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Looking for a microphone which goes to 50kHz or so

There is an allegation that a particular aviation headset causes tinnitus.

It is one of a number of active noise reduction (ANR) headsets on the market.

An ANR headset uses basically a mike, a tiny speaker and some op-amps inside each earcup, and the speaker is driven so as to generate a zero SPL inside the earcup.

That would just give you noise reduction, and the audio would get cancelled too, so they feed the audio into the summing node of the op-amp, so you hear it OK.

So... all the components required for oscillation are in place!

And it's true that some of the cheaper ones do "whistle", or give out an audible hiss.

The headset in question (I have two - $1000 each) certainly doesn't whistle, to my hearing, so if it does it will be high up, and because some pilots are teenagers, it would have to be well above 20kHz otherwise there would be loads of complaints.

I can't see any possible route to ear damage other than a very high pitch oscillation, possibly confined to when there is an incoming audio.

So I would like to test for this.

I have a TEK digital scope which can do FFT, and have a Marconi 2024 spectrum analyser which goes from 10kHz to 2.4GHz :)

Can anyone suggest what sort of microphone would work well? I don't want to spend loads of money on it, say $100 max, and it needs to be easy to interface.

I was also going to get a ribbon tweeter (loads on Amazon for about $40) to test the frequency response of the mike with, using my HP3314A signal generator. But of course the tweeter won't be flat-output anyway. But then I am only testing for *any* component above what the headset is supposed to be emitting...

Any suggestions would be much appreciated.

Reply to
John-Smith
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On a sunny day (Sun, 04 Jan 2015 20:20:58 +0000) it happened John-Smith wrote in :

I think many cheap electret microphones go way up above 20 kHz, maybe to 40 kHz or higher with some attenuation.

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I have some of the small ones, dirt cheap.

Reply to
Jan Panteltje

Can't you just check the _circuitry_ with an O'scope to view oscillations? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

On a sunny day (Sun, 04 Jan 2015 20:33:51 GMT) it happened Jan Panteltje wrote in :

PS maybe it is PWM audio and the RF not filtered. PWM audio is quite common these days for celphones, there are specifc driver chips for that. Circuit diagram would tell you... Chip numbers.

Reply to
Jan Panteltje

ISTR 40kHz ultrasonic transducers that used to be used for intruder alarms, remote controls etc worked upto far above 40kHz. If all else fails its not hard to make a crude carbon mic with metal foil as a diaphragm.

NT

Reply to
meow2222

+1 
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www.wescottdesign.com
Reply to
Tim Wescott

As already mentioned some electret microphones do respond well to frequencies above 20kHz, but others do not. The manufacturers do not normally specify this, although they will sometimes tell you if you ask. Knowles were good when I wanted extra data about their mics.

If you don't have a mic with some sort of calibration, not seeing anything proves nothing. B&K will be happy to supply you with a calibrated ultrasonic microphone, but you will not like the price.

Probing the circuit could stop the oscillations (if they exist). However, as you have an RF spectrum analyzer, you could make a coupling coil, connect it to the spectrum analyzer and fit it inside the headset. This would have negligible effect on the circuitry but would detect any ultrasonic activity in the voice coil of the headset. You could confirm the operation of the coupling coil by driving a second coil with your signal generator and placing it close to the measuring coil.

Ultrasonic exposure is just one hypothetical possibility for causing timmitus. Simply being exposed to high sound levels (which must happen a lot to users of aviation headsets could be enough. John

Reply to
jrwalliker

On a sunny day (Sun, 4 Jan 2015 13:38:43 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

Its easy to test those mikes if you have some old acoustic remote control or piezo actuator from it, these are around 40 kHz:

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Reply to
Jan Panteltje

Hows about put that scope across the transducer and look at it. if you can't get at the wires and it's magnetic, use a pickup coil. Start small. Determine if it's oscillating before you go all out to accurately measure it. Shouldn't be hard to take just any old electret microphone element and a tweeter and a function generator to determine if it has any response up there. If it does, and the headset measurement shows none, you're done.

