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Lapel Mic/Speaker impedance...in helicopter


Premium Subscriber
Dec 26, 2004
New York
EMS helicopter, XPR 6550 installed and tied into our audio systems so we can use it as a radio with our helmets. Has worked for years however the audio quality has always been poor, my guess is the portable radio's built in microphone or a lapel mic/speaker setup is not matched well with our helmets microphones. They came with PMMN4024A mic/speakers and I tried to take one apart and try to either measure or find a part number for the microphone to gather info, got nothing.

Helmets are Gentex SPH-5 with 5060-4 microphones. Here are the specs on it.
SENSITIVITY @1 KHZ: -28 dBV/Pa +/- 3 dB
Frequency response: 100 Hz to 10 KHz
Distortion: Less than 3% THD @ 120 dB SPL across
specified frequency response
Impedance: Less than 50 Ohms
Rated load resistance: RL : 100 to 1800 Ohms
Rated supply voltage: 8 to 30 VDC (not polarity sensitive)
Rated Current drain: 40 mA max, Ta = 25oC

Any thoughts on how to match or what the problem would be?


Lots and lots of watts
Dec 5, 2008
Sonoma, CA
Sensitivity is a bit low. You'll need a good -20 to -15 dBV to drive a Motorola correctly.

They also use electret mic elements that have a DC voltage applied to them, kind of like a preamp for the mic for higher levels.


Nov 5, 2020
Does the audio quality sound like an electric guitar with the gain too high? (Grunge sound type stuff?).

Many electret mic rigs, which the 5060-4, can drive the input on a commercial radio too hard.

How is it wired? Intercom box going to the mic rig on the helmets and the commercial radio, or are the helmets direct connected to the XPR?


Mar 14, 2010
Ft Myers, FL
That happens on spk/mic wire too close to the antenna, cheap mics wires like Baofeng are not shield, rf from the antenna get in the mic wires.