I have made a recording inside a quite office (office noise) with the Zoom H6 mobile recorder and XY microphones and got the following spectrum from the signal:

Spectrum of office noise with Zoom H6, XY microphones

There is a peak at about 19.8 kHz. Even though I am not able to hear such high tones, I am pretty sure the tone is not present in the acoustic environment. I have seen the same tone from a USB audio interface connected to a smart phone.

I can use a filter to get rid of the tone, but I was curious why it is there.

Does anybody know where the tone comes from? And what is the purpose of this tone?

Thanks in advance

  • Did you try to make a recording with the same setup in another place? Is the peak at 19,8 kHz still there?
    – Nils
    May 18, 2017 at 11:22
  • Hi Nils, Just after I posted the question, I started to doubt if the 19.8 kHz tone was present in the environment. Then we used a Bruel & Kjaer spectrometer and saw that indeed the 19.8 kHz tone is present in the office environment. I was surprised to see such a strong tone, and its present on the entire floor in our building. I don't know where it comes from, but it solves my question because now I know it is not coming from the recording equipment.
    – Costello
    May 18, 2017 at 11:51
  • Please tell us if you find out where it's coming from.
    – Nils
    May 18, 2017 at 12:13
  • I'm glad you figured it out - but in SE terms this isn't really an answer & should have been posted as a comment. The answer could [& still can] subsequently be tweaked from the further information provided.
    – Tetsujin
    May 18, 2017 at 12:41
  • 1
    Rather unlikely, but it is right in the same frequency range that google nearby uses... :P developers.google.com/nearby/messages/overview
    – Scorb
    Jun 20, 2017 at 15:34

2 Answers 2


This 19.8kHz tone is usually produced by emergency audio systems, at least in my case (modern office building in Prague).

It's purpose is to comply with EN 54-16 standard which dictates that every emergency audio loop should be checked for integrity at least once per 100 seconds.

This is usually done using ultrasonic frequency (where the impedance is high enough so it's not influenced by contact resistances and temperature coefficient of resistance in wiring, is non-audible, and is still inside the usable bandwidth of PA). But, many (if not all) audio system manufacturers usually check it continuously, causing this annoying "phenomenon".

And yes, it's also repelling rodents and martens out of the building :-).

Just put the microphone close to the speaker in the ceiling, I bet this might be your case as well...



Sources for such a tone are DC/DC converters (these days, a whole lot of switching power supplies are likely candidates) and heating valves.

CRT monitors are no longer en vogue so probably not involved unless you are working in some CAD/artwork department still employing them.

If it's an entire office floor, you'll likely have several pieces of equipment with the same specs: LED lighting, WLAN routers, desktop computers, office laptops. On of the power supplies on them is likely to produce that frequency.

Or you have some computer virus doing air-gapped communication using almost-ultrasound.

Or you have an actual anti-infestation device designed to fend off rats, teenagers, and breast feeding.

  • Thanks, for the suggestions, we are still investigation this.
    – Costello
    May 26, 2017 at 8:52

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