A wristwatch phone that lets you listen by sticking a finger in your ear, an MP3 player that vibrates the bones in your skull to play music that only you can hear -- these are some of the products being developed using a technology called bone conduction that sends sound waves through the bones around the ear.

Bone-conduction technology has long been used in hearing aids and other products for the hearing impaired, as well as in military headsets. Recently, several commercial companies have embraced it for products aimed at the general public.

Carrying on a conversation in a busy restaurant can be a challenge for anyone who wears a hearing aid. The devices amplify speech, but they amplify the general racket in the room, too.

But a microphone that imitates the remarkably acute hearing of a tiny fly - and gives it a boost with the latest in miniature lasers and signal processing - may one day help solve this problem.

Hearing enhancements could include the following:

• Extended frequency range
• Translated frequency ranges
• Increased sensitivity to low amplitude sounds
• Automatic dynamic range compression to protect against too loud sounds
• Improved directional sensitivity
• Enhanced sound discrimination
• SONAR capability
• Private/covert listening capability
• time compression/expansion

Listen to bats, dolphins, insects
Diagnostic for machinery, bearing failure, compressed gas leaks, etc.
Hear ultrasonic proximity/intrusion detectors
Hear quartz crystal in a watch

Methods:
Down-coversion
High-fidelity, but narrow bandwidth
Frequency division
Lose fidelity, gain bandwidth
Time expansion
Record and then play back at lower speed

The "cocktail party effect" - the ability to focus one's listening attention on a single talker among a cacophony of conversations and background noise - has been recognized for some time. This specialized listening ability may be because of characteristics of the human speech production system, the auditory system, or high-level perceptual and language processing.

Sugically implanted into the cochlea of people whose inner ear hair cells don't work.

Audio is picked up by a microphone outside the ear, processed and converted to typically twenty-one audio channels and electrical signals passed to nerves.

Twenty-one channels is very limited compared to the range of frequencies normally detected by the hairs, and it is estimated that about one hundred channels will be needed to hear near-natural quality speech.

Sound arrives in the pinna, the visible part of the ear on the outside of the head.

Sound waves travel the 1-inch length of the ear canal and stimulate the tympanic membrane ("eardrum").

The vibrations of the eardrum are passed on to the three bones of the middle ear, which amplify the sound and send it into the inner ear, a snail-shaped tube—the "cochlea"—filled with liquid.

In the cochlea, the sound becomes a fluid wave, stimulating approximately 7,000 "hair cells" that line the cochlear walls.