In this topic you will learn about how sounds move from one place to another.

A sound is made by something moving back and forth. This back and forth movement is called a vibration. Unless a thing vibrates, there can be no sound. Musical instruments make sounds by causing columns of air or surfaces to vibrate.

Sound is a vibration that travels through matter. Matter is anything that has mass and takes up space. Matter can be a solid, liquid, or gas. A molecule is the smallest piece that matter can be broken into without changing the kind of matter.

Vibrating molecules squeeze together, then spread apart. A vibration that spreads away from a vibrating object is a sound wave. It carries the energy from the vibrating object outward in all directions. The part of a sound wave where molecules are crowded together is called compression. The part of a sound wave where molecules spread apart is called rarefaction. The vibration continues to spread.

You hear sound when a sound wave reaches your ear. Sound waves are collected by the outer ear. These sound waves cause the eardrum to vibrate. The vibrating eardrum makes tiny bones in the middle ear vibrate. These vibrating bones pass the sound vibrations to a fluid in the inner ear. Tiny hair cells in the inner ear tube vibrate. The hair cell vibrations are passed to a nerve and then to the brain. The brain interprets the vibrations as sound.

Sound can travel through gases, liquids, or solids. Sound waves need matter to travel from a vibrating object. Sound cannot travel through a vacuum. A vacuum is a place where there is no matter.

Some people cannot hear very well. Loud and clear sounds are soft and muffled. Soft sounds may not be heard at all. This is known as deafness. A hearing aid in the outer ear can make sounds louder and may help overcome deafness. Close-captioned television lets deaf people watch television and know what is being said. Sign language is a way for deaf or hearing-impaired people to communicate using their hands.