Sorry to take so long with the answer. I'm not on AK very often these days.
gonzomeep said:
Since the Heil diaphragm is an accordion pleated transducer, it would seem that the phase of the rear radiation would be identical to the front.
Nope. It's an oversimplification, but you can think of it this way: As the front blows, the back has to suck.
gonzomeep said:
In conventional cone/dome di-pole speakers, there are two identical sets of drivers wired out of phase intentionally, right?
Right. Strictly speaking, these are not "true" dipoles, but close enough for gummint work. An ESS speaker with a big Heil on top is a half-dipole-- the tweeter's a dipole with a figure-8 radiation pattern, but the woofer is cardioid-shaped at the top of its range and omnidirectional from about 100Hz on down. Kind of a mess, really.
gonzomeep said:
There are companies that produce bi-polar speakers too, like Definitive Technology, that use an identical set of drivers (in an MTM configuration--BP3000tl) wired in-phase intentionally. Two schools of thought.
Prezackly.
gonzomeep said:
Now to the Heil, it presents a strange situation because it is one diaphragm that squeezes the air. It seems that the phase would actually be the same because the air is being squeezed in the same direction WRT emanation from the diaphragm, which seems in-phase, just facing opposite directions, unlike a conventional cone/dome di-pole arrangement that would have one driver pushing and the opposite/identical driver being pulled in.
The accordion model will lead you astray if you take it too literally. In the Heil, let's assume we can watch the pleats move during a simple sine wave. On the positive side, every other pleat "wall" (say, the even-numbered ones) is headed toward one magnet, while the alternate "walls" (the odd-numbered ones) are heading toward the other magnet. Then as the signal dips into the negative, the walls pull back and then head to the opposite magnet -- very different from a simple bellows. Each U-shaped pleat can itself be compared to a bellows, but the coordination of all the pleats in their magnetic field is more complex.
As a given pleat squeezes out air, its neighbor, facing backwards, is pulled open-- that is, it draws in air. Dipole. A peculiar dipole, I'll give you that, but a dipole nevertheless.
Don't take my word for it-- get two pieces of cardboard and reflect the backwave to one ear while your other ear gets a bounce from the front of the Heil. To do this experiment optimally, you'd want to hook the Heil directly to a small amp and carefully drive it with midrange frequencies, but I think you'll be able to hear that the front and back are out of phase even with the Heil still mounted on the speaker. Turn down the bass and treble to bring the midrange to the fore. Stick your nose right on the middle of the Heil's side.
gonzomeep said:
Also, as I understand the basic method of measuring a speaker's sensitivity, the builder will put the speaker in an anechoic chamber, place a microphone at a calculated height and 1 meter away. One watt (2.83volts into 8 ohms) of pink or white noise is fed into the speaker and the resulting output level is measured and expressed in DB's. You wrote that when the Heil is used with an enclosed rear, it cuts it's sensitivity in half?
Yep, the rear radiation is turned to heat inside the cabinet. Half the output is being thrown away. No problem in the real world, since the Heil is so much more efficient than the woofer it's likely to be paired with that this maneuver might just save the cost of a tweeter-level control.
gonzomeep said:
If the standard test for sensitivity is measured in an anechoic chamber, the rear output would have no/nil bearing on the outcome, right?
Theoretically right. But nobody listens in an anechoic chamber, which is why measurements have been done in normal rooms for many years now using gating on the mics and other such trickery.
Woofers, being largely omnidirectional, always have "rear" output. The lower the frequency, the "rearer" it gets.
Outdoor speakers for concert reinforcement-- that's a different story. They operate in what's effectively an anechoic space, and to put tweeters on the back of such a speaker would obviously be... ..well, to be kind, let's call it uneconomical.
And if you take your speakers outside and play them on the lawn, you'll notice the bass seems to be missing.
A real room acts as a collector of the total output of a speaker, so for it to sound "flat", there should be just as much treble and midrange energy pumped into the room as bass, and phase doesn't matter, since all the reflections in a real room will scramble that anyway. What we end up measuring is what the speaker mavens call the power response of the speaker in a defined chunk of space. If you've heard me rant about "room boom", that's what I'm talking about. Using a Heil or any other kind of dipole or bipole tweeter is a handy way of adding some treble to the power response of a speaker without making the speaker sound too bright. A dipole tweeter will have the added effect of making the stereo image spread out a bit, giving an extra sense of space, which may or may not accord with the sound originally recorded.
Does that make sense? By the way, thanks for pointing out that ESS is finally out of business. The website takes you to Simply Speakers now. Hard to believe. End of an era. Hats off, gentlemen, and hang onto your Heils! Oskar, we hardly knew ye.
