I've got an example of this: Some Blackpink songs (cannot remember which ones, possibly Playing With Fire or DDU-DDU-DDU) were mastered and released on Spotify/iTunes [unsure if has been removed now] with a 18-19kHz carrier tone (iirc it was actually a number of tones) modulated. The signal was very loud. I first noticed this in a friend's car and assumed it was because the song was ripped from YouTube, but later realized that all of the versions have this.
The reason for this is because at concerts, you can buy a piece of merch called a lightstick. Essentially, it is a STM32 and a mems microphone that is demodulating the carrier signal, so it can flash in time with the music. The tone has to be <ultrasonic because it has to work with consumer speakers and the mic in the device.
I assume the theory goes that anyone who has been to enough concerts to have a lightstick is no longer capable of hearing the noise.
The reason for this is because at concerts, you can buy a piece of merch called a lightstick. Essentially, it is a STM32 and a mems microphone that is demodulating the carrier signal, so it can flash in time with the music. The tone has to be <ultrasonic because it has to work with consumer speakers and the mic in the device.
I assume the theory goes that anyone who has been to enough concerts to have a lightstick is no longer capable of hearing the noise.