There is no resistor trust me, this is a behavioral schematic representation there is only one extra diode from gate to source. There is no small scale IC technology for making multi GOhm resistors.
I can say with reasonable certainty that my mics are conv. foil type. There is no plastic coating on the backplate, and the diaphragm is metallized both sides.
Pardon brevity, my left hand is half paralyzed ATM. Flurging wrist drop. =/
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The panasonic is not ruler flat, it actually has roughly 3dB of peaking at the top end, then drops back down, like any small pressure microphone.
If you have some of these capsules, or even some similar, you must look at: John Conover: Using the Panasonic WM61A as a Measurement Microphone
See figure 3 for a HF typical response.
BTW, if you are planning to use this for measurement, there is little reason to DIY, a calibrated mic from Cross Spectrum or other sources is much more cost effective unless you have a lab grade standard available.
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Then they would need an external polarizing voltage, I doubt these do. Without the electret (the coating can be virtually invisible BTW) or an external applied E field there is no output. There is no such thing as a metal diaphragm and metal backplate that acts as a microphone.
BTW I completely disassembled a WM61 and posted the pictures on the micbuilders yahoo site with some computations of the parameters from measurements with a micrometer. This is all you see the electret is not visible to the naked eye.
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They definitely do require an external voltage, but not for polarization. This is the whole point of an electret - it retains a semi-permanent EMF, like an electrical analog of a permanent magnet.
The external voltage is used to operate the internal FET impedance converter. That's what the 2K2 and 3-5VDC external power is for.
I'd like to see those photos, do you have a reg-free link?
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I for one am not. My present interest in ECMs is driven interest in building a parabolic mic, which I can use to listen to forest sounds at night.
BTW, I assume that a unidirectional mic would be best for use in a parabolic, but that might be totally wrong. Thoughts?
Speaking of, what are the main differences between omni and uni-directional capsules? I see the additional holes on the back of the uni, but that's about it, externally.
The directionality of a mic capsule probably assumes plane waves, so same direction and same intensity. Put it in front of a parabolic reflector and things change. How much they change I don't know. It would depend on exactly how the capsule obtains its response.
Well, you certainly don't want rear response from the mic. That does suggest a cardioid response might be a good choice.
If you consider the diameter of the (parabolic) reflector, and its focal length, you can estimate the maximum off-axis angle at which sound waves, after reflection off the reflector, will enter the mic. This will tell you roughly how wide a "lobe" you want the mic to have in its forward response. (Cardioid vs hyper-cardioid, etc.)
The relevant math is basically:
tan (max angle) = radius of mirror / focal length.
The attached image is for light and a concave mirror, but it works in exactly the same way.
-Gnobuddy
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@All
So about the obsolete Panasonic WM61 parts.. there are plenty on offer by Chinese / Taiwanese eBay sellers, at very fair prices. Are these to be trusted, or are the WM61 also faked by the thousands?
@scott wurcer & All
I found a set of pics on another site, depicting a disassembled WM-61A. Assuming that it's a genuine part, I can see what you mean about the electret layer (deposited on the back plate) being effectively invisible. Aside from different hole stamping patterns, I can't tell the WM61 back plate from the one removed from one of my generic ECM capsules.
Likewise, there isn't any discernible difference between the diaphragms, either. At this point, I simply cannot say whether or not the caps I got are back or foil electret types.
So about the obsolete Panasonic WM61 parts.. there are plenty on offer by Chinese / Taiwanese eBay sellers, at very fair prices. Are these to be trusted, or are the WM61 also faked by the thousands?
@scott wurcer & All
I found a set of pics on another site, depicting a disassembled WM-61A. Assuming that it's a genuine part, I can see what you mean about the electret layer (deposited on the back plate) being effectively invisible. Aside from different hole stamping patterns, I can't tell the WM61 back plate from the one removed from one of my generic ECM capsules.
Likewise, there isn't any discernible difference between the diaphragms, either. At this point, I simply cannot say whether or not the caps I got are back or foil electret types.
My photos are on another computer I'll post them if I boot it up. My capsules are from Panasonic long before they were obsolete. BTW there really is nothing to cheapen there are lots of folks making decent mics from Chinese capsules. The Transound ones in particular.
AFAIK the electret is some kind of sputtered on teflon that is exposed to a huge e-field which it retains its charge for some sources say >200yr. I have 40yr old capsules that still have exactly their original sensitivity. The coating is far thinner that plumbers tape I see no reason that it would not be virtually transparent.
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For my application, the 'best' capsule has a high s/n and sensitivity, but I don't much care about bandwidth. If I can get a solid 100-10KHz response, that would be just fine. Again, this is for a parabolic that will be used to listen / record forest sounds at night.
It's not hi-fi work, and and resultant recordings can be eq'd as needed. The goal is to listen to birds and other aquatic animals from across a lake.. among other things.
As for pattern, it seems that only omni and uni are available. No mention of cardioids, etc. For the parabolic, I'd think an omni might be fine so long as it's mounted in such a way as that any sounds other than from the reflector are damped out.. like, mount it in a foam rubber block.
It's not hi-fi work, and and resultant recordings can be eq'd as needed. The goal is to listen to birds and other aquatic animals from across a lake.. among other things.
As for pattern, it seems that only omni and uni are available. No mention of cardioids, etc. For the parabolic, I'd think an omni might be fine so long as it's mounted in such a way as that any sounds other than from the reflector are damped out.. like, mount it in a foam rubber block.
If you want good S/N, get a larger diameter capsule as they have lower self noise. Larger diameter capsules are also available in a lot of patterns.
BTW Unidirectional and Cardioid are the same thing.
