Diaphragm loading with no phase plug

[longdrive03]

Any problems with loading a 7 x 10" tractrix horn with a diaphragm without a phase plug? How would that affect the dispersion/spl? Thanks.

[boom3]

Phase plugs seek make all paths from all points of the diaphram to the throat equal. Up to a given frequency, that works. Because I don't know the driver or horn, I can't really speculate what effect this will have on dispersion or spl. Omitting the phase plug will, however, make a dip in the response at some point, due to destructive inteference between different parts of the diaphram.

[WMcD]

It has been done. Bruce Edgar published an article about his first midrange tractrix horn in SpeakerBuilder. He used a four inch JBL (?) conventional midrange driver. No phase plug of course. He did not like the result until he added a sandwich of scotchbrite donut between the driver and horn.

I take it that the midrange driver was not a good acoustic match for the throat and the Scotchbrite acted as a resistive element.

Usually the need for a good match between the driver and the throat is descibed as being necessary for power transfer from the driver into the throat. Certainly this is correct.

But what is less well described is the function of the driver in absorbing the mouth reflection, if there is one. There should not be much reflection with a proper mouth. In any case, a matched driver absorbs the reflection when it travels back to the throat. A poorly matched driver causes the reflection to bounce back up the horn, so you have a resonant tube effect.

People may have noticed a comment in PWKs patents on how small a mouth one can get away with in a bass horn. There is a note about how smaller mouths can be used with a proper driver. I think this "absorb the reflection" is why. He may have gotten this from Wente and Thuras.

But in any case. I understand that the Avante Guard horns use a largish diaphragm which is convex. The theory is that the wavefront going down the horn naturally becomes convex because of drag at the edge walls. Therefore a convex diaphragm is a proper source as far as geometry.

I believe a phase plug is needed when you use say, a two-inch diaphragm to feed a one-inch throat, or like that. You want to avoid the pressure from the edge of the diaphragm from getting to the center with a delay, and therefore out of phase as frequency increases.

On the other hand, it seems to me this does not hold when the diaphragm is the same size as the throat. Now I'll admit there is some potential for interference effects. But my thought experiment is that with a small throat and phase plug, the horn cross section eventually gets to be the diameter of the driver. So why are there no interference effects at that point? Maybe there are, but no one seems to worry about them.

If you want that article I can probably find it and send it to you. The forum owner has asked us to refrain from posting copyrighted material. So I've not been doing that.

Gil

[boom3]

I intuited this convex wave several years ago and posted a thought about it here, under a different name, that account was lost in a crash, hence the suffix 3 on my name. Did not and don't have the instrumentation to explore it. Be glad to see someone else do that.

Gil If I follow, you say that the horn becomes the diameter of the driver at some point so why sweat a coherent wave front. The notion is that if the entire wavefront starts off in phase at the throat, it will retain that relationship down the horn to the mouth. Is that really true at all frequencies of interest? I doubt it, but it is a starting point. The phase plug, IIRC was invented in the Teens or Twenties at Bell Labs (natch) for their "loudspeaking telephones" used as public address for speeches over landlines, which was done long before audio broadcasting. I don't have an JAES or IRE index handy, but I am sure somebody here has access and can find the original articles.

[longdrive03]

Thanks for the responses guys. I guess I could get off my lazy butt and hook it up and listen, huh? I think my problem may be solved anyway. I have quite a few 7 x 10" tractrix horns used in some Klipsch models which don't have a built in phase plug. I have the screw in driver assembly with a phase plug but more horns than assemblies. I have some drivers assemblies without phase plugs so I was going to give it a shot but tonight I got more drivers with phase plugs so I'm happy for now. I saw on the PE board a reference to a dome tweeter loaded in a round "waveguide" which was interesting. However, the waveguide didn't have the efficiency of a horn. Thanks for the help.

[boom3]

I saw on the PE board a reference to a dome tweeter loaded in a round "waveguide" which was interesting. However, the waveguide didn't have the efficiency of a horn. Thanks for the help.

This is like the old Dhorm tweeter...Klipsch used a clone for a while and then went back to phase plugs. Jim Hunter at Klipsch knows about that topic. If the throat is small compared with the wavelength of the highest frequency of interest, a phase plug is not needed.

[ZAKO]

The Cohearent wave phase plug in the JBL 2250 is a true cohearent phase plug going from a 4 inch diaphram down to a 1 1/2" opening expanding to a 2" conicle tube...the design extends the bandwith....In the Khorn the woofer fires into the 3X13 slot also extending the upper bandwidth Paul got that idea from Olson. The Community M200 driver does not have a phase plug but a so called phase ring. A very different concept.