Thanks for the suggestion, Brandon.
@Patrick Bateman meta-absorber files for sb26:
https://www.diyaudio.com/community/...amaterial-may-28-2023-123d-model-zip.1180352/
https://www.diyaudio.com/community/...tamaterial-may-28-2023-stl-model-zip.1180353/
@Patrick Bateman meta-absorber files for sb26:
https://www.diyaudio.com/community/...amaterial-may-28-2023-123d-model-zip.1180352/
https://www.diyaudio.com/community/...tamaterial-may-28-2023-stl-model-zip.1180353/
So I did exactly as you suggested & retested with foam + meta-absorber. Virtually no change in DATS result.
Tried changing the foam to a small ball of wool. Again, virtually no change.
Tried without any back at all -- wide open without foam. This dropped Fs to 350Hz. Foam changed impedance at Fs from 6.56 to 5.37 ohms.
All the post-mod results showed the same small peak at 1.2 kHz about 0.5 Ohms above the stick curve.
Possible reasons for the additional peak:
1. Some subtle damage/change to the dome, VC, former and/or slot?
2. This wrinkle is native to the moving assembly & is compensated by the stock back/wool?
Finally, I took the part that I cut away from the stock back, lined it back up as best as I could, and taped it up back to some semblance of normal: Amazingly, the 1.2 kHz disappeared, and the DATS result ended up being nearly identical to before the mod.
This suggests I should try to replicate the foam lining from the stock back cavity to eliminate that 1.2 kHz peak.
@Patrick Bateman -- Did you test your meta-puck modded SB26? Did you notice this phenomena?
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Some (Helmholtz?) resonance could be at play here. The hollow pole piece being the duct. You showed that the original chamber has a sort of cone that just might broaden that resonance.
@Patrick Bateman replicated that cone in his mod. The extra resonance doesn't look good, but whether it needs a solution will be clearer after I do some FR runs. I expect I'll be playing with different stuffing if it proves to be problematic.
Maybe I missed it but did anyone try to see how low the SB26ADC can be crossed stock before all the hacking and waveguidery?
I would place a cotton ball in the throat of the Puck, and likely use the foam or felt from the original. If there is no fill in the pole of the stock tweeter, a cotton ball wedged in there has always been beneficial in my experiments, and may allow absconding with the foam or felt.
If you check the test data from hificompass, distortion starts rising below 2khz tho FR is flat to <1khz. With passive I'd say 2khz is the practical limit, probably best with 2nd order. With active, 4th order might be ok pushed to 1.2khz. depends how clean you want it to sound.
Yeah, will start with that or similar. Waiting for the cold snap to ease. It's -11C out, and studio/workshop is too expensive to heat adequately.
Yes. I have not measured yet but @Patrick Bateman has. Will add link to his relevant posts about this in a couple minutes.
Here are those posts:
https://www.diyaudio.com/community/threads/unity-horn-with-metamaterial-tweeter.380104/#post-6949987
https://www.diyaudio.com/community/threads/metamaterial-acoustic-absorbers.380113/#post-6866563 --
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Just circling back on this:
The stock is capable of acoustic LR4 1.2KHz...
Do you ever get a chance to measure how your larger chamber and/or lower Fs changed things?
I wonder how would perform melamin foam (aka magic sponge for easy sourcing) when putted in the stock back chamber and tunnel ?!
Afraid not. It's been a busy year with lots of non-audio projects keeping me away from the studio. 🫤 Soon, I hope.