"Into The Abyss", Tweeters in low places

[paulspencer16]

I had to make temporary PS from an old hotel room key card but it helped the top octave but obviously much to be desired until it looks good

[paulspencer16]

On this baffle in the WG and the PS and a 10uf cap

[guitar maestro]

On this baffle in the WG and the PS and a 10uf cap

That's a nice corner countour, similar to the Legacy Audio design, which I think looks great.

[guitar maestro]

But throat geometry is critical to top octave behavior. It needs to match the tweeter to avoid standing wave buildup between the dome and the guide walls.

Does the frequency at which it becomes problematic occur when the throat diameter (or one axis for rectangular apertures) is larger than the size of the radiator as well as smaller?

I noticed what I believe to be similar behavior when mounting the Dayton H110 waveguide onto an AMT. with the throat dimensions being "off" and not matching the AMT diaphragm aperture, I noticed the top-end wasn't as "nice" as when I matched the throat to the openings on the front plate of the AMT.

non-matched throat-to-AMT aperture (red/purple with waveguide, blue/green originals):




With throat-matching, the bump from 10-15k smoothed out, as well as most of the FR curve (red with waveguide, blue without):




It seems that freq = c / d, where c = speed of sound, and d= diameter of the waveguide throat, matches my results closely. The non-matched rectangular throat had a short dimension of about 1.25", so that gives a "problematic" starting frequency of about ≈10.8kHz, right where the peak started to occur in the graph. Correct, or coincidence?

[paulspencer16]

Hi Mr Maestro. Are you saying the throat has to be an exact match for the tweeter (dome in my case) ? Looking at the fit here I would need to add approx 1mm to the inside diameter of the throat. Maybe I could Bondo it sand it back and try that? Do you think this is the issue with the top octave and if that is rectified I wouldn't need the phase guide?

[paulspencer16]

Sorry same photo try this

[paulspencer16]

Here we go

[guitar maestro]

Well actually it was PeteS who stated it should be a match. How much of an "exact" match, I'll leave that for him to answer. There is no doubt in my mind that tight & smooth transitions from the radiator/aperture to the actual throat do provide desirable results. Or you could answer it yourself by doing the testing and collecting empirical data to validate/invalidate your hypothesis.

[TN Allen]

Looking at the caliper reading, .5mm (.02") clearance between the surround and face plate opening doesn't seem like much, given there probably is some variation in the surround diameter.

It makes sense to do some testing to evaluate how critical the throat match to the surround may be. I don't recall that this has been done and conclusive data posted on this forum. Most tweeters with non-removable face plates couldn't be tested, unless the faceplate configuration, and the distance between the surround and face plate opening is sufficient to sneak a wg throat between.

[guitar maestro]

I agree on the testing needed. Given the fact that the majority of the surround (at least I think that's a fact, lol) doesn't move all that much, I hypothesize that a tight throat by about 3-4mm in ID (34-35mm ID) so that the throat of the waveguide covers about ½ of the surround would prove useful. Unfortunately I only work with AMT's so I can't test my own hypothesis on a dome, but I am hoping to test my hypothesis on my AMTs next month. AMT diaphragm edges are fixed around the perimeter, so I'm gonna try mating up the throat of the waveguide to the 1st pleat on each L-R side, and inboard of the top-bottom edge where the aluminum conductors make a U-turn.

[paulspencer16]

HI so I added some Bondo and got to work with some sandpaper and managed to get it pretty close to round. Measured it and it had no effect on the on frequency response. So I think in this instance the 1mm ish difference between the throat of the WG and the size of the tweeter dome is negligible.

[TN Allen]

That is very interesting.

[paulspencer16]

Hi there so carried on looking at this and found all of the amazing work done in a different thread on here and other forums by augerpro, TN Allen, Dan P and many others. I am way behind the curve on this and didn't appreciate how much had already been done on this subject.

Thanks to all you guys.

I looked back at the measurements and decide to run a new set and carry on with this little project. The upshot is that yes the throat diameter definitely impacted the FR of the tweeter in the WG. I'll post all the photos in one dump. I Also hacked the RST-28f with a razor saw and played with the parts to integrate them into the WG. Think it's sounding pretty good but will wanna live with it for awhile to gain a better impression.

[paulspencer16]

Tight/loose

[paulspencer16]

With / without PS