If you want to achieve highly accurate results, a precise sim of the dome tweeter becomes very important.
For instance: does ABEC (AxiDriver) take the aperture effect into account? > probably not, since it assumes an ideal membrane.
So I'm guessing modeling the TW29RN dimple dome isn't possible with ABEC? Would be a bummer since my measurements today are pretty wacky, and the iterative process might take awhile.
BTW I hope to finish up measurements tomorrow and start posting results. I think I have the SB26 nailed on the 5" waveguide (now version F).
Should be possible with a optimized/custom driver script.
This is one of the many features in ABEC, without images of the formula's and electrical diagrams:
The standard model of an electro-dynamic transducer models the electrical, mechanical and acoustical parts.
The electric network parameters are the voltage U and current I. The mechanic parameters are the force F and velocity v. The acoustic parameters are the pressure p and volume-velocity V, one pair for the front and one for the rear side of the diaphragm.
The electrical part consists of the Voice Coil Impedance Zvc and the electro-mechanic motor.
The mechanical part is modeling only one rigid body motion, which is the translation in axial direction. Rotations and eigen-vibration of the diaphragm are ignored.
The vacuum allows to define certain technical parameters:
- Resonance frequency
- Mechanical quality factor
- Electrical quality factor
- Total quality factor
- Mechanical compliance.
Looking at the Diaphragm Shapes and the Case Tree, it seems only regular cone & dome shapes are supported:
WG design in progress...
If the dome could be specified as a convex dome with a concave centerpiece, it should theoretically be possible.
Christoff might shed a light on this.
However, the membrane if the TW29RN is still fixed in the center, making it a 'ring dome tweeter'.
A good indication is the given Sd in the spec sheet: Sd=9.6cm2
The corresponding diameter of a circle with Sd 9.6cm2 is around 35mm. From the datasheet we can roughly take these dimensions:
The diameter of the dome is 29mm, corresponding to the voice coil diameter given in the datasheet.
There is a relatively wide suspension: Dia with suspension is around 43mm, i.e. the suspension is 7mm wide.
Usually, one would assume, that roughly the half of the suspension is radiating, i.e. contributing to Sd as given in the datasheet: Radiating surface = Dome + Suspension/2 = 36mm.
If the full area would contribute to radiation of sound, Sd would be around 10.2cm2.
The Sd-Difference is 10.2cm2 - 9.6cm2 = 0.6cm2. This area corresponds to a circle of 8.7mm (i.e. are of the dome not contributing to radiation because of fixed dome center. Though the datasheet indicates a diameter of 4mm, this might be still reasonable.
Note that is a bit more of the suspension would contribute to radiation, the non-radiating dome center would be less accordingly.
So if I would try to model the TW29RN with ABEC, I'd model it as a ring-dome tweeter and construct a dome (by 3D-CAD) with the dimensions given above and import it to ABEC.
I might give it a try, if you like...
Here are results for the SB26 5" waveguide version F, and the SB19ST 5" waveguide version A. I'll get the TW29RN and N26CA out over the next couple days.
The midband is much smoother over previous results due to the .02" lip I incorporated after shims and putty showed the faceplate was not sealing well against the mouth of the waveguide:
Speaking of sealing, in actual use you want to seal a couple gaps on the tweeter that do not seal like you might expect. I would use blue tack putty, easy on ,easy off:
Unsurprisingly the phase shield was slightly worse with a fabric dome. OTOH I didn't expect that on axis dip even without phase shield. I believe the shape of the dome requires another look at throat angles as this a small diameter but tall dome. Nearly a 180 degree hemisphere. I'm *guessing* I need to relax the throat angle some.
One thing I've noticed so far is that everything looks much better on the 8" waveguides (results coming soon). I guess I assumed that a little waveguide would cause little problems, but that does not appear to be the case. In fact just going to say, 6" wide waveguides might solve a lot of the top octave issues I've had to solve. But I had to start somewhere!
Just now actually looking at the results...I'm really happy with the way the lip smoothed the response of version F, but now the plots (say 10-40 degrees) are not as straight as version C in post 1. They have more arch. I didn't want that and not exactly sure I how I got it. The only other difference was the flare radius vertically: C is 1", F is 1.1", and E (below) was 1.25", which I didn't care for and should have even worse curving of the responses if that was the cause, and it doesn't.
Dagnabbit, I had a flawed phase shield that I *thought* I had corrected before this went to the printer, but had not. Rotating one direction vs the other produces a different response above about 8khz, with one direction being much straighter and the overall response lifted >8khz. The corrected phase shield *should* be an average of the two differing directions.
Now I have to print to verify. I can do other tweaks while I'm at it, like changing that lip depth, which impacts the very top of the range near the soundcard's cutoff. The dome's resonant spike is near there and some tweaking could get it to be nulled down.
Christoph> if you get a chance, work the sim more on the SB26ADC. Of interest: a shim at the throat to move the dome back, vertical wall radius, and to a lesser extent the horizontal wall radius - though I feel that is pretty right on.
Christoff, if it's not too much of a hassle, it would be interesting to see the effect of a small area of "low curvature" (or even flat) in a (rather deep) waveguide for a hemispherical dome tweeter.
Some space of 2-4mm around the surround "to breath" > might eliminate/cancel interferences.
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