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Old 2nd January 2012, 08:19 AM   #491
SM7UYJ is offline SM7UYJ  Sweden
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alexberg:

I put a 1mm rubber sheet under the PET-film and a 1kg weight on top of my tests probe => I get quite consequent readings (+/- 5-10%)

During the holidays a did a few tests with different coatings, and tried a few new mixtures of Elvamide.
One thing I discovered was I had used the wrong type of ink before!
It said "carbon black" on the bottle, conducted good when I drew a test line on a piece of paper but in solution it did not get the results I was looking for. After reading posts here I bought some small ink cartridges from Rotring and now thing started to behave as I wanted!
A 10% mixture of Elvamide and a couple of drops of ink. and the resistance was in the Giga ohm range. The coating sticks extremely well to the Mylar if you heat treat the film after applying the solution.
The downside is that the coating is "grayish" and not transparent.

Roger
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Last edited by SM7UYJ; 2nd January 2012 at 08:22 AM.
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Old 2nd January 2012, 11:14 AM   #492
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Roger,
would not you like to dry polyamide completely...
Elvamide can hold as much as 2.5% of water thus making the former conductive.
I cant' recall when I tried this particular ink brand, and whether or not I've posted the results here, but:
1. My "elvamide" (one of the many alcohol soluble polyamides) would not adhere well.
2. Resistance varied a lot simultaneously with the moisture content
Definitely "no go" for me.
I doubt that conductivity is due to carbon in the ink...

Alex
P.S. I'm too lazy to make real measurement tool like yours - it's much easier to spray paint two contacts each time you need them. Not so good for checking evenness of the coating though.
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Old 2nd January 2012, 11:32 AM   #493
SM7UYJ is offline SM7UYJ  Sweden
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Alex:

I think it is Elvamide 8063 that I have used, and it realy sticks to the PET-film!

Also did some tests mixing Elvamide with a permanent ESD product (not Staticide) and got good results but I get the same results using the Rotring ink!?

I have abt. 1kg Elvamide left, so if you would like a sample I'll be happy to send you some!

Roger
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Old 2nd January 2012, 05:24 PM   #494
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Now I've got som licron crystal I've also done some tests, for me it works best to dilute the stuff 1:5 with methanol and then I apply two layers of coating with a paintbrush.


For measuring the surface resistance it's difficult to make a good contact. The easiest way I found is to just drop some water on the layer of coating. (of course this only works well if your coating is not very sensitive to water)
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Old 2nd January 2012, 10:35 PM   #495
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Thanks for the report,zweetvoetje,I had figured that methanol would be a good solvent to dilute it with.
When I run out of my spray stuff I will just get a Gallon of it !!!

jer
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Old 3rd January 2012, 09:45 AM   #496
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Do you directly spray the licron crystal on the diaphragm? I think that that adds a little bit too much weight to the diaphragm.
I've measured that two layers of diluted coating adds around 1-2 grams/m^2. But the undiluted coating is much heavier. Probably around 10g/m^2. This will lower the sensitivity somewhat at 20kHz.

(I've done measurements by looking how a high note (18kHz) got through a layer of diaphragm. For 2 micron mylar with 2 layers of diluted I only see a phase-shift of 15-20 degrees. (10-15 degrees without coating). When the phase shift is so low there still is no considerable damping, but when it's 45degrees you're already at -3dB.
So if you have a thicker membrane and a thicker layer of coating this might become a problem.
(note that my measurements weren't very precise. My soundcard doesn't produce very clean sine-waves at this frequency and my tone-generator is currently broken down...)
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Old 3rd January 2012, 10:26 AM   #497
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Yes,I just spray it on to wet the mylar and then swab it around with a paper towel that has been wetted with the Licron Crystal.

I have found that just one application works well as long as there is complete coverage of the mylar.

I have done two coats as well just for good measure of having enough but as long it is covered well it is probably a waste of material.

I'm sure the stuff from the spray can is probably already thinned as most aerosol's are.

It makes alot of sense of your description of phase shift and the added mass,it explains why I can hear the difference compared to the original Licron Formula as it is much thicker.
Anywhere from .15mil to .40mil depending on how much I applied with the Original stuff.
I have found it to crack around the edges so I would put on a second coat around the perimeter.
The Cracking has caused a couple of diagphrams to not charge at all, Though it was an easy fix once I figured out what was happening.

