I would like to know if there are known risks for hearing or any other possible health issue in case of long usage of ultrasonic (parametric) speakers (see an example) at low volume. For long usage I mean about 8 hours every day for years.
I found some medical articles, mostly dating a few years ago, some are optimistic some are on the "it depends" side. It is clear that high volume and very high frequencies can be harmful. I do not have the medical experience needed to understand if the concerns expressed on those articles can be applied to my scenario (low volume, 8 hours each day)
Here are the most optimistic articles:
Here the less optimistic: