The frequency where things start to differ goes up with throat diameter going down. Is this some far field beaming, but then closeby?
18sound ND4015Ti2 (1.5")
(I haven't checked that I accidentally didn't place the mic againt the phase plug wall.)
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).
(I haven't checked that I accidentally didn't place the mic againt the phase plug wall.)
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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Extension duct with an entropy raiser 🙂
Should have printed it a tiny bit smaller... This was 0.6mm nozzle - not the best choice for this but I'm not going to change the nozzle now.
Should have printed it a tiny bit smaller... This was 0.6mm nozzle - not the best choice for this but I'm not going to change the nozzle now.
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)
More points:
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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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.
I recently noted the Beyma T25S tweeter with its removeable diecast waveguide
https://beyma.com/nmf/beyma-speakers-data-sheet-direct-radiation-tweeter-T25S.pdf
I wondered if an ATH4 replacement is worthwile for a little more control
https://beyma.com/nmf/beyma-speakers-data-sheet-direct-radiation-tweeter-T25S.pdf
I wondered if an ATH4 replacement is worthwile for a little more control
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Beyma CD171Fe/PT also looks good for a 1"
https://beyma.com/new-compression-drivers/tsheets/CD-171Fe-PT-8-OH.pdf
"DEPLOCEX Technology"
https://beyma.com/new-compression-drivers/tsheets/CD-171Fe-PT-8-OH.pdf
"DEPLOCEX Technology"
If the wall slopes were not equal, then this could also cause a reflection very close to the incident.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.
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.
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 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.
- 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.
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.
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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Provided that they are smooth enough on the inside there are capillary tubes that may be of interest to you then:
https://www.amazon.com/CCTVMTST-Stainless-Capillary-Tubing-Diameter/dp/B07KV2JCYL/ref=sr_1_14?crid=2PXKUQDAW86K4&keywords=Stainless+Steel+Capillary+Tube+OD+1mm&qid=1679647217&sprefix=stainless+steel+capillary+tube+od+1mm,aps,362&sr=8-14
https://www.amazon.com/CCTVMTST-Stainless-Capillary-Tubing-Diameter/dp/B07KV2JCYL/ref=sr_1_14?crid=2PXKUQDAW86K4&keywords=Stainless+Steel+Capillary+Tube+OD+1mm&qid=1679647217&sprefix=stainless+steel+capillary+tube+od+1mm,aps,362&sr=8-14
BTW, this is the Makarski's work, using a thin probe: http://publications.rwth-aachen.de/record/61507/files/Makarski_Michael.pdf
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.
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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Pretty much what I had in mind.
A r/Theta CNC type table might be easier.
Goes up to 14kHz with not such a small mic assembly.
A r/Theta CNC type table might be easier.
Goes up to 14kHz with not such a small mic assembly.
⌀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).
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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With a two input soundcard you could do that in one go with the Makarski arrangement.
If he's using two tubes together, that might be why the mic "column" looks way larger than 1 or 2mm.
If he's using two tubes together, that might be why the mic "column" looks way larger than 1 or 2mm.
No, one probe at a time 🙂 (I may missed what was finally used but there's a comparison in the paper.)
This is also referenced: https://www.bksv.com/pdf/Bp0659.pdf
This is also referenced: https://www.bksv.com/pdf/Bp0659.pdf
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