Open source Waveguides for CNC & 3D printing!

[natehansen66]

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.

[Gaga]

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.

BW, Christoph

[Rokytheman]

Originally Posted by augerpro

No I haven't. Are the diaphragm dimensions the same as the TW29RN?

Not sure if the dimensions is exact the same. The piston area on both tweeters is 9,6 cm^2. The normal holes for mounting the tweeter (with faceplate) is quite close.

Of course the two tweeters are not the same, but maybe a waveguide for the Satori-tweeter will work okay for the SB29.

[Gaga]

Early on in the thread at PE, we discussed this, and it would be fairly easy to design the phase plug as a standalone item with cylindrical keys at the end, with a matching depression in the waveguide wall which would be easily made with the end mill of the CNC. I "hope" with some optimization the phase plug won't be necessary.

I did some simulations with 3 different phase plugs. I think that the specific design of phaseplugs/phaseshields for a waveguide is a very good tool to achieve (as good as possible) constant directivity up to the high frequency range.

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):

SB26 in infinite baffle.png

and the respective directivity plot (normalized to 0°):

SB26 in infinite baffle directivity norm0.png

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:

Waveguide Augerpro eliptic phase shield 1 image.png

The respective SPLs from 0-90°, 15°-steps, normalized to 0°:

2018-02-03 eliptic WG phaseshield 1 SPL 0-90deg 1 .png

The same for a phase-plug with higher diameter:

Waveguide Augerpro eliptic phase shield 2 image 1.png

SPL 0°-90° (as above):

2018-02-03 eliptic WG phaseshield 2 SPL 0-90deg 1.png

And a phase plug version 3, a ring with central hole:

Waveguide Augerpro eliptic phase shield 3 image 1.png

Again the directivity 0-90°:

2018-02-03 eliptic WG phaseshield 3 0-90deg 1.png

Where does the phase plugs impact directivity? As a comparison, the directivity 0°-90° of the waveguide without phase plug vs phase plug V2:

Waveguide eliptical without vs with phase shield 2 0-90Gr horiz 1a.png

For better comparison the horizontal directivity without phase plug:

2018-02-03 eliptic WG Brendon wo phaseshield SPL 0-90deg 4.png

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.

BW, Christoph

[Ro808]

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.

[augerpro]

Originally Posted by Gaga

Before performing more simulations of different phaseplugs, I'd suggest to start with the WG-contour instead, if you're interested.

BW, Christoph

I agree.

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.

[Gaga]

Interesting!

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 did simulations to compare the impact of a simple 'disc' vs a 'ring-type disc' in the waveguide thread here. Diameter and placement of the 'disc' and 'ring' were identical.

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:

ring phase plug image.png

Directivity -90° to 90°, normalized to 0°:

disc phase plug directivity norm0.png

Note the 'widening' aroud 17kHz.

Same simulation with 'ring' phase plug:

disc phase plug image.png

Directivity, normalized to 0°:

ring phase plug directivity norm0.png

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.

[augerpro]

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.

[Gaga]

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....
BW, Christoph

[augerpro]

Doh, I knew that, sorry!