How audio watermarks are captured
In the U.S., a company called Nielsen has its watermarking technology encoded into broadcast content (Radio/TV/etc), while some 65k individuals called "panelists" wear a small pager-like meter called a portable people meter (PPM) which "listens" for and reports the watermarks that it "hears" (as the individuals go about their daily life). The "panelists" are essentially a statistically significant sample of the larger population of Radio/TV consumers.


  Nielsen’s watermarking involves codes, and each PPM transmits both
  complete and partial codes back to Nielsen. In situations where
  partial codes exist, Nielsen has edit rules that “fill in the blanks”
  by validating against nearby complete codes. Broadcasters are
  currently updating their station’s encoders with the Enhanced CBET (Critical Band Encoding Technology)
  technology. Prior testing and preliminary results from the roll-out
  show that there is an increase in the quality of the code detection
  and an increase in average quarter hour audience estimates.


Here are some links for you:


http://www.nielsen.com/us/en/press-room/2016/nielsen-to-increase-portable-people-meter-sample-size-by-10-percent-across-48-radio-metro-areas.html
http://sites.nielsen.com/newscenter/making-noise-nielsen-audio-gets-into-the-spirit-of-total-audience/