interest - the resonant frequency of the PR, it is in phase with the output of the woofer. Indeed at that frequency, almost all the energy of the woofer is transferred to the PR, and the woofer's excursion is reduced almost to nil. This can be seen in the excursion graphs for any PR (or ported) box, as the excursion at the resonant frequency of the PR (or port) is substantially reduced as compared with frequencies above or below.

The PR or port is a rather high Q thing, meaning that it is a bandpass function that tends to be narrow and easy to excite at the center frequency, falling off on either side relatively quickly. Compare to the woofer itself which can be said to be a "bandpass" function, but one that is flat for many octaves. The PR (or port) tends to be narrow and function over less than one octave, usually about 1/2 octave in an effective manner.

So, neither the port or the PR is really "out of phase" with the woofer at any frequency where there is significant output from them. Just like the differential in phase between two drivers at the crossover point, as the PR comes into play, and then goes out, there is a phase differential between the two, but this will sum to a flat response, IF the alignment is set up that way.

The term "out of phase" here is taken to mean 180 degrees out of phase, and so cancelling when summed. Again this does not happen with a PR.

It is easy to think that it would be out of phase, if you assume a simple "pumping" from the reverse wave of the woofer - but this is not what happens!

Hope this explains...

_-_-bear

Great info, Bear! Thanks for sharing.