Patent US 5,457,807

A surveying technique transmits a combined signal made up of a programming signal and a survey signal, both of which are acoustically reproduced by a speaker in the audible range. The survey signal is uniquely coded to identify a signal source such as a radio station or television channel. At the receiver, the acoustic survey signal is controlled so as not to be heard at an appreciable distance from the speaker, and is converted to a non-acoustic signal. The converted non-acoustic signal is transmitted for detection by a portable unit worn by a person being monitored for his listening and/or viewing habits. The detection of the converted survey signal by the portable unit identifies the signal source to which the person was tuned. The conversion avoids the possibility of disturbing the monitored individual by the acoustic survey signal.