Sound

Sound can also mean a body of water, like a bay or channel.
Sound is caused by sound waves. It can be heard when it goes through a medium to the ear. All sounds are made by vibrations of molecules. For example, when a person hits a drum or a cymbal the object vibrates. These vibrations make air molecules move. Sound waves move away from where they came from. When the vibrating air molecules reach our ears, the eardrum vibrates, too. The bones of the ear vibrate in way the object that started the sound wave vibrated.
There are three different mediums. They are solids, liquids and gas. Sound travels the fastest through solids because the particles in a solid are closer together than they are in gas and liquid.
These vibrations let you hear different things. Even music is vibrations. Irregular vibrations are noise. People can make very complex sounds. We use them for speech.
Sound waves are longitudinal waves with two parts: compression and rarefaction. Compression is the part of the sound waves where the molecules of air are pushed (compressed) together. Rarefaction is the part of the waves where the molecules are far away from each other. Sound waves are a sequence of compression and rarefaction.
Vacuum
Since sound is a vibration of a medium, it cannot go through a vacuum. A vacuum is a place where there is no medium, for example in outer space. The word comes from the Latin adjective vacuus for "vacant" or "void".
Speed of sound
Sound waves can travel through solids, liquids, and gases. The thicker the medium, the faster it goes, but they cannot travel through a vacuum (a place with nothing in it). This is why astronauts cannot talk to each other in space: they need a radio to hear each other. Sound can travel through water faster than through air; and even faster in solids like stone, iron, and steel. At room temperature and normal atmospheric pressure, sound travels at 344 m/s (1134 ft/s), 761 miles per hour). Because the temperature and pressure change with altitude in the atmosphere, speed will vary as well.
Pitch and Intensity
Pitch is the highness or lowness of sound. Pitch is how humans hear different frequencies. Frequency is determined by the number of vibrations per second. The highest key on a piano, for instance, vibrates 4,000 times per second. It has a frequency of 4000 hertz (Hz), or 4 kilohertz (kHz). Lower keys have lower frequencies. A note an octave higher than another note has twice the frequency of that note.
The intensity of a sound is how much sound energy goes through a square meter in one second. Sound waves with higher amplitude (bigger vibration) have higher intensity. The intensity of a sound is higher closer to the sound source. Farther away, it's less intense. The inverse-square law shows how sound intensity becomes smaller, farther from the source. "Inverse square" says that when distance gets multiplied by a number, sound intensity gets divided by that number squared (the number times itself). Thus, twice the distance means a quarter the intensity.
Sound intensities can be very different. They can range from 0.000000000001, which are barely heard, to 1 W/m2 (painfully loud). The decibel scale makes sound intensity numbers easier to work with. A 0.000000000001 W/m2 intensity is 0 dB (decibels). It is an exponential scale, so when the decibel number increases by 10, the intensity is ten times as much. So, a 1 W/m2 intensity is 120 dB.
Loudness is how people sense the intensity of sound. Loudness depends on sound intensity, sound frequency, and the person's hearing.
Heard and not seen
Audible sound has frequencies between 20 Hz to 20 kHz. Human beings can hear audible sound. Sound waves with frequency above 20 kHz are called ultrasound waves. Sound waves with frequency below 20 Hz are called infrasound waves. Human beings cannot hear ultrasound waves and infrasound waves, but some animals, like bats and dolphins, use them. Older people have an even smaller hearing range. People are best at hearing sounds between 1000 Hz and 6000 Hz.
The Doppler Effect
When a sound source is moving towards someone, the frequency seems to increase. The same thing happens when someone moves toward the sound source. Frequency seems to decrease when someone moves away from a sound source. It also seems to decrease when the sound source moves away from someone. This is called the Doppler effect.
References
Halpern, Alvin, Erich Erlbach (1998). Beginning Physics II: Waves, Electromagnetism, Optics, and Modern Physics, pg. 50-56
Other websites
Conversion of sound units and levels
Sound calculations
Basic English 850 words