Ears
This was the first type of audio surveillance device, and has been used by spies and civilians alike since the advent of listening. These are the device of choice for most listeners, because of their ease of use, availability, and portability. Ears stay attached comfortably to each side of the head for the duration of life, and are very light-weight, making them ideal for hands free listening to a variety of sources. They are easy to get as well. In fact, ears are a natural part of humans' anatomical apparatus, and involve no cost. An added benefit is that thanks to the workings of the human brain, ears are entirely automatic, and require no manual control by the listner.

UD[photo]
"http://udel.edu/~smoser"
last updated April 15, 1999