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Old 2nd Jan 2014, 6:53 pm   #1
Martin G7MRV
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Default Electret microphones with very high frequency response? (~120kHz)

Hi All,

Ive drawn much of a blank elsewhere as far as this goes, so hoping someone on here will have some insight,

Im looking for an electret microphone insert that will have a usable frequency response up to the ultrasonic region, around 120kHz. This is to improve on my 'Project Fleidermaus' bat detector that I designed and built many years ago, which currently uses a 40kHz ultrasonic transducer. This is fine for the pipistrelles living around my back yard, but not much else!

I have been told the MCE-2000 and MCE-2500 electrets are usable, but cannot find any actual response data, nor at the moment any actual microphones!

Any suggestions welcome
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Old 3rd Jan 2014, 1:19 am   #2
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Not a direct answer to your question, but you would need to be looking at a transducer with a very small diameter diaphragm.

Maybe some of the miniature electrets would be worth a try although I would be surprised if they went up that high.

Acoustic measurement mics for those frequencies are typically 1/8" diameter (and very expensive)
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Old 3rd Jan 2014, 2:35 am   #3
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Default Re: Electret microphones with very high frequency response? (~120kHz)

You'll just have to imagine the prices!

http://www.bksv.com/Products/transdu...one-cartridges

These are true capacitor mikes, not electret and so they need 200v polarisation sources as well as matching preamps.

Going to very small diaphragms puts up the frequency response at the expense of sensitivity, and affordable mikes were all aimed at the audio market, so common mikes weren't made that small until they started putting them in small places.

The electret and preamp shouldn't put any serious limit on HF response, but manudacturer used masks with small holes to shape HF response. Maybe a little microsurgery?

David
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Old 3rd Jan 2014, 12:11 pm   #4
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Default Re: Electret microphones with very high frequency response? (~120kHz)

I made a bat detector a few years ago and I went through my large collection of electrets to find the best, which turned out to be a no-name type which looked very cheap. I'm struggling to get much above 26 KHz though.
A friend tells me that those piezo HF speakers (tweeters) work well. I once had a massive collection of those disc-type piezo sounders, but like a fool I chucked them
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Old 3rd Jan 2014, 12:26 pm   #5
Martin G7MRV
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Interesting Andy, you should be able to do much better than that unless the mic has a very poor response. The common pipistrelle uses about 45kHz and the soprano 55kHz. Have you heard any bats on it?

The problem I have is that since mine currently uses a transducer for 40kHz, common pipistrelles are about all I can hear on it!

Ive seen a source of one of the suggested suitable mics, the panasonic WM61, so might get those and try them. AT £6 for a pair its not cheap, but if they work then for another tenner I can build another detector to use the second one. For the cost of those capacitance mics I could build another hundred detectors!
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Old 3rd Jan 2014, 1:21 pm   #6
Martin G7MRV
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Well, bit the bullet and ordered the WM61s, will see how they perform when I get them,

If anyones interested, my detector is based on the designs here http://bertrik.sikken.nl/bat/ne612het.htm
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Old 3rd Jan 2014, 3:50 pm   #7
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Default Re: Electret microphones with very high frequency response? (~120kHz)

I'd be tempted to unsolder a mic insert out of an old cassette recorder, and try that. The worst it can do is not work .....
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Old 3rd Jan 2014, 7:59 pm   #8
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Quote:
Originally Posted by Martin G7MRV View Post
Interesting Andy, you should be able to do much better than that unless the mic has a very poor response. The common pipistrelle uses about 45kHz and the soprano 55kHz. Have you heard any bats on it?
I have actually heard bats, but only at fairly close range. At the holiday cottage we stayed at last year, a couple of bats were zooming around in the garden one night. I was standing outside 'satellite spotting' when I became aware of them, so I nipped inside and grabbed me box.
Up at the HF limit, I could hear a ticka-ticka-ticka sound whenever a bat faced me in flight. It was quite interesting, as the repetition rate increased as the range reduced.
Another sound I heard was coming from the far end of the garden. It was a kind of pinging sound and I kept hearing it faintly when in certain spots. I traced it to the ditch just over the fence at the far end. I couldn't hear anything without the batphone. Must have been insects of some description.
I really must put my little box on the bench and make it better!
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Old 3rd Jan 2014, 11:38 pm   #9
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Default Re: Electret microphones with very high frequency response? (~120kHz)

How about the very weeny microphones in 'in lead' mobile phone headphones. Very small.
 
