I know at least one group that has used mobile-phone ultrasonics for sonar imaging of breathing. http://apnea.cs.washington.edu/ http://apnea.cs.washington.edu/

I know at least one group that has used mobile-phone ultrasonics for sonar imaging of breathing. http://apnea.cs.washington.edu/ http://apnea.cs.washington.edu/ In terms of entropy, breathing rate is a lot of information: several breaths per minute. So if something of that rate can be noncooperatively recovered, the claim in your comment about cooperative recovery is dubitable.