Reply to
mike

A spark gap, like from a fire starter thing, should generate a pretty decent acoustic impulse. That could be used to see if a microphone has high frequency response.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

By any chance, was this claim made by a class action plaintiffs attorney (also known as an ambulance chaser)?

More like high level oscillations, but the high frequency angle is worth checking. Incidentally: "Prolonged exposure to sound or noise levels as low as 70 dB can result in damage to hearing". Flying without the noise canceling headset seems a more likely cause of tinnitus than flying with the headset.

Searching Google Images for electret microphone frequency responses: I get a wide variety of plots, only a few of which seem usable to

100KHz and none higher. Most are characterized to 20KHz and stop. Use the frequency plots to find a likely electret microphone.

There are also MEMS microphones which might work. These are commonly used for ultrasonic transducers and are usable to

1MHz. However, the frequency response not flat like an electret mic. For example: Knowles has a line of good and cheap ultrasonic transducers. One was used on a Mars rover. I played with this one building a bat detector and a buried traffic detector loop finder. Highly recommended.

Incidentally, you'll get quite a bit of info on wide band ultrasonic microphones and circuitry by reading about bat detectors:

The problem with all these mics is that they operate in the RF frequency range, including the low end of the AM broadcast band. You'll need to pay attention to shielding and probably use differential balanced amplifiers to reduce any common mode pickup.

Good luck.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Or just hold a scope probe, X1 or X10 close to the unit and if it is oscillating, any field should show up.

Reply to
Tom Miller

Tom Miller

** Much better is to use a coil with a hundred turns or so on a 20mm former placed where the listener's ear would be.

Make you own or try a 0.2 mH choke made for speaker x-overs.

If you cannot see at least 50mV p-p on a scope you have no chance with a microphone.

.... Phil

Reply to
Phil Allison

** Well, definitely NOT a dynamic mic !!!

In order to test a headphone, the mic capsule has to be VERY close - preferably linked with some kind of artificial ear to seal the volume between it and the transducer. An omni pattern electret or condenser mic is best and will be unaffected by the magnetic field radiated from the coil of the transducer coupling to the coil of the mic capsule.

I just tried a Sony hi-fi headphone and a Shure SM58 dynamic mic with the grille removed - the combination showed good response out to 200kHz !!

... Phil

Reply to
Phil Allison

Jan Panteltje wrote

The bit driving the headset is an aviation radio i.e. an AM modulated VHF thingy e.g. KX165, KX155A, KX165A. Or the radio part of a GPS like a GNS430 etc. I can't see any point in PWM audio and looking at say a KX165A MM I can't see anything like that.

I think it would play absolute havoc with ANR headsets, not to mention interference with other avionics.

Reply to
John-Smith

Jim Thompson wrote

The suggestion is that the high freq oscillation is generated inside the headset.

Reply to
John-Smith

mike wrote

That was my plan...

Reply to
John-Smith

Jeff Liebermann wrote

No; it's in a US pilot forum.

But yeah I get your drift.

I agree totally.

It would knacker your hearing within hours. But nobody is suggesting that, because you would not be able to communicate with ATC, etc.

Very many thanks for these tips!

I am going to start with a ribbon tweeter and with that I will test my miniature mike's frequency response (and its preamp).

Reply to
John-Smith

On a sunny day (Mon, 05 Jan 2015 20:22:07 +0000) it happened John-Smith wrote in :

Did you not say there was electronics inside that headset?? That is what I am talking about. Else where should that RF come form?

No way, you think wrong. it will be just a few kHz above audible audio, I have done that sort of thing:

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here I deliberately leave out any filtering, see the warning. It is OK if you use a small speaker (like in a cellphone), but not so good if you plug it directly in or beam into your ear.

The thing probably uses a DSP.

Reply to
Jan Panteltje

I'm not sure if you're objecting, agreeing, or expressing confusion.

If there's an oscillation anywhere in the system, either inside of the headset circuitry or because of audio traveling in free air or conducted or whatever, it'll show up on the speaker leads as a distinct tone (and I mean the SPEAKER leads internal to the headset, not the audio to the headset).

You'll have to hack into the head set to get at the speaker leads, which may be a more invasive approach than you want to take -- but if you do, you'll sweep a whole lot of uncertainties off the table.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

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