Inactive speakers...some measurements

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.

sean...It isn't easy to figure out what to make of the data, so what I did was to determine what kind of power level we are talking about. I assumed that this power was all radiated (or absorbed if you like). And my conclusion is that inphase or out of phase it's just too damn small to be heard.

The frequency sweep you suggest sounds like the obvious thing to do if you had equipment, but, in practice I doubt that any instrument would be sensitive enough to even get close to the power level in question. I believe that the ear is much more sensitive than any instrument, but when an effect is large enough for the instrument to detect the instrument gives a description that the sensitive ear cannot provide.

The speaker I used was an aperiodic (damped vented) enclosure containing two LF drivers (8" and 10") which is good to about 30 Hz when powered. It was placed about 2 ft in front of a MG1.6 + subwoofer source. The 100 dB environment was measured right at the inactive speaker.

Regarding passive radiators, I generally do not like speakers that use them, but I can't be sure it is the fault of the PR. Supposedly a PR is functionally similar to a vent but behaves in a more controlled manner and without wind noise. (I don't like vents either). One idea that sounds interesting is to use an actual (inactive) driver instead of a PR, and tune it by L/R/C loading of the voice coil, instead of by the usual way of mass loading the cone. This can be extended to actively driving the "inactive" driver with some sort of signal not the same as the primary signal going to the "active" driver. (In fact I have such an experiment in process).

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.