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Sound Bite

Science Snack
Sound Bite
Tune in to tunes with your teeth.
Sound Bite
Tune in to tunes with your teeth.

When you listen to a radio or music player, you normally hear the sound coming from the speaker or headphones. But sound doesn’t have to be transmitted through the air to be heard. In this Snack, you pick up sound vibrations through your teeth!

Tools and Materials
  • Radio, boombox, or other portable music player with a headphone jack (older radios may work better than newer ones; see To Do and Notice below)
  • Audio cable with a 1/8-inch phone plug (sometimes called a mini jack) on one end and two alligator clips on the other
  • Small DC motor (e.g., Kelvin 850647)
  • Plastic wrap
  1. Turn the radio on and set the dial to a station with a clear, strong signal.
  2. Insert the phone plug into the headphone jack. (This disconnects the external speaker.) Connect the alligator clips at the other end of the cable to the two terminals of the motor.
  3. Tear off a piece of plastic wrap. Wrap the motor completely in plastic. This keeps the motor sanitary between one person's using it and the next, since you will be putting it in your mouth!
To Do and Notice

Set the radio volume to a medium-high level, then put the plastic-covered motor in your mouth and bite down on the metal case. You should hear the radio playing! You may have to listen very carefully. If you have trouble hearing the radio, try plugging your ears with your fingers to drown out any competing external noise. You can also try biting on the shaft of the motor instead of the case.

Note: Some radios have a weak headphone output, so they may not work with this Snack. If necessary, try other boom boxes or radios until you find one that works; older radios are likely to work better. Mobile phones and MP3 players also work, but can be harder to hear and there is a small risk of damaging your device if you spin the motor shaft rapidly.

Experiment with the following:

  • Try pressing the motor against your skull behind your ear, then hold the motor against your forehead. Which position produces the clearest sound?
  • Try pressing the motor directly against other bones in your head.
  • Try different types of songs or other sounds. Do some work better than others? Do you hear some frequencies more clearly?
  • If you have earbuds, you can try biting on those (after wrapping them in plastic) instead of using a motor.
What’s Going On?

The output from the radio, in the form of a changing electric current, is sent through the audio cable to the motor. When the electric current passes through the coils of the motor, these coils act as electromagnets. Since the electric current is constantly changing, the strength of the electromagnetic fields constantly changes too, in synchronization with the radio output.

The interaction of these constantly changing electromagnetic fields with the permanent magnets in the motor causes the motor to vibrate. When you bite on the motor, these vibrations are transmitted to your teeth and jaw bone. In turn, the vibrations stimulate the nerve endings in your cochlea, which is part of your inner ear. The nerve impulses sent by the cochlea along the auditory nerve to your brain are then interpreted as sound, just as if they had been caused by sound waves entering your ear. (Click to enlarge image below.)

diagram of inner ear (cochlea)

Normally, sound waves transmitted in air spiral through the ridges of your outer ear, bounce around in your ear canal, vibrate your eardrum, wiggle the tiny bones of your middle ear, and finally set the fluid in the cochlea, in your inner ear, in rhythmic motion. This rhythmic motion is then transmitted via nerve signals to the brain, where it is interpreted as sound.

From the external ear to the cochlea, the sound waves travel in the form of tiny motions of molecules. These motions are amplified more than 100 times by the time they reach the fluid inside the cochlea.

The sound waves you hear when you bite the motor, in contrast, are not amplified nearly as much because they are conducted directly through teeth and bone to the cochlea.

Going Further

Motor Autopsy 

If you have never seen the inside of a small electric motor such as the one used in this Snack, try taking one apart. Use a broken one if possible, but a new one is not very expensive and its loss is well worth the experience.

Check Your Hearing: Two Kinds of Hearing Loss

If you have access to a tuning fork, you can perform two simple hearing tests, the Rinne Test (pronounced reh-NAY), and the Weber Test, both of which distinguish between conductive and sensorineural hearing loss. Look them up to learn how to perform them.

In conductive hearing loss, sound vibrations are not being transmitted from the outer ear to the cochlea because of an ear-wax blockage, fluid accumulation in the ear (e.g., due to an ear infection), a damaged ear drum, or arthritis of the tiny bones of the middle ear. When you stick your fingers in your ears, you are simulating conductive hearing loss.

In sensorineural hearing loss, your brain is not receiving nerve signals from the inner ear, even though sound vibrations may be reaching this organ. In the majority of cases, hair cells in the cochlea are damaged, often by exposure to loud noise.

So, what does this mean for improving your hearing by rerouting audio through your jaw? If you’ve got a cold or an injured ear drum, you might be in luck, although the signal lacks the amplification it would otherwise get. But if you’ve been spending a lot of time in mosh pits or blaring Rachmaninoff through your earbuds at top volume, you may be listening to the sound of silence.