I'm working on a project where we're recording audio from the intercom output of an aircraft. Essentially, this lets us record air traffic and cockpit audio.

Aircraft audio is stereotypically high impedance. On the order of 500Ohms.

The USB audio IC I'm currently looking at is a TI PCM2903. The datasheet lists the ADC input impedance as 50K. My electrical deign would probably very closely resemble the one implemented for the evaluation board for this IC, which is available here: http://www.ti.com/lit/ug/sleu117/sleu117.pdf

I'm fairly sure I'll need to impedance match to get the volume level reasonable (not excessively quiet), but how do I go about calculating that?

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    \$\begingroup\$ Aircraft audio is stereotypically high impedance. On the order of 500Ohms For a line level I would call that a fairly low impedance. I would avoid using the word stereotypically in conversations about audio as this introduces confusion (mono/stereo). \$\endgroup\$ Mar 13, 2018 at 16:16
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    \$\begingroup\$ What makes you convinced you need impedance matching? You can load the 500 ohms signal output with 50 k input impedance and that should work fine. Impedance matching is generally only needed for telephone landlines (long distance) or at RF (Radio Frequencies). If you "impedance match" a 500 ohms output with a 500 ohms input you get about half the signal compared to using a a 500 ohms output with a 50 k ohms input. \$\endgroup\$ Mar 13, 2018 at 16:21
  • \$\begingroup\$ The loss due to impedance match with a 500R source into a 50k load is ~0.01%. I would not worry, gain errors and other losses are going to bury that. You may be getting confused with the 600R used in analog telephony, that is to do with the shared two wires for tx and rx that would echo without matching. \$\endgroup\$
    – Jay M
    Nov 15, 2019 at 17:26

1 Answer 1


I designed a full duplex headset for 4 users using throat-microphone with squelch, AGC and headset for wind-positive pressurized air suits in the 70's for low level nuclear service in maintenance of 2ndary heat exchangers. These were all conducted on a shared single wire pair without echoes for excess sidetone.

I have left out a few details on sidetone cancellation, but characterizing the coughs , pops and noise from the audio, is a matter of spectrum analysis to detect the best SNR trigger that begins with true speech. A cough is noise and a throat mic has very insensitive to medium and high frequency yet very sensitive to low frequency such as a cough. So detecting bass impulse noise like a pop or cough is good for detection, but then use a mid-range bandpass filter to attenuate that noise. This will improve the poor quality of throat microphones immensely.

So a precise answer without precise headset specs is impossible.

If you get stuck "a little cough" is a good way to trigger the mic ON in exceptionally noisy environments.

Forget about impedance matching for audio electrical impedance.

  • ** Pay attention to matching frequency spectrum for band pass, band stop and trigger matching..**

You only need to consider mic gain, speaker damping factor and amplifying to match the signal/noise ratio spectrum. ie match receiver theory

Forgive me on details, I designed it 45 yrs ago.

  • \$\begingroup\$ IF you can't make an intelligent comment, it is just a troll vote -1 \$\endgroup\$ Nov 17, 2019 at 19:25

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