Vibrations and sound

1. A sound wave and the vibration of a solid object are two forms of the same disturbance, and each can be turned into the other.
2. A vibrating surface in contact with air pushes on the air as it moves outwards and pulls back as it moves inwards.
3. Moving outwards squeezes the nearby air particles together into a compression.
4. Moving back inwards leaves extra space, so the air particles spread out into a rarefaction.
5. Each cycle of the surface's vibration produces one compression and one rarefaction, so the sound wave has the same frequency as the vibrating object.
6. The reverse also happens, because the changing pressure of a sound wave pushes and pulls on any solid surface it reaches and makes that surface vibrate.
7. In both directions energy is transferred, but no material travels from the source to the receiver.

Solids producing sound

1. A loudspeaker contains a light paper or plastic cone that an electrical signal forces to vibrate back and forth, sending sound waves into the air.
2. A plucked guitar string vibrates, but the thin string moves very little air on its own.
3. The string passes its vibrations through the bridge to the hollow wooden sound box, whose large surface pushes on far more air and makes the sound much louder.
4. A tuning fork has two metal prongs that vibrate at one fixed frequency when struck, producing a pure note.
5. Pressing the base of a vibrating tuning fork onto a wooden table makes the note louder, because the large table top is forced to vibrate and pushes on more air.
6. A drum skin vibrates when it is struck, and its large area creates strong compressions and rarefactions in the air.

Sound making solids vibrate

1. When sound waves reach a microphone, they make a thin, flexible diaphragm vibrate, and the microphone turns these vibrations into an electrical signal.
2. A loud bass note from a passing car can make a shop window rattle, because the sound waves force the glass to vibrate.
3. Music from next door reaches you because the sound waves make the dividing wall vibrate, the vibrations travel through the bricks, and the far side of the wall then produces new sound waves in your room.
4. Your own ear works in the same way, using a chain of solid parts to turn sound waves into vibrations and then into electrical signals.

Structure of the ear

1. The outer ear, or pinna, is the visible flap that collects sound waves and funnels them into the ear canal.
2. Sound waves travel down the air-filled ear canal to the eardrum, a thin membrane stretched across its inner end.
3. The changing pressure of the sound wave makes the eardrum vibrate at the same frequency as the sound.
4. In the air-filled middle ear, the eardrum is linked to three tiny bones called the ossicles: the hammer, the anvil and the stirrup.
5. The ossicles act as levers that pass the vibrations along and amplify them, increasing the force of each vibration.
6. The stirrup presses against a small membrane called the oval window, which is the entrance to the inner ear.
7. The vibrating oval window sets up waves in the fluid inside the cochlea, a coiled tube in the inner ear.
8. Tiny hair cells lining the cochlea are moved by the fluid and produce electrical signals.
9. The auditory nerve carries these signals to the brain, which interprets them as sound.
10. Along this route the disturbance changes form several times: a sound wave in air, vibrations of solid parts, waves in the fluid of the cochlea, and finally electrical signals.