Ultrasonic investigations in shopping centres

I can't remember how I first came across these near-ultrasonic 'beacons' ubiquitous in PA systems. I might have been scrolling through the audio spectrum while waiting for the underground train; or it might have been the screeching 'tinnitus-like' sensation I would often get near the loudspeakers at a local shopping centre.

Whatever the case, I learned that they are called pilot tones. Many multi-loudspeaker PA systems (like the Zenitel VPA and Axys End of Line detection unit) employ these roughly 20-kilohertz tones to continuously measure the system's health status: no pilot tone means no connection to a loudspeaker. It's usually set to a very high frequency, inaudible to humans, to avoid disturbing customers.

However, these tones are powerful and some people will still hear them, especially if the frequency gets below 20 kHz. There is one such system at 19.595 kHz in my city; it's marked green in the graph above. I've heard of several other people that also hear the sound. I don't believe it to be a sonic weapon like The Mosquito; those use even lower frequencies, down to 17 kHz. It's probably just a misconfiguration that was never fixed because the people working on it couldn't experientially confirm any issue with it.

Hidden modulation.

Pretty quickly it became apparent that this sound is almost never a pure tone. Some kind of modulation can always be seen wiggling around it in the spectrogram. Is it caused by the background music being played through the PA system? Is it carrying some information? Or is it something else altogether?

I've found at least one place where the tone appears to be amplitude modulated by the lowest frequencies in the music or commercials playing. It's probably a side effect of some kind of distortion.

Here's a spectrogram plot of this amplitude modulation around the strong pilot tone. It's colour-coded so that the purple colour is coming from the right microphone channel and green from the left. I'm not quite sure what the other purple horizontal stripes are here.

But this kind of modulation is rare. It's more common to see the tone change in response to things happening around you, like people moving about. More on that in the following.

Doppler-shifted backscatter.

Look what happens to the pilot tone when a train arrives at an underground station:

The wideband screech in the beginning is followed by this interesting tornado-shaped pattern that seems to have a lot of structure inside of it. It lasts for 15 seconds, until the train comes to a stop.

It's my belief that this is backscatter, also known as reverb, from the pilot tone reflecting off the slowing train and getting Doppler-shifted on the way. The pilot tone works as a continuously-transmitting bistatic sonar. Here, the left microphone (green) hears a mostly negative Doppler shift whereas the right channel (purple) hears a positive one, as the train is passing us from right to left. An anti-aliasing filter has darkened the higher frequencies as I wasn't yet aware I would find something interesting up here when recording.

A zoomed-in view of this cone reveals these repeating sweeps from positive to negative and red to green. Are they reflections off of some repeating structure in the passing train? The short horizontal bursts of constant tone could then be surfaces that are angled in a different direction than the rest of the train. Or perhaps this repetition reflects the regular placement of loudspeakers around the station?

Moving the microphone.

Another interesting experiment: I took the lift to another floor and recorded the ride from inside the lift. It wasn't the metal box type, the walls were made of glass, so I thought the pilot tone should be at least somewhat audible inside. Here's what I got during the 10-second ride. It's a little buried in noise.

Scooter calculation.

For the next experiment I went into the underground car park of a shopping centre. I stood right under a PA loudspeaker and recorded a person on a scooter passing by. A lot of interesting stuff is happening in this stereo spectrogram!

First of all, there seems to be two pilot tones, one at 19,595 Hz and a much quieter one at 19,500 Hz. Are there two different PA systems in the car park?

Second, there's a clear Doppler shift in the reverb. The frequency shift goes from positive to negative at the same moment that the scooter passes us, seen as the wideband wheel noise changing color. It looks like the pattern is also 'filled in' with noise under this Doppler curve. What all information can we find out just by looking at this image?

If we ignore the fact that this is actually a bistatic doppler shift we could try and estimate the speed using a formula on Wikipedia. It was pretty chilly in the car park, I would say 15 °C. The speed of sound at 15 °C is 340 m/s. The maximum Doppler shift here seems to be 350 Hz. Plugging all these into the equation we get 11 km/h, which sounds like a realistic speed for a scooter.

Why is it filled in? My thought is these are reflections off different points on our test subject. There's variation in the reflection angles and, consequently, magnitudes of the velocity component that causes frequency shifting, down to nearly zero Hz.

What now?

What would you do with this ultrasonic beep all around you? I have some free ideas:

  • Automated speed trap in the car park
  • Detect when the escalators stop working
  • Modulate it with a positioning code to prevent people getting lost in the maze of commerce
  • Use it to deliver ads somehow
  • Use it to find your way to the quietest spots in a shopping centre


  1. Now I know. One shopping center, west, in my city, I always feel uncomfortable in it's hallways. I can't wait to get out, felt freaked out completely....