You might try a shotgun mic, perhaps easier to aim, not sure how easy they are to DIY. I get the feeling they are just a tube with slots cut in it, in front of an omni, ot perhaps a cardioid.
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Interesting, I had no idea. I mean, I figured that larger caps were 'better', but self-noise wasn't part of my thought process. Again, I make no pretense as to understanding mics.
I was semi-suspecting this, so what you say makes sense. Thanks for filling it in, though, much appreciated.
Isn't a shotgun mic another form of highly-directional mic? Need to look that one up. Would a shotgun be good receiver element for a paraboilc?
Yes, it is. 🙂
No, I don't think so. As mentioned earlier, the sound from a parabolic will arrive at the microphone capsule from a fairly wide range of angles; you want a "unidirectional" capsule, but one with a fairly wide-angle response.
You might try to visualize a car headlight (bulb + reflector) if the diagram I posted earlier isn't making sense yet. Light coming off the bulb in all sorts of directions bounces off the reflector, and gets aimed forwards. If you reverse every light beam, and replace it with sound, you get the microphone + parabolic reflector case.
-Gnobuddy
Here is a teardown of a Primo EM23 capsule. This is a larger capsule with no FET so you can do your own optimized pre-amp. This capsule is one of the prefered ones for nature recordists because you can achieve 8-10 dBA noise with it. It is not cheap at $60 in ones, and I'm not sure they sell to individuals anymore. The EM21 is the Uni version. I have built two of these and have made recordings of music and nature, you would have a hard time doing better.
6 is the front viewed from the back, there is a grill and 8 ports, 5 is the diaphragm with integral spacer around the outside edge, 4 is the backplate with the acoustic loading hole array, 3 is the hardware the gold nipple goes up through 2 (the back of the capsule) and is the contact for the pre-amp, everything stacks and 1 screws into 6 with a spanner wrench to hold everything together. The actual electret is not obvious by visual inspection.
6 is the front viewed from the back, there is a grill and 8 ports, 5 is the diaphragm with integral spacer around the outside edge, 4 is the backplate with the acoustic loading hole array, 3 is the hardware the gold nipple goes up through 2 (the back of the capsule) and is the contact for the pre-amp, everything stacks and 1 screws into 6 with a spanner wrench to hold everything together. The actual electret is not obvious by visual inspection.
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A parabolic gives you gain, and some directionality. You would probably want to optimize focal length and diameter along with mic polar pattern to get something usable at the frequencies of interest. First thought is just a cardioid placed at the focus.
Heh, you may have misunderstood my question - or more than likely, it was just poorly presented. 😉
I'm aware of the behavior of a parabolic reflector - my question (using a shotgun) had more to do with the pickup pattern of the mic than the behavior of the dish. I was just wondering if the shotgun had a pattern that would would be easier to focus on, vs. a cardioid / hypercardioid.
(Obviously, the shotgun pattern isn't a coherent cylinder, it must have some angular spread.. see what I was on about?)
Thanks for that - once again, interesting and informative.
Fwiw, I have a variety of different ECM capsules & complete mics on order.. all of them are cheap-ola China stuff, but I figure they will be good for experimentation. Hard to beat the prices, 10pc. for $1USD.
It's funny.. I figured out that I could cut a trace on the PCB of my current capsules, and separate the FET source from the case ground, converting it into a "3 wire" capsule. This way it can be used as a source follower (common drain). As ever with the net, someone else already did it long ago - apparently it's called the Linkwitz mod. 😀
Shotguns are extremely directional - they will only receive sound from almost directly ahead of the mic.
This is absolutely, definitely, NOT what you want for a microphone you plan to aim at a large nearby parabolic reflector. Depending on the diameter and focal length of the dish, it will aim sound at the microphone from a fairly wide range of angles.
Ergo, you want a microphone which can respond roughly equally well to a wide range of angles, but all from the front of the microphone. Unidirectional, but not very unidirectional, in other words. In practical terms, cardioid is probably what you want.
Ideally, you'd want the half-angle of the forward response lobe of the microphone to be arctangent(reflector radius/reflector focal length), or something close to that. But "ideally" is not too significant in this case, practicality is what matters.
To put numbers on it: if you had a reflector dish that was, say, a foot in diameter (6-inch radius). Suppose it has a focal length of 1 foot. The reflected sound will arrive at the mic over a range of angles from zero degrees (straight ahead) up to roughly 27 degrees off-axis; a microphone with a 54 degree wide forward lobe would be a good match.
That number (27 degrees) comes from the equation I posted earlier, namely, arctangent(0.5 ft/1 ft).
A shotgun mic aimed at a parabolic reflector wouldn't pick up the sound bouncing off most of the reflector. Except for sound bouncing off the very centre of the dish, the rest would be entering the microphone at too wide an angle, and the shotgun mic would not respond to it. Most of the area of the parabolic reflector would be wasted, and would not help improve the sensitivity of the shotgun mic.
I'm still not sure I understood your question, as we appear to be going round and round in circles. If I still haven't understood you, sorry! 🙂
-Gnobuddy
Here's a site and an article on parabolic mics, it should answer most of your questions. The site might still sell everything you need.
http://www.froglogger.com/Documents/Parabolic.pdf
EDIT - yes the EM172 is a good capsule for this at $10. The EM23 is still in production but now $90 in ones. The link to their Documents directory seems to have gone missing but it works, there is some more stuff there.
http://www.froglogger.com/Documents/Parabolic.pdf
EDIT - yes the EM172 is a good capsule for this at $10. The EM23 is still in production but now $90 in ones. The link to their Documents directory seems to have gone missing but it works, there is some more stuff there.
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