Acoustic Hearing

1.  A police siren moves through air as Sound Waves

A police car coming down the road switches on its siren. The sound of the siren travels as sound waves, or vibrations in the air molecules, eventually reaching our ears (Figure 1).

Figure 2: Police car siren moving as sound waves. These waves vibrate the air which reach our ears.

2. The Sound Waves send vibrations through the ear to the cochlea

As the sound waves from the siren enter the ear, they go through the ear canal and vibrate the eardrum. The eardrum vibrations move three tiny bones in the middle ear called the ossicles, sending vibrations into the cochlea (Figure 1)

3. Sound vibrations activate hair cells in the cochlea, sending signals to the brain

The cochlea is a fluid-filled, spiral-like structure that houses tiny hair cells. The sound causes vibrations in the fluid, causing hair cells to move. As the hair cells move, they send neural signals through the auditory nerve and into the auditory cortex of the brain (Figure 3)

Figure 3: Hair cells convert sound wave vibrations to neural signals and into the brain.

Each hair cell detects a specific frequency or pitch. An example of high-frequency sound would be a bell ringing, while low frequencies could be a kick to a drum. The hair cells are organized in tonotopical order; cells closer to the entrance of the cochlea detect high frequencies, while the hair cells at the end of the cochlea detect lower frequencies (Figure 4)

Figure 4: Example of Tonotopic organization within the cochlea.

As the sound waves vibrate the hair cells, those hair cells send neural signals to the brain, allowing us to identify the sound as a police siren.