Old 4th Jan 2014, 4:37 am   #10
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Anything with a diaphragm in the 5mm and down region is promising. But watch out for a cavity in front of the diaphragm. Often the cavity has a small hole in its top, and it acts to shape the frequency response, tailoring a phone to the voice. It might be an interesting challenge to cut off the filter without damaging the diaphragm proper.

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Old 4th Jan 2014, 10:29 pm   #11
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Hi,
I cobbled a simple bat detector together using an ultrasonic remote control transducer from a scrap TV, a VLF converter and my Saisho SW5000 reciever. Very rough & ready, but it worked well enough to 'see' what was flitting about the trees in the garden opposite. I can't remember exactly, but I think that the output of the converter in MHz equalled the input in KHz. The transducer seemed most sensitive at around 28KHz.
Cheers, Pete.
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Old 6th Jan 2014, 9:41 am   #12
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Many small electrets will work at frequencies much greater than their data sheets suggest. The difference here is that very few, if any manufacturers either measure or specify their products above around 20 - 25kHz unless it is specifically intended for ultrasonic applications.
At work we use B&K reference microphones which have a response that extends to around 100kHz or so but as already pointed out, they require a high bias voltage and they cost silly money.
I have used ordinary (less than 5mm dia) electrets for things like bat detectors and leak detectors in the past and they invariably do work but the catch is that the sensitivities will be well down at 40kHz and above and the sensitivities will vary significantly not only from one type of microphone to another but between the same types of microphones.
At work we use one commercial electret microphone in an ultrasonic application and it works well but the downside is it's sensitivity is around 40dB or so lower than that at mid band audio frequencies and in order to calibrate it a variable element in the gain path is also required.
So for a one off application you can tailor the gain to suit but don't expect anything like a flat response at those frequencies.

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Old 28th Feb 2014, 2:18 pm   #13
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Just resurrecting this thread to report that after continuing poor results with all manner of electret inserts, I ordered some 40 KHz TX and RX piezo transducers from CPC. I'm now actually getting very good sensitivity up around 40KHz plus and minus 5KHz, but of course the response drops off quickly beyond these limits.
I stuck one of the TX ones on the output of my sig gen for the initial test and almost blew the little batphone speaker to bits! Cor, it came through LOUD.
I still haven't found any 'ordinary' transducer that works over a wide range.
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Old 28th Feb 2014, 6:55 pm   #14
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Default Re: Electret microphones with very high frequency response? (~120kHz)

I built this Maplin kit bat detector some years ago. It appears to be no longer available but it does tune upto 160 KHz. The mic insert is a tiny rectangular thing.
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Old 26th Nov 2014, 9:35 pm   #15
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Hi martin,
replying to an old post but the following might be of interest.

I co designed the Maplin bat detector you showed with Maplin after many years of studying bat sonar and making bat detectors.

The Maplin detector used a Knowles microphone EK 3132 , which although specified for audio had a usable response into the ultrasonic region. From my log book notes 13 June1994 I measured the output of the EK 3132 at 5 mV on a pipistreal bat flying round my garden but did not specify p/p or rms. This microphone had a usable response up to 160 Khz . As mentioned many of the small electrets work into the ultrasonic range but accurate measurements of sound pressure levels in this range are very tricky as you are working in the millimetre wavelength range giving standing waves all-round the microphone.

Unless you are trying to track the lesser and greater hourshoe bats which use 110 Khz and 85 Khz you can put a ceramic 25 Khz and a 40 Khz Ceramic resonator capsules in series this gives a broad response from 20Khz up to 60 Khz. Not a flat response by any means but very usable in field trials I have attached a photograph of one of the detectors I made showing the different diameter resonators behind the grid.

