Acoustic Horn Design – The Easy Way (Ath4)

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#12,021
Celestion CDX1-1425 (1")

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#12,023
18sound ND4015Ti2 (1.5")
(I haven't checked that I accidentally didn't place the mic againt the phase plug wall.)

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This kind of exit will need some further propagation distance to average everything out (which seems to always happen inside a horn). That's the problem with the wave-shaping plug - there's no time to let the different ring areas to average out when they are still close together.

Which leads me to an idea of placing an additional segment of a duct between the driver and the shaping plug, perhaps filled with foam or something, just to enable this early averaging (and hopefully also dampening the higher order modes a bit).
 
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#12,027
Now I got lost. I made a 15mm extension for the CD131 (#12,024) and now there's a sharp cancelation near the center.
Is this to be expected / can this happen in a wavefront progression? I guess so but this seems a bit too eratic to me...
I checked the seal, etc., the results are pretty robust and repeatable. What to take from it?

0,5,10 mm off center (blue, red, green)
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More points:
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#12,032
mabat said:
Well if the above was caused by a reflection from the edge/corner under the mic plate, it would spoil all the effort. But then it would be strange that the length of the duct could have so strong effect.
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If the wall slopes were not equal, then this could also cause a reflection very close to the incident.

The technique shows promise - it can differentiate different driver in a critical range (I suppose, but > 10k is not really much of an influence on normal sounds.) But questions, like your test above remain.

I should think that leakage would be a big concern (but maybe not at such high frequencies.
 
#12,033
Maybe I don't understand what you are trying to do but you could reverse your set up:
- The ECM is fixed at the end of a long vertical tube (50cm?) with the minimal diameter possible.
(Many MEMS have nasty HF (>15kHz) resonances...
  • You add a conical termination to the Driver you want to measure with matched angle of a TBD depth
  • then you use a simple X/Y or maybe even easier r/Theta CNC type table.

You are then able to sweep any surface within the conical termination at a given z coordinate the limitation would be that no measurement could be made at less than (tube diameter)/2 from the walls.
 
#12,034
That's basically what Kees suggested, only he recommended MEMS. I'm a bit sceptical about diving an electret mic into the throat without disturbing the sound wave - the little electrets have ~5mm diameter, isn't that too big for that? Perhaps it would need to be a probe with an extension of a small tube but then I'd expect all kinds of different problems with that (maybe not**). Not mentioning that for every exit angle and radius you would have to make a different termination (it doesn't have to be very long though, as the end reflection can be gated out).

Now I try to figure out what's the problem with the conical extension I made above. It may not be so easy to actually make a smooth continuation to the segment inside a driver...

** A small extension tube (a needle for glue) could be attached to an ECM and any reflection within the tube could be gated out as well. For 0.3 ms time window (which seems enough) it would have to be ~5 cm long, perhaps still practical.

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#12,036
BTW, this is the Makarski's work, using a thin probe: http://publications.rwth-aachen.de/record/61507/files/Makarski_Michael.pdf

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I guess I should try it at least to verify my approach. The issue I have with this is that when connected to a (general) horn, typically there will be diffractions generated near the throat - how can this not affect what's measured? I should think that a conical extension is a must.
 
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#12,038
⌀1 and 2 mm tubes are mentioned.
BTW, he calculates particle velocites from pressure differences, i.e. measures at two slightly different heights. I guess that I would have to insert a 2-4 mm spacer for that with my technique.

Anyway, the differences across the cut plane that Makarski measured are ~3 dB maximum for a 2" driver... (and pretty non-axisymmetric which is also interesting).
 
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