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Spark calibration of microphones
Spark calibration of microphones
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Old 2nd August 2021, 03:03 PM   #341
scott wurcer is offline scott wurcer  United States
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The one above is 70kHz or so (not that far off), Ryberg shows the null frequency vs spark energy. Mine were around 35kHz so I just added a scaled version of the theoretical curve, close enough for me (<-+1dB).
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Old 2nd August 2021, 03:23 PM   #342
keantoken is offline keantoken  United States
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Here is an interesting article on ultrasonic condenser microphones:

Microphones

Interesting idea to use a watch spring and/or balance wheel for the backplate. Also, CRT shadow mask material.
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Old 2nd August 2021, 05:30 PM   #343
1audio is online now 1audio  United States
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Spark calibration of microphones
Interesting work. Keep in mind that most of the work is almost 50 years old. Still not much has changed. I could not figure out where the commercial microphones he described were sold.

In doing a little research a found this https://www.knowles.com/docs/default...5h-qb-revh.pdf Here Heterodyne Ultrasonic Bat Detector – MarcelMG Blog – electronics, engineering and more available here Digikey for $0.76. Things have moved a lot in 50 years. One huge virtue of Mems transducers is very high reproducibility. The variations in sensitivity and response are quite small.
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Old 2nd August 2021, 11:22 PM   #344
1audio is online now 1audio  United States
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Spark calibration of microphones
I have the coil drive pretty well optimized now. This is with 50 Ohms between the coil and the cap. The 87 Ohms might be better but this is close enough I think. I'm getting very clean sparks it seems. The current duration is about 50 uS. Does that mean the period between the pulses of the doublet is around 50 uS? That would translate to around 20 KHz? maybe including the finite nature of air etc. could explain the 30 KHz limit?

I could build a different sparker using a resistor, caps and a high voltage supply but I'm not sure it would be better. Here is a youtube on such an effort How to model a spark gap in LTSpice - YouTube The challenge is staying awake. Still its pretty good.
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Old 3rd August 2021, 02:49 AM   #345
keantoken is offline keantoken  United States
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Well going on the theory, the more energy in the discharge the longer the period of the N-wave, meaning a lower HF limit. This is because when the maximum compression of air is reached, the only other way to expand is in wavelength.

The fact that the coil continues to ring after the spark suggests that it is no longer shorted by the spark. There may be some glow discharge extending the doublet period, perhaps smoothing the negative tooth of the doublet and therefore filling in the nulls (but maybe N-waves don't work that way). If it works this way then this may be why a coil seems to give different results than a direct discharge from a capacitor.

If the ringing is fast enough without decay then the next peak may keep the spark alive.

It is the bright plasma which creates the gaseous cavity in which the doublet expands and implodes (if I interpret the literature correctly), so it is the current spike and not the trailing voltage decay which produces the main peak of the doublet.

I had a similar thought that since the discharge event had a 2-3 sample width in the recording, maybe that could be extrapolated to the HF limit of the doublet.
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Old 3rd August 2021, 03:38 AM   #346
1audio is online now 1audio  United States
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Spark calibration of microphones
These waveforms are on the primary of the coil. Tomorrow I'll measure the current in the spark and see how long its conducting.
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Old 3rd August 2021, 07:20 PM   #347
1audio is online now 1audio  United States
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Spark calibration of microphones
At the spark gap. The upper tace is a Hantex ignition probe and the lower trace is the current from a current transformer. I am not sure what to make of the upper trace. I guess I'll set up the p6015 to compare. The lower trace is around 50 mA/div. Not a scaling this scope supports. In any case the current is gone in less that 200 nS. No second arc (I checked).
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Old 3rd August 2021, 07:44 PM   #348
1audio is online now 1audio  United States
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Spark calibration of microphones
Hanteck probe going back not suitable for this. may work for older car ignition.

Upper trace is current not sure on the calibration. Lower trace is the voltage at the arc from P6015.

Now that its all working time to rework the spark gap and remove all the stuff that will degrade the quality of the shockwave with near field reflections.
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Old Yesterday, 06:04 AM   #349
keantoken is offline keantoken  United States
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So the discharge is over much faster than the blip I see on the recordings. I figured this might be the case but it is obvious now.

It looks like there is some 60MHz oscillation at the beginning of the discharge. That is a quarter wavelength of 1.25 meters, maybe related to the coax.
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