Problems testing a microphone 1

[KirbyWan]

Hello all,

I am working on a test setup to check the microphone on an airline crew oxygen mask. This is a carbon microphone (spec sheet provided below) and it says the supply voltage is 16 to 24 VDC. Our setup provides a signal using a Goldline TS1 signal generator through a speaker. The microphone is then plugged into a Goldline DSP-30 to measure the response. The microphone level from the DSP-30 is 12 VDC. As a mechanical engineer this is not my strong suit, but our problems are that at lower frequencies (~500 to 850 Hz) our respons is somewhat attunated and we are getting multiple peaks like there is an reverberation in the test box, but only with the test mic. We also have a calibrated mic in the same test box and it reads fine.

I am changing the insulation in the box to something softer with an eggcrate shape to inprove the reverberation, but I think my problem is the difference in microphone voltage levels. Can someone support or correct my conception? Also is there a way to change the voltage signal to the microphone? Thank you for your help in this.

-Kirby

 

[VEBill]

If the test calls for 24 volts, then you should use 24 volts. If you test equipment doesn't support 24 volts, then you might have to build the circuit shown and use a scope manually.

Also, can you try it without the test box? Out in the open, in a large open area? You might have to stay late to run this test if the facility is noisy during the day. But it's a quick way to determine if the test box is causing the frequency response dips and peaks.

Once you've replicated the test set-up exactly, then if it still fails then perhaps the mic is old and/or broken. Give it a good whack (hey, it's a carbon mic) and retest. If it still fails, then it fails.

 

[KirbyWan]

VE1BLL,

Thanks for the response. We have many of these mics in house and they all exibit similar response, but they are old mics that we are testing to verify they still are up to snuff so I don't know if one or most of them are bad. I will try the percusive maintenance as you suggested and try testing this when it's quiet out in the shop.

-Kirby

 

[itsmoked]

Percussive maintenance is often called for with carbon mics. The carbon packs up and locks tight.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KirbyWan]

Additional information.

We swapped out some insulation and got some better results, then worse. While moving the speaker I noticed that the tone changed, which led me to fiddle with the speaker. If I put pressure on the speaker (a labtec which looks like it was designed as a computer speaker) the tone would become more pure. it seems like below 800 Hz or so the speaker case was vibrating giving the additional tones. I don't know why the calibrated mic (which is of very good quality or at least high expense) did not pick up the added tones as much as the carbon mic did. Could anyone comment on carbon microphones? What are the benefits/problems with this type of mic? How does it compare to a dynamic mic?

Thanks for the response itsmoked.

-Kirby

 

[itsmoked]

Well, every mic has its own acoustical response! That means they may work in a tuned enclosure differently.

Your very small changes could change the box/mic 'system' response dramatically. Boxes with square corners can easily result in standing waves. 800Hz wavelength is about one foot!! You need a completely lossy set up. Sponge egg crate and the lowest power levels you can manage. Try to do your tests on a sweep so standing waves have no chance of setting up.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

It's important to keep in mind that the insulated sound box is intended to simulate a large, quiet and anechoic environment, but on a bench top for convenience. But it's not a fundamental requirement for the test. So if it is suspected of giving trouble, then it might be worth going back to an ACTUAL large, quiet and (reasonably) anechoic environment - just long enough to confirm what's what.

Carbon mics were invented about 130 years ago. I believe that the only reason that they're still used in aircraft flight deck Smoke Masks is that nobody has ever bothered to move the standard.