What is sound?

According to The New Penguin Dictionary of Music, sound is "The substance or the medium of music. Music is an art of sound unfolding through time, and an art of time unfolding through sound." (http://www.credoreference.com)

Sound is made from vibrations, transmittede from the object making the vibrations to the ear through fluctuations in air pressure.

Every sound has a distinct makeup of sounds from the sound spectrum. There is a fundamental pitch or frequency that dominates the sound, along with a large number of further sounds (called Harmonics) that we perceive but don’t distinguish from the fundamental.

Pitch is directly proportional to sound vibrations; the faster the vibration the higher the pitch.

“Timbre is a term that relates to the perceived type of sound coming from a sound-generating source”:

A violin timbre is percieved as the sound coming from a violin. A cat meow timbre is percieved as coming from a cat.

The number of harmonics in a sound, and their strength in volume is dependent on the size, shape and materials of the sound producing object. In the majority of tuned musical instruments there is a standard set of harmonics known as the overtone series. The strength of harmonics change from one instrument to the next, allowing us to perceive the difference between a note played on a violin and the same note played on a piano.

The most simple sound has a fundamental and no harmonics, referred to as a Sine Tone. Sine Tones do not occur in nature but are synthetically and electronically generated

Sine tones are used as a basis in hearing tests. At birth we can hear frequencies up to around 20,000 Hz. Many sounds or timbres contain frequencies as high as this within their harmonic spectrums.
As we age we lose the ability to hear high frequencies, this furthered through any damage we do to our ears.

An important characteristic of sound is the way that it is shaped over time, referred to as the sound’s amplitude.

"The amplitude of a wave is the maximum displacement, or the maximum value, it attains from its mean position during a cycle. It is both positive and negative. With sound waves, the greater the amplitude, the louder is the sound." (http://www.credoreference.com)

Amplitude is most often described with four terms, known as ADSR:
~Attack
~Decay
~Sustain
~Release

Attack is the period leading up to the sound reaching full volume. In many instruments it is virtually instantaneous.

Decay occurs after the sound has peaked. The decay occurs before the sustain.

Sustain is a 'constant' volume that the sound remains at until the key (or whatever is producing the note) is released.

Release is how quickly the sound fades once the note has finished the sustain period (the key has been lifted).