    1. Uneasiness can also be caused by low frequency sounds. See A Ghost in the Machine by Vic Tandy: https://www.researchgate.net/publication/24236415_A_Ghost_in_the_Machine

  2. Very cool. They are doing a similar thing to your location beaconing idea with led lights in stores, spam prevention is killing the link so if you search “Supermarket LED lights talk to smartphone app” you can find a bbc article.

    1. When I worked at Acuity (Lithonia) Lighting, we were working on a system to do this for Publix, Walmart, etc. I think Philips had the patent.

      Would use the selfie camera on your smart phone to detect your position by which lights were above you, and at what angles. 3cm precision ish.

    2. Philips has several patents on using LED lights for both transmission and reception of light based data. More information at https://www.signify.com/global/innovation/trulifi

    3. something similar i investigated was that ALDI in the UK recently started using e-ink price tags that seemed to update on their own, i theorized that they may be modulating the shops LED lighting since there was what appeared to be a light sensor on every tag.

      That was until i managed to find one that someone pulled off, google the name on the back (hanshow nebular) and it's a 2.4 GHz radio link, the thing on the front is just a status light, my theory still holds true though, since some digital shelf labels do use modulated light.

  3. I guess all iphones listen to it, with no option to turn it off.
    as it reminds of some tech of for finding lost keys.

  4. "use it to deliver ads somehow" you monster

  5. What software and hardware are you using to capture the sounds?

    1. Hi, I used a couple different recorderds (Tascam DR-05 and a Zoom one). The spectograms were made in a home-baked software using the fftw3 library.

  6. Could it be a motion detection system? Used during closing hours to detect if someone is moving along the hallways? Some of these images remind me strongly of the sonagrams I received from the Graves radar in France, also a bistatic radar.

  7. Very cool! I sometimes walk around work with an FFT app on my phone to find motors and bearings that are about to fail. They produce ultrasonic squeals quite a while before they produce audible ones.

    1. Hey that's really cool. I am gonna walk around and see if I can find anything like this. Not that I can do anything about it haha

  8. Modern PA systems work with class D amplifiers. They do pulse width modulation at a much higher frequency than max frequency to be transmitted. This way power transistors are only switching (at high frequencies) instead of burning power. A last low pass filter stage turns the PWM into sound. Depends on the quality of the filter how much high frequencies are left.

    Since the lowest frequencies need the most power it looks like AM modulation – I assume. The PWM frequency depends on the chipset but is often around 50kHz (cheaper ones) to 150kHz.

    1. I does not seem realistic for me that the speakers would be able to emit the 50 kHz from the PWM, to say nothing about its harmonics, so it does not seem to be the case.

      On the other hand I'm only used to these amplifiers on personal audio systems, perhaps the "high-power, low quality" ones for PAs are a lot different, but I don't expect higher quality speakers in these systems anyway. The 19 kHz pilot tone seems to be at the limit of their frequency response.

  9. Apropos the subway ("underground" in the UK), the repeating pattern could be diffraction from the tracks.

    Many thanks for your ever-interesting writeups.

  10. I've always thought that these ultrasonic sounds were there to prevent the young from hanging out there for hours. Since they can possibly hear some of those sounds and the elders cannot.

  11. "Use it to deliver ads somehow" subliminally into peoples' brains.
    Gawrsh I could use some Heinz baked beans right about now.

  12. 2 pilot tones in the garage... my guess would be multiple speaker chains powered from multiple amps. Mind you, I've only ever dealt with one PA system years ago but I do remember speakers are constantly status monitored. Impedance measurement I believe. Digging through the old logs I also see some ERROR 22kHz CHECK messages.

    Last time I did some home experiments my hearing tops out around 16 kHz so I have no idea how anyone notices these things :)

  13. Very interesting investigation, i never knew about these pilot tones used in PA systems or how they could be used as a doppler sonar.

    In a similar vein i've found that HDMI harmonics (like the common 445.5 MHz or 297 MHz) can actually exhibit changes in amplitude if someone is moving in front of or near the screen, i first noticed this when my brother was sitting in my chair with my SDR running, i've seen similar modulation in the wild before (i.e from other peoples houses) but i can't confidentally say it's a good way to know if someones watching TV as for all i know it could just be interferance from other HDMI cables or to do with the image on screen changing.

  14. My initial thought was that an approaching train changes the air pressure slightly and that might account for those changes you are seeing.


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