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Old 2nd December 2019, 05:03 PM   #31
impuls60 is offline impuls60  Norway
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Thanks for a very entertaining thread! I would like to learn how phase plugs in horn actually work so I've done some light reading on Wikipedia regarding phase plugs. I would like to find out why different phase plugs in your thread behaves like they do, and I think this Wikipedia article gives some good clues:

"At high frequencies, the diaphragm does not act as a perfect piston; instead, it displays rippling, modal properties related to its stiffness and density. Because of the speed of wave propagation through the diaphragm material, the center of the diaphragm moves slightly later than the perimeter. Radial slots in the phase plug do not correct for this small time difference, which affects the highest frequencies. Concentric circular slots may be able to correct for the diaphragm's rippling behavior but the positioning of the slots is critical. Circular slots may allow resonances to build up between the diaphragm and the phase plug, ”resonances which cause wave cancellations and a corresponding reduction in frequency response at the resonance frequency."

-Source: Phase plug - Wikipedia

So based on this info and you thread the center of the soft dome produses an acoustic phase change above 10k Hz? It might not, but I'm going to assume it does . If so, I think this out of phase energy should be dampened so it doesn't affact the direct sound coming from the outer tweeter area attached to the voice coil(which is what we want to listen to).
I remember you printed a small phase horn on your guide and the level dropped off. That would indicate that the energy was reflected back towards the cone. On the Vifa,the plug cant reflect energy back since there isnt any volume between it and the membrane. Also, the extra mass of the plug must affect the phase of the center of the tweeter(added weight causing further delay?), but also stop wave energy from traveling laterally from one side to the other.


The Wiki article also says:
The combined area of the phase plug slots is typically about one-eighth to one-tenth of the area of the diaphragm. This gives a pressure-to-volume velocity change ratio in the range of 8:1 to 10:1, which serves to match the impedance of the diaphragm to the horn throat.[8][14] A larger slot area admits more sound wave energy but also reflects more energy backward onto the diaphragm. A smaller slot area traps more wave energy between the phase plug and the diaphragm. In researching the diaphragm/phase plug interface, David Gunness found that only half the wave energy, at best, travels directly from the diaphragm through the phase plug slots and out to the listener. The other half (or more) causes cancellations within the space between the diaphragm and the phase plug, or causes temporal anomalies (time smear) upon leaving the phase plug later than the direct sound. To minimize the problem, Gunness modeled the behavior mathematically and used digital signal processing to apply a polarity-reversed version of the undesired wave behavior to the original audio signal"

-Source: Phase plug - Wikipedia

So it seems that the varying footprint in the printed phase plugs would reflect different amounts of accoustic energy back towards the membrane. This would cause at lot of nonlinearities.. It now seems like a phase plug should reflect as little as possible energy back towards the cone and one would deal with the unwanted energy in other ways..

One solution to deal with the potential unwanted energy from the tweeter center area could be to experiment with the diameter of the inner annular ring. Would this create an high pass function into the center tube if the diameter is larger than 1/4 of the wavelength?
I also wonder if the wavefront radiating from the center tweeter could be causing cancellation at some point? If one got rid of that energy the total high frequency response could get even better?
Would it be possible to absorb this short wavelength energy with acoustic foam inside the inner annular tube?(This would probably change the acoustic impedance so more energy would be reflected back).
Another alternative might be to design small diffusors into to the inner tube? Utilizing compression or diffusion to dampen the level of the unwanted energy?
When I looked at Kef's design with the hard dome it strikes me that the phase plug dampenes the center of the dome and the high frequency break up. I think it also stops laterally moving wavefronts inside the waveguide.

What do you think of my guesses of whats happening?
I'd love to see your further experiments and hopefully a printable file since I have a 3d printer .
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Old 2nd December 2019, 06:47 PM   #32
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

Click the image to open in full size.

IMHO, pathlength is mostly the only thing that matters.

With a 1" dome tweeter, even that tiny bit of pathlength difference, from the tip of the dome to the edge of the dome, that matters.

If you put a dome tweeter on a flat baffle, the sound radiates in a sphere. Because it radiates spherically, that pathlength difference from the tip of the dome to the edge of the dome, it doesn't matter as much.

But as soon as you stick that dome on a waveguide or a horn, that pathlength difference becomes a problem. A phase plug corrects that problem.

For instance, on page two of this thread, I showed illustrations and measurements which indicate that putting a tiny little cone in front of the dome, it equalizes the pathlength difference from the tip of the dome to the edge of the dome. Here's a cutaway, to show what I mean. That cone in the center, it equalizes the pathlength.

Click the image to open in full size.