I don't yet have a scale to measure mass and weights,But Charlie was the first one so far to measure its thickness and he used the same application technique that I did except he used a foam brush to swab it with.
He said that it was on the order of a few microns or less.

I am very interested in how you performed your measurements so that I might be able to duplicate and see the results as well.

I have a very good 24bit sound card as well as a signal generator and I just got a Dayton Measurement microphone with the calibration data.
I used to have some nice Audio Technica pen microphones but they came up missing one day.

jer
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Old 3rd January 2012, 12:00 PM   #498
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For the measurement I used my stereo-set connected to my computer. On the computer I used jaaa to generate an 18kHz sine-wave. (which doesn't look anything like a sine at an 48kHz sampling rate)
I put a microphone in front of my speakers (5cm) and looked at both the speaker signal and the microphone signal on my oscilloscope. (trigger on speaker-signal)
This way I can look at how much phase-difference there is between the speaker and the microphone when there is only air in between. Now when I put something between the speaker and the microphone I see a phase-difference and/or attenuation of the microphone signal. To measure a diaphragm I held a piece of mylar in between the tweeter and my microphone. (best to hold it quite close to the microphone)

When you know the amount of phase-difference and attenuation you can in theory calculate the mass of the diaphragm.
But I used a little trick, I knew the mass of the mylar was ~2.5g/m^2 and with coating the phase-difference was around 1.5 times as large, so the mass with coating is also around 1.5 times as large. (this trick only works for phase-differences of less than +/-30 degrees)

In the past I've also done the correct calculation to directly calculate the diaphragm mass from the phase-difference, attenuation, sound-velocity, frequency, and air-impedance, but I've got to search some time to find my calculations again...


The measurements are actually not that easy, because for me there was a lot of background noise (cars etc.) which made it difficult to measure the phase-difference exactly. With many modern digital oscilloscopes you can however average that out, I however only have an analog oscilloscope...

It should also be possible to do these measurements solely with the computer, but I think this can only be done reliably when the sampling rate of your soundcard is 96kHz or above. But I don't know if there is a lot of good software to do stuff like this. In theory you can record a wav-file (no MP3!!) with the speaker and the microphone-signals simultaneously and do some math with Mathematica (expensive) or Sage (free).
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Old 3rd January 2012, 01:30 PM   #499
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I hope there are no mistakes in here. I might be missing a factor 2 somewhere...

From solving the wave equations of a wave colliding with a membrane of mass m I get a transmission coefficient of:
Click the image to open in full size.
(Latexform: $S_{12}=\frac{1}{1 + \frac{m \omega i}{2 Z}}$)
with:
m: membrane density (kg/m^2)
omega: radial frequency. (2 Pi times the frequency)
Z: Impedance of air: +/- 400
i: imaginary sqrt(-1)

But what does this transmission coefficient mean?
It means that if a wave travels through a membrane, the amplitude of the wave will be reduced by a factor |S21| (the absolute value) and that it's fase will be retarded by an angle of arg(S21). (the rest of the wave will be reflected back)


This can also be used to calculate how well a diaphragm of certain mass will radiate high frequencies. Therefore however you should fill in half the mass of the diaphragm. (because the diaphragm sends a wave forward as well as backward)


(with a good calculator you can use imaginary numbers. I don't know if windows calculator supports it, but if not, use Wolfram|Alpha: Computational Knowledge Engine )
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Old 3rd January 2012, 05:54 PM   #500
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Very Cool,zweetvoetje !!

I Guess I will have to spend the next summer brushing up in my math skills some more.

Windows calculator won't hand this type of stuff as it is just a simple calculator.

I have a much more advanced programable type scientific calculator that I have found,But I haven't spent any time yet to learn how to use it.

I think excel can do this stuff and it is a program that I wish to learn well someday as well as Java.

But just knowing the measurement technique with a microphone will be of some use as well.

My sound card is a GINA24 and it does 24/96 and I think it will do 192k as well.
In some programs it does pass the test and in some programs only in 16 bit mode.
I have yet to physically test it to make sure.

Again,Thank You !!

jer
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