Is sleep tracking with sonar safe for your health?
Ultrasound is generally considered safe if it is at normal volume. Regarding health effects, it works in a similar way to normal audible sound, i.e. very loud ultrasound can damage your hearing, whereas at low volume it is safe to hear. When using speakers, smartphones are nowhere close to be able to produce such loud sounds as to damage your hearing. We also use ultrasound that is very close to the hearing range (around 20 kHz), so the effects of the ultrasound are almost identical to hearing a high pitched sound at the same volume (expect you can’t hear it at all). The ultrasound volume we use is around 40 dB – which is lower than normal speech volume. You can measure the sound level yourself using e.g. this app.
This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org//faqs/sonar_safety.html