You can also do some strange things with phase plugs, such as make a converging wavefront or a diverging wavefront. On one of my Unity horn projects, I accidentally made a phase plug with a converging wavefront, and the net effect was that it screwed up the polars because it basically "focused" the sound at single point in space (because the wavefront converged, instead of diverged.)

And of course, the most important thing here, is that all of these designs can be copied and scaled. For instance, if you're using a 1" tweeter instead of a 3/4" tweeter, you can simply 'scale it up.'

If I had more time to kill, it might be fun to create a phase plug that matches the waveguide it's attached to. I think that was a big part of what Geddes patented, IIRC:

US7095868B2 - Phase plug with optimum aperture shapes
- Google Patents
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Old 2nd December 2019, 07:22 PM   #33
ErnieM is offline ErnieM  United States
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Quote:
Originally Posted by Patrick Bateman View Post
Here's the frequency response and distortion of the BMS at a level comparable to where I measured the Vifa. The BMS just *crushes* the VIfa in the distortion department.

For $90, the BMS is killer, but I like the Vifa better because of:

1) better polars

2) smoother response

3) a fraction of the price

But if low distortion is important to you, and you don't need a low xover point, the 4526HE is REALLY nice.
I'm having a hard time with these numbers. Most compression drivers have much higher 2nd harmonic distortion at those levels(110-115db). More like 10-20%.

Last edited by ErnieM; 2nd December 2019 at 07:42 PM.
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Old 2nd December 2019, 07:41 PM   #34
Patrick Bateman is offline Patrick Bateman  United States
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That's probably due to a few things here:

1) I'm too lazy to use my SPL meter to find out what the actual levels are. I basically measure everything at the same level, which is about 90% on my amplifier. Because the BMS is so efficient, I lowered the measurement level by 20dB, and then I increased it by 10dB when I did the "oh **** my neighbors are going to call the cops" measurement. I got some serious stink eye for doing that measurement, hopefully the HOA doesn't fine me. It was LOUD.

2) The BMS is 113dB efficient with one watt : Overview

3) I paid extra for the spiffy version, with a shorting ring. Most compression drivers under $200 don't have shorting rings.
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Old 2nd December 2019, 07:56 PM   #35
ErnieM is offline ErnieM  United States
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Could be. Don't need more than a watt for most rooms.

Art Welters test with a TC9 vs. Eminence N314 was an eye opener. IMD was much lower and sounded much cleaner at high levels than any compression driver on the same horn. More of a concern at p.a. levels than home levels.
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Old 3rd December 2019, 10:57 AM   #36
impuls60 is offline impuls60  Norway
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I agree completely with pathlength difference is the most important thing to get right. But since you are designing for a soft dome and not a pistonic driver, the design should account for the different behavior of a soft dome vs regular horn membrane if going for perfection . I looked at Geddes design an I assume that wave guide is for regular horn usage. If I had a high speed camera I would look at the soft domes behavior at high frequency to find out when the center of dome would start lagging behind the voice coil. Estimating the radius from the center and outwards could be possible from looking at the film, and that would give a good starting point for designing a very good waveguide.
My next project is to print a waveguide for a Fountek ribbon so I can get better off axis performance and the tips for making a waveguide really helped me out!
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Old 3rd December 2019, 02:27 PM   #37
kipman725 is offline kipman725  United Kingdom
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Quote:
Originally Posted by ErnieM View Post
Could be. Don't need more than a watt for most rooms.

Art Welters test with a TC9 vs. Eminence N314 was an eye opener. IMD was much lower and sounded much cleaner at high levels than any compression driver on the same horn. More of a concern at p.a. levels than home levels.
Do you have a link to that? it sounds interesting.
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Old 3rd December 2019, 06:09 PM   #38
ErnieM is offline ErnieM  United States
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High Frequency Compression Driver Evaluation
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Old 3rd December 2019, 06:30 PM   #39
weltersys is offline weltersys  United States
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High Frequency Compression Driver Evaluation

Post #60 shows some distortion comparisons between the TC-9 paper cone and Eminence compression driver.

The relatively large diameter (68mm) of the TC-9 makes a smooth upper polar response difficult, if not impossible, to achieve, but the lower distortion due to the Xmax being nearly 3 times that of a typical compression driver is a tradeoff to be considered.

Last edited by weltersys; 3rd December 2019 at 06:42 PM.
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Old 3rd December 2019, 08:23 PM   #40
pelanj is offline pelanj  Czech Republic
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Interesting. Maybe TC9 with a 3D printed phase plug and back cover could work as a nice compression driver...I need to get a 3D printer soon.
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