Commercial capsules designed for ultrasonic frequencies (B&K ¼ inch) are very expensive but one area you can explore is making capacitance microphones , if you look at the other photo I attached this is one I made from 6 uMeter capacitance film 4 uMeter film can also be used but is quite delicate. ( from old capacitors or contact a capacitor manufacturer they have sheets of it ). You form a small capacitor by stretching the film over a porous conductive backplane I tried various backplanes from sandpapered , sand blasted , machine turned surfaces. These all worked but the simplest is 12mm dia brass sintered filter material fine grade as identified by professor Pye. So referring to the photo lathe turned capsual you can see the bronze disk and the film disassembled.
Not having a lathe I made mine as in the photo showing the aperture in an aluminium strip.

V out = Q/C where Q is held constant via a 100 Meg 160 V charge and C is varied by the ultrasonic pressure wave.
Specification of the round microphone is better than -70 dB ref 1V/ubar at 180 Khz and -50 dB +/_ 3 dB over 20 to 110 Khz.
Check out

http://www.echolocation.co.uk/index....es/microphones

Proffesor Pye of Queen Mary Collage London referenced in this 2011 artical is someone I spoke to numerous time in the 80’s whilst working on bat microphones and I note from the web page diagram a capsule very similar to the round one professor Pye sent me.
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Old 28th Nov 2014, 5:55 pm   #16
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Default Re: Electret microphones with very high frequency response? (~120kHz)

To Conclude,

I thougt it might be of interest to show my bat detector collection with a brief description of the technologies used , including one of the first comercial detectors sold to the public .

apologies as It was Panrock who showed the Maplin detector not Martin.

Attached pdf.
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File Type: pdf Bat Detectors and Microphones.pdf (1.24 MB, 1486 views)
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Old 30th Nov 2014, 6:34 pm   #17
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Default Re: Electret microphones with very high frequency response? (~120kHz)

Wow, what an interesting thread! I built a bat detector to my own design back in the 90's; I used a 40kHz transducer and an electret capsule that were selectable one or the other, followed by a low noise preamp and a diode ring mixer (built using miniature audio transformer at one 'end' and push-pull drive from the local oscillator tuning 20 to about 60 kHz at the other); then an LM386 to drive the output.

To test it, I used a second 40 kHz transducer built in a separate box with a CMOS IC oscillator as a test source. The first thing I noticed about this was that our cat could very obviously hear it.

Up to about 30 kHz the electret was pretty good- you could 'hear' the 32kHz crystal in my watch with it. Above this, it fell off very sharply and at 40kHz was 20-30dB down on the Murata 40kHz transducer. The detector, switched to the ultrasonic transducer, heard bats over a limited range (they had to be close enough to be obvious to the eye) and actually the range using the CW test source was disappointing too at only around 30 metres out in the open; the signal-to-noise ratio of the detector was just not good enough. You can actually hear the effect of the resonant transducer as you tune through 40kHz.

With the test source, it was fascinating to hear the effects of Doppler shift on moving the detector around though; especially if there were reflections from other nearby objects as well. You could make some very spooky sounds! But that was as far as the project went at the time; I simply needed a better microphone.

Now I've read this, and especially having read about different bats working at 55 and 110 kHz, it might be time to revisit it. It looks as though DigiKey stocks the Knowles EK3132 mic, £10.80 in one off's. They mention 'ultrasonic applications' for it.

http://www.digikey.co.uk/product-det...1008-ND/458519

Cheers,
Alan
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Old 5th Dec 2014, 2:16 am   #18
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Default Re: Electret microphones with very high frequency response? (~120kHz)

With equalised amp I was using the small size electret up to about 60KHz. Sensitivity was poor. I was using FM Audio and an FM IF IC for audio link. The earphone was a small piezo disc about 12cm which was also used as PSK 40KHz low speed reverse data link. 1989
It implemented a combined cordless mouse and phone handset shaped like a pen. No Bluetooth or WiFi then!
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