Passive radiators seem like a great idea, but they must be out of phase with the driver. Yet, you see them on some pretty good speakers, such as the Sunfire subs. All I can think is they provide a spring-board, storing energy for the driver. Still, any sound produced by the PR is out of phase though. Any thoughts?
Wow, this is cool. Thanks to you guys and your willingness to suffer us neophytes, I actually have some understanding of what's going on with PRs. Before this I just thought of PRs as doing the exact opposite of the driven speaker.......oh contrare moose-breath. Thanks!
Ports, slots or any other type of bass reflex design with a "hole" in it allow out of phase information to leak out of the box. The output from the port that is "in phase" is only very near the region of tuning. Stuffed transmission lines also do this, but the output level is drastically reduced. Passive's do not suffer this problem since there is no "leakage" from the backwave of the drivers BUT passive's can contribute lower levels of output at frequencies other than their point of resonance. EVERY design has some trade-offs to them. Sean >
The volume of the cabinet is a spring - while the property of the PR is mostly mass. The whole thing is merely a relatively simple resonant system. Of course there are losses and interactions and the issue of "Q" (the bandwidth of a resonance compared to its maximum, etc. But when you are all done it is a resonant system, that's all.
As far as filling with "gas" (other than "air") that's not of much use as the cabinets are awfully leaky, even when sealed to the limit of your ability to seal them! The only effect that changing the gas - at normal atmospheric pressures - would have is to change the spring which would effect the resonance and of course change all the parameters along with that. But they all tend to change when you change the mass and you change the volume of the box, the driver, the stiffness of the PR's suspension, etc. So, there's probably not much benefit, if any, to be had there.
The Dayton Wrights used gas as a dielectric, without which they would arc through rather instantly. Btw, they also used an awful piezo electric tweeter, so go figure!
Thanks Bear. I think I'm beginning to get it. The PR is tuneable with the volume of the cabinet (due to the compressing nature of air) and the properties of the PR stucture. I wonder, have there been any speakers with pressurized cabinets or maybe gas filled? ........ How about liquid filled (he,he)?
BTW, on my website there is a page with my version of subs with PRs, 18" PRs. Two per channel. I call them Quadripole, there are now some graphs and a better discussion than I had up a few months back... I do show the output of the PR and the woofer in separate graphs (iirc). I did not put up the impedance curves. FYI the pair (each channel) comes out now to 8 ohms... I can get some more "snot" if I rewire the drivers so that it ends up as 2 ohms, but I have to put back the Crown Macro Techs to drive that low impedance properly.
The cabinets shown are actually a bit too small - these are the original pairs that I built and they should have an extra cubic foot (a few inches back and one or two here and there gets you an extra cu ft!). IF it did have the extra foot the F3 would be at 20 Hz exactly and the overall response without EQ (I don't usually use any) would be a bit flatter...
_-_-
(Dickason's Loudspeaker Cookbook covers some of this theory, btw...)
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!
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 >
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 : )
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.
I've never spent much time playing with or learning about passive's although i have owned a pair or two over the years. As you probably know from some of my posts, i am a "sealed" kinda guy when it comes to boxes with TL's coming in second. Someone that is far more knowledgable in this area would have to help you out.
All i can say is that passive's allow a lower percentage of "out of phase" signal to play with speakers reproduction above its point of resonance than what a port or simple "slot loading" allows. Below the point of resonance, they are all about equal. Sean >
At what cabinet size and/or radiator size does hysteresis become an issue? I would think that in a big cabinet or with a big passive radiator, the response of the radiator to movement of the active would begin to lag, resulting in some potentially smeary sound. Is this an issue? Inquiring minds wanna know these things....
I don't understand the nature of the question, other than your possible concern of buying a speaker which you find inferior, due to a passive radiator inegration. I personally owned Thiel 2.3's, that used Passive radiator, and the things were fantastic sounding from top to bottom! I do tend to notice, however, that my favorite subwoofer designs DON'T employ passive woofers! Hummmmm...beyond me
Besides the phase considerations, you should look at the transient response of all vented designs. Passive radiators are the slowest of the slow but can still sound "good" if done right. I much prefer a well tuned passive radiator to that of a port for multiple reasons. Sean >
The drivers of most speakers except for Meadowlark, Vandersteen and a coupl of others are out of phase due to the high order crossovers most commonly used.
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