Sound and hearing

Sound is a vibration passed from particle to particle through a medium such as air, water or solid rock. A vibrating object, like a guitar string or vocal cords, pushes on nearby air molecules, creating regions of higher pressure called compressions and lower pressure called rarefactions.

Sound waves are longitudinal: the particles vibrate back and forth in the same direction the wave travels. Because sound needs particles to travel through, it cannot cross the vacuum of space.

In air at about 20 degrees Celsius, sound travels at roughly 343 metres per second. It moves faster in warmer air, and much faster in water, around 1,500 metres per second, and in steel, over 5,000 metres per second, because particles are more tightly coupled.

Light is far faster than sound, which is why you see lightning before you hear thunder. Counting the seconds between the flash and the rumble and dividing by three gives a rough distance in kilometres.

Pitch depends on frequency, the number of waves passing a point each second, measured in hertz. Young, healthy human ears can typically hear from about 20 to 20,000 hertz, and the upper limit falls with age. Dogs hear higher frequencies, and bats and dolphins use ultrasound for echolocation.

Loudness depends on the amplitude of the wave, how large the pressure changes are. It is measured on the logarithmic decibel scale, where every increase of 10 decibels represents ten times the intensity. Normal conversation is around 60 decibels; long exposure to sounds above about 85 decibels can damage hearing.

• Reflection: sound bounces off hard surfaces, producing echoes and the reverberation of large halls.
• Diffraction: sound bends around corners, which is why you can hear someone in another room.
• Interference: overlapping waves can reinforce or cancel, the principle behind noise-cancelling headphones.
• Doppler effect: a siren sounds higher as it approaches and lower as it moves away, because the waves are squeezed or stretched.
• Resonance: an object vibrates strongly when driven at its natural frequency, as when a singer shatters a glass.

Sound waves are funnelled by the outer ear into the ear canal, where they vibrate the eardrum. Three tiny bones in the middle ear, the malleus, incus and stapes, the smallest bones in the body, amplify the vibrations and pass them to the inner ear.

The inner ear's cochlea is a fluid-filled spiral lined with thousands of hair cells. Different positions along the cochlea respond to different frequencies. As the hair cells bend, they generate nerve signals that travel to the brain, which interprets them as sound. Damaged hair cells do not regrow in humans, which is why noise-induced hearing loss is permanent.

Ultrasound imaging sends high-frequency sound into the body and builds pictures from the echoes, allowing doctors to examine unborn babies and organs without radiation. Sonar uses sound echoes to map the seafloor and detect submarines.

Architects design concert halls to control reflection and reverberation, and engineers design quieter aircraft and cars. Cochlear implants can bypass damaged hair cells by stimulating the hearing nerve directly, giving many deaf people access to sound.