Cyclical signals in the very high frequencies, above most humans’ hearing, are often continuity test signals rather than a sign your recording chain is at fault.
More and more, I see super-high-frequency information in recordings from clients (I do post-production audio for very diverse projects). You can see the sort of thing I mean at the top of this iZotope RX screenshot. Do you know what kind of information this is? Is there some HVAC or alarm system that puts high switching notes above human hearing? It’s probably not added by the recorder, because I sometimes see it switched on when there is a recording in another building by the same recordist and machine. If anything, it looks like it is more common in larger corporate buildings.
SOS Forum Post
SOS Technical Editor Hugh Robjohns replies: It’s an increasingly common problem, with many potential causes. But, given that you’re talking about modern public and corporate spaces, it’s probably the continuity checking system for the PAVA (public address and voice announcement) system. There are different ways of doing it, but one common system sends a very high-frequency signal (20-25 kHz) through the whole signal path, out through each speaker in a 100V line chain.
At 20-25 kHz, few people will hear it — but small-diaphragm mics are usually able to pick it up.
At the end of the chain there’s a frequency doubler that sends the doubled signal back down the chain to a detector at the amp. If the doubled signal comes back, the system is working. If not, it raises a fault alarm. Sometimes the test signal is continuous and sometimes it’s intermittent. At 20-25 kHz, few people will hear it — but small-diaphragm mics are usually able to pick it up. These kinds of tone are, thankfully, easy to notch out without affecting the wanted audio.