You may already be aware of this issue, but it is new to me. I first learned about it today form:
http://www.zdnet.com/article/hundreds-of-apps-are-using-ultrasonic-sounds-to-track-your-ad-habits/?loc=newsletter_large_thumb_featured&ftag=TRE17cfd61&bhid=21920984823979904861134656766145.
One...
