Gaga, could you post the ABEC files for this sim? I've made pretty good progress with ABEC but I've hit a road block that I think may be due to my old pc. I'd like to try to run this and see if it works.
Nate, I'll post the project here or give a download-ling via PM. It will need some time, as I have to clean up the scipts - I did not have in mind to share, when setting up simulations for these waveguides.
Of course the two tweeters are not the same, but maybe a waveguide for the Satori-tweeter will work okay for the SB29.
Reason is that a 1''-membrane already starts to beam >6kHz. This simulation (done using AxiDriver) shows a 25mm membrane in an infinite baffle (with SB26 TSPs):
and the respective directivity plot (normalized to 0°):
Now, where do 'phase-shields' (often seen as small, round, transparent plastics fixed at the back of protecting grids in front of metal domes) impact the directivity of dome tweeters?
This is a simulation of Brandons eliptical waveguide 'C' with a small phase-plug V1:
The respective SPLs from 0-90°, 15°-steps, normalized to 0°:
The same for a phase-plug with higher diameter:
SPL 0°-90° (as above):
And a phase plug version 3, a ring with central hole:
Again the directivity 0-90°:
Where does the phase plugs impact directivity? As a comparison, the directivity 0°-90° of the waveguide without phase plug vs phase plug V2:
For better comparison the horizontal directivity without phase plug:
The effects of the phase plugs appear around 13-20 kHz, where the WG-contour has no effect (may be beside bad match of tweeter membrane and WG-throat).
The suggested phase plugs are not meant to be the best possible solution here, they should just help to demonstrate the impact of phase-plug variations in this waveguide.
Before performing more simulations of different phaseplugs, I'd suggest to start with the WG-contour instead, if you're interested.
Christoph, your conclusions with regards to the phase plug are valid, based on experiments with optimization of horns by Scandinavian scientists. They found the impact of a phase plug is significant in the highest octave (albeit with some transmission losses).
I'm currently trying to replicate a similar setup but this will take a while (or even a long time ;- ).
Some optimization functions are available in Comsol, based on earlier work done by the Scandinavians. They have published more advanced methods recently, but:
- these involve a lot of scripting complex math in Matlab
- and might be overkill for this project.
BTW I did some comparisons of phase shield shapes back in the PETT thread, but photobucket pulled the pics :( Anyway, I did four shapes: disc, cone (like Vifa/Scan), scalloped disc, and a ribbed plug similar to the ones Eton uses on their woofers. They were all terrible except the disc and scalloped disc. The scallops offered no advantage, so I further investigated the simple disc. I tried diameters of .5", .38", and .25". The .25" offered the best balance of properties.
I was curious about the ring type you simmed, since this is pretty common. I never really understood it, why only allow the parts of the membrane furthest from each other to radiate? And your sim reinforces my intuition. (maybe with a fabric dome this could work where the center of the doming is moving out of phase with the outside, but with the geometry involved puts the radiated sound back into phase at any typical listening axis).
I should be able to get measurements of the new waveguides taken tomorrow.
Here, the ring had a positive impact in that it resulted in a more constant directivity between 15 and 20 kHz.
Simulations done using AxiDriver. Disc phase plug in front of SB26 dome:
Directivity -90° to 90°, normalized to 0°:
Note the 'widening' aroud 17kHz.
Same simulation with 'ring' phase plug:
Directivity, normalized to 0°:
The 'widening' smears over a larger frequency range and shifts to higher frequencies, resulting in a more constant directivity.
Note that AxiDriver (and ABEC) assumes an ideal membrane without break-up at higher frequencies. Simulation of fabric domes is therefore not really possible. Simulation of metal domes is (hopefully) more realistic though.
Your disc diameter is close to .5" which I found to be inferior to a smaller one - though I haven't looked at the plots to see if they match your sim. Try a .25" diameter disc and see how that sims.
Version 1 shown above is the phase shield with a diameter of 0.25'' as you suggest.
Version 2 has a diameter of 0.56''.
Version 3 has an inner diameter (hole) of 0.318'' and a ring of 0.127'', i.e. a diameter of 0.572''.
May be I should have given the diameters of the simulated phase shields....
Doh, I knew that, sorry!
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