Low Pass Filter?

[ric8138]

Recently ran into an issue with ultrasonic security system messing with Sennheiser G3’s. Does anyone make a low pass filter to go between mic and transmitter to avoid this issue?
We got around it with a wired boom and 2 hardwired lavs, but it was not easy. Please don’t tell me to just buy “Lectro”, I know that’s the long range answer, but when working on micro budget indies, it’s just not practical. 

[Rick Reineke]

I have seen an in-line high or low pass filter for unbalanced lavs, (which most are). If the noise is getting in through the mic/wire connection,I would try another mic (and/or channel.. It would likely be the same feeding a Lectro (or whatever) if it's getting in through the cable  . I have never encountered security system interference noise working on many ENG with the G series Sennheiser or others on thousands of ENG and docu shoots (prior to sound for movies),.

[Patrick Farrell]

You could build an RC filter into an adapter cable to roll off everything above 20kHz. However, having a resistor in line could potentially affect the bias voltage that the mic sees so choose values carefully and test beforehand.

[ric8138]

11 hours ago, Rick Reineke said:

I have seen an in-line high or low pass filter for unbalanced lavs, (which most are). If the noise is getting in through the mic/wire connection,I would try another mic (and/or channel.. It would likely be the same feeding a Lectro (or whatever) if it's getting in through the cable  . I have never encountered security system interference noise working on many ENG with the G series Sennheiser or others on thousands of ENG and docu shoots (prior to sound for movies),.

This was a new one on me, but apparently the motion detectors “chirp” in the 40k range. The mic picks it up and it messes with the compander circuit in the G3, causing a pumping volume sound. First time I’ve run into it in 30 years of sound.