A website uses the AudioContext API to send a low frequency audio through the browser to the computer. It then measures how the computer processes this sent data. Based on how this signal is processed, the results from the AudioContext API can help identify the same user across different browsers.
This process doesn’t require access to the device permissions like microphone or speakers. No audio is recorded, collected or played by any means. It gathers the audio signature of a user’s device and uses it to create an identifier to track that user. It simply relies on the difference in the way these generated signals are processed on each device.
Since this technique utilizes the device’s (hardware) capability differences and not just the browser’s — it can be used to track users across different browsers as long as they’re on the same device. Browser compartmentalization (a method where a person uses two or more dedicated browsers for different Internet activities) can be used to escape cookies set by different trackers.