Inactive speakers...some measurements

PR's have the slowest transient response of any vented design that i'm aware of. Just like a port, they also contribute energy outside of their resonant frequency, but do so in different manner.

Rather than having to move air through a tube and having to worry about varying flow velocities, PR's have the added drawback of increased mass. Since the PR isn't moving perfectly in time with the active piston due to this reciprocating mass, and the movement that it has is a direct effect of the active piston having already moved, this is what causes the increase in ringing that one can hear and measure.

In plain English, the PR responds to the active piston AFTER the fact, which is why we have delayed damping and increased ringing. While PR's are great for adding extension, output and bass weight ( perfect for HT ), they lack proper definition and attack / decay characteristics when it comes to music.

Using a passive with a brake on it will probably work better if properly applied, but it is still going to respond to the pressure changes inside the box at any given frequency. In effect, this would be like having a sturdy cabinet that had a small section that was far flimsier in one given area. Tuning that part of the "flexible box" could give you added output when excited, but it is still going to flex ( to a lesser degree ) outside of its' resonant range. When it does this, it will contribute output outside the box according to the pressure changes inside the box.

If you want to start playing games using a PR without killing your transient response, try mounting a passive radiator INSIDE a sealed box on a barrier wall that divides two active drivers. This works best if you use the same approach but with two ( or more ) different drivers with varying sized sealed chambers for each driver.

While each driver / chamber has its' own natural resonance, the other driver / chamber, which is tuned to a different frequency, is now altering the resonance via pulling on the passive. This now changes the volume of each chamber and the associated tuning and pressure inside each chamber. In effect, each driver pulls / pushes the passive according to its' own needs, but, there is always another driver trying to muscle the passive at a different rate. This helps to null any specific peaks or characteristics from any of the individual driver / chamber resonances on their own. Since the individual peaks are minimized and spread out, bass is both more linear and delivered in a more controlled fashion. If everything is done right, you can also gain some extension from this arrangement.

You now have multiple drivers for increased surface area, higher maximum spl's with greater power handling due to sharing the load, minimal peaks at resonance, greater extension, less distortion due to reduced excursion, increased linearity, etc... All of this due to using passive radiators INSIDE the cabinet. The PR does this without contributing any sonic characteristics outside the box.

When you start looking at stuff like this, it is very confusing to say the least. This is especially true when running multiple drivers with multiple chambers and multiple passives. It took me a long time to understand how this works, but believe me, it does work IF you put the time and effort into it. Sean
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PS... No, i did not invent this, but i wish i did : )

I agree that watching the cones on a woofer / PR system can be pretty interesting. If you have two woofers with a passive ( like Polk 10's, 12's, etc... ), it almost seems like the woofers themselves are out of phase with each other.

As a side note, the active woofer also has to deal with the "back-lash" inside the box from the PR being excited. Obviously, the PR has mass, so it will ring once excited. That ringing translates into vibration that is fed back into the active drivers through the acoustic coupling within the box. With the method that i mentioned, the passive is ALWAYS actively driven by one of the woofers. This is true even if one of the woofers is resonating and has limited excursion since each of the woofers has different tuning / output characteristics. The drive to the PR is always varied as it is being pressure fed by multiple signals of various tunings.

I hear what you're saying about variances in designs, etc... but give this one some thought. It is pretty mind-boggling and quite advanced. This is probably the reason that not too many people mass produce something like this. A lot of math / trial & error involved. Sean
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You're missing the point. Not only are the non-driven speakers being excited, they are re-radiating the energy that they absorb out of phase. Depending on the suspension of the drivers and spl's in the room itself, the amount of excitation and re-radiation will vary at a non-linear rate. This is not to mention that some drivers have far looser suspension than others, making them more susceptible to excitation and re-radiation. Using a lower efficiency speaker of the sealed variety as a point of reference, all other speakers will produce greater variances into such a situation.

If you want to do this and do it simply, take a few frequency sweeps in the room at various repeatable volumes and chart the frequency response. Then do the same thing after introducing some good sized speakers that aren't going to be used into the room. These speakers should be relatively efficient and use either a vent or a passive radiator. Duplicate the previous test and then compare the results.

If you really want to do this and do it quickly, borrow some type of device that does digital room correction and run the above tests. It will give you all the variables in a matter of minutes. Sean
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