can't get microphone signal into oscilloscope

[imperial-aero]

Hi,

I'm working on a LabVIEW VI to acquire signals from 16 microphones. However, once I set everything up I find that I'm only able to obtain the input from 1 channel. I've also noted that the channel that works keeps changing as well, for example on my VI it was channel ai4 that works, and on a LabVIEW example it was channel ai8. This is very strange.To check whether all the channels were working, I first sent individual signals (from a function generator) to the input terminals and they did produce the waveforms as expected. Secondly I connected four signals from the function generator to the input terminals, and these four channels produced the waveforms as expected.

To check my microphones, I connected them upto an oscilloscope and found out that I could not get any signals. However, when I connected the microphones to my computer directly, I was able to get signals from it.

I think my problem is not with the NI Devices or LabVIEW. If I can figure out why I'm not obtaining any microphone signals on the oscilloscope then maybe I can figure out why I'm obtaining inputs from only 1 channel. To be honest, I'm really confused now and don't know what else I could do. Any suggestions will be helpful. 

The devices I'm using are PCI 6229, BNC-2090 terminal and the microphones are simple tie-clip ones.

[rpursley8]

Most microphones require a DC power source to work.  For 3-pin microphones the power is separate from the signal line.  For 2-pin microphones the power and signal are on the same line.

Take a look at this web site http://www.epanorama.net/circuits/microphone_powering.html#soundcard

[imperial-aero]

Thank you again for the quick reply Pursely.

The microphone I'm using comes with it's own portable batteries. And the thing is I'm actually able to input signals to my computer directly using the microphones. But can't understand why I can't get any on the oscilloscopes (I actually tried two oscilloscopes, just in case one wasn't working). Also it doesn't explain why I can get readings from 1 channels when multiple microphones are connected to the BNC-2090 device. It's really strange.

[rpursley8]

Check the input impedance of the scope.  If it's set for 50 ohms, the microphone may not be able to drive it.

That's the only thing I can think of.

[imperial-aero]

Ok, I think that's a good point. I'll check that. Also, I was wondering whether the signals were too low and maybe it needed a pre-amplifier or something. But then again,microphones do have built-in amps that amplify the signals i guess. I'll check the impedances.

Cheers.

[imperial-aero]

Here are some specifications of the microphones. The impedance does seem very high (1000 ohms). Do you reckon this might actually be too large for it to drive the scope?

Main Specifications

Element: Condenser
Polar Pattern: Omnidirectional
Frequency Response: 50 - 18,000Hz
Open Circuit Sensitivity:  -54dBm +/-3dB, 1kHz @ 1Pa
Impedance: 1000 Ohms +/-30%

[rpursley8]

Most scope have an input impedance that can be switched between 1 Mohm and 50 ohms.  Do you know what your's is set to?  It needs to be set to 1 Mohm.