Inactive speakers...some measurements

sean...There probably are enough different ways to configure a speaker system so that every one of us guys on Audiogon (plus Elizabeth) can have their own way to argue is best! Vance Dickason in his "Loudspeaker Design Cookbook" talks about "Augmented PR" where a PR connecting two internal volumes is mechanically coupled to a cone working to the outside. And then there are Bandpass enclosures where all the cones, passive or otherwise, are hidden away inside, and the sound comes out through a hole.

I have always been amazed that PRs behave as they do. With two drivers in a box, and only one powered, it really is difficult to determine by visual means or by ear, which one is powered and which one is passive. In particular the fact that they are in phase (move in and out together) seems counterintuitive. At first thought you would expect the PR to get pushed out when the driver pulls in. That's what happens if you push (at near DC frequency) on one of the drivers. However, at audio frequencies, the pressure variation at the back of the active driver lags driver displacement by 90 degrees and the PR displacement, observed from inside, lags the pressure it receives by 90 degrees. 90 + 90 = 180. But this is 180 degrees on the inside of the enclosure, so the pressure variation outside the enclosure caused by the PR motion is back in phase with the active driver. There is a time delay of one half of the period of the signal. For a steady sine wave signal this would have no effect, but when the signal varies, and is composed of a complex waveform, having many frequencies, all with different half-period delays, the result is a bit mushy.

I have said that vented speakers make excellent bird houses. IMHO, speakers with PR's are little better. However, an idea that interests me is an "active enclosure" where an internal driver, rather than air compression alone, determines the pressure behind the active external driver. The isobaric configuration, where the internal driver sees the same signal as the external one, is the most simple version of the "active enclosure" but the idea can be taken much further. When I get the details figured out, Slappy and I will go into business and become another Bose.

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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Sean...You are correct that the classical mathematical analysis of loudspeaker designs is ridiculously complex. I prefer to copy designs that I like, and then tweek things a little at a time. Not too appropriate for an engineer like me, but this is a hobby, not my job, where I get lots of practice doing it the other way. I think that a different analytic approach, made possible by development and easy availability of powerful computers would be effective for loudspeakers.

Simulation. The fidelity of simulations has become amazing with the development of generic tools for creating simulations, and the availability of computers with enough memory for the program and the computational speed to run the simulation in minutes rather than days. The performance of the simulated speaker could be output in terms of the same parameters that could be measured on a physical version of the design. For icing on the cake, the program could play music (simulated or prerecorded real) through the simulated speaker system and output an audio signal that the user could listen to on headphones, and, in a matter of a few minutes, hear the effect of design changes that might take a week to try out in the real world. (Of course the headphones would contribute their own signature, but changes would still be recognizable).

Someone, maybe Bose, may have already done this.