Sound is a vibration in an elastic material that is capable of being heard by the human ear.
Vibration is a back and forth motion.
Elastic is the ability of a material to compress and expand .
Sound energy moves as a wave, thus is called sound waves. These waves cause a material to compress, then expand as they travel through it.
Sound waves start from some vibrating source.
For example, the strings on a guitar only produce sound when they are moved, such as when a person pulls or strums them. The motion of the strings push against the air around them causing the air to to compress. When the strings move in the opposite direction, the compressed air particles then separate, expand. As sound waves move through air, the air molecules inside your ear vibrate. When this happens, they strike your eardrum, causing it to vibrate, which causes the fluid in your enter ear to move. These movements are transmitted to your brain which interprets them as certain sounds. In other words, when an object vibrates, a sound is made. Vibrations are not always visible, such as those in air. But, if sound is heard, something is vibrating.
Pitch is how high or low a sound is and depends on frequency (number of vibrations per second) of the sound. The greater the frequency the higher pitch of the sound.
The loudness of sound depends on the energy of the sound and is measured in decibels (a number that tells how loud a sound is). The greater the decibel the louder is the sound. A decibel of 0 means no sound. Most people talk at 20 to 40 decibels. A sound greater than 120 decibels can hurt the inside part of your ears, resulting in reduced or total hearing loss. Even listening to loud sounds higher than 100 decibels, such as loud music and loud TV can cause hearing loss. Loud machines, such as lawnmowers can affect hearing. Workers around loud machines should wear protective ear muffs to block the sounds.
All materials can vibrate. The less a material vibrates the more it absorbs sound and the less it reflects or diffuses sound. For example, if you walk into a room filled cloth covered furniture, carpet on the floor and curtains on the windows, you do hear the sound made by your steps. But if the same room were empty of furniture and no carpet covered the floor, the sound of your steps would be loud. This is because the furnishings, curtains and carpet absorbed the sound while the walls and floor of the empty room reflected the sound. Echos are reflected sound. In the empty room, you hear the original sound produced by your steps plus all the echoes. Thus you hear more sound in the empty room.