Rear fireing Tweeter - in/out of phase ?

[Bernard Wolf]

Hi everyone - I am doing a tiny 'experiment' on my S3100 with a rear fireing pieso tweeter and am wondering what is the correct phasing - in or out ? Any help would be appreciated.

Thanks - Bernard

[4313B]

Originally posted by Bernard Wolf
I am doing a tiny 'experiment' on my S3100 with a rear fireing pieso tweeter and am wondering what is the correct phasing - in or out ?

Whichever sounds better? I know the answer lacks glamour but it's the best I can come up with.

[Bernard Wolf]

Well, I have only tried it in phase so far and yes I can certainly try out ,which I will do this evening..... just wondering though which is correct from an electro-accustical perspective. I mean, ordinarily out of phase would have a cancellation effect but I guess with such a small output that may be a moot point.

Bernard

[Don C]

A dipole like an electrostatic could be simulated by putting the tweeter out of phase. But that would only work if the two tweeters in the system were located at the same distance.

[Tom Loizeaux]

It seems logical to me that, if you're pairing the rear-firing tweeter with a front-firing tweeter, that they should both be "in phase"...meaning that they should both push away from the center point. This is what a real sound making device would do. If you bang a triangle, the sound pulses would radiate outwardly (in a sphere). Of course, assuming you set up your speakers in a conventional way, the rear-firing tweeter will be heard as it bounces it's sound off the rear wall.
Does that make sense?

Tom

[Don C]

Hmmm, maybe, but I don't think so. Lets use an easier example. Say a drum. When you hit the drum, the drum head moves with the stick, both sides move in the same direction, away from the stick. Recreating that with a dipole speaker system, you would want both diaphrams of the drivers to also move in the same direction, with each other, not away from each other. Since they are pointed in opposite directions, they would have to be wired in opposite phase to move in the same direction at the same time. Make sense? Well, that's how a dipole like an electrostatic would work, sound would be coming from both sides, but when the front surface moves forwards, so does the back surface, they are one.
Of course it is not going to be so simple in practice, because with two diaphrams instead of one, there is going to be a difference in time delay due to any difference in distance between the two tweeters and the listener. I get a headache pretty fast trying to imagine what that would do to the sound at different frequencies. Maybe that's why I am not a speaker designer. And Giskard was probably right in the first place, just wire it whichever way sounds best to you.

[Bernard Wolf]

Well, thanks for your input guys. I ended up wiring them out of phase so the tweeters would move as one like a diapole. I can't say that I can really notice much of a difference though as the rear tweeters are quite limited in their output. I feel though that they do work a bit better for my intended purpose which is, as Von Swiekert (sp?) calls it "ambiance retrieval". My speakers are very far out from the walls and thus the experiment. Also, the speakers I have read of that use a rear firing tweeter use a tweeter-mid combination which would be much more complicated to reproduce - don't think I will try that just yet.

All in all I am happy with the results I am gettting as the sound stage is larger and more alive sounding.

Bernard