This is called a Waveform Displacement Graph of simply a Waveform . All sounds, no matter how complex, can be expressed as a waveform. If we view the entire waveform of a sound, we can see the change in amplitude over time. This is the envelope of the sound. Since waveforms are usually symmetrically above and below the 0 line, we normally graph envelopes by displaying only the maximum positive values of the sound over time.
Most synthesizers allow the envelopes of their voices to be modified.
For example, you could prolong the attack of a piano sound, or alter the release time of the instrument so that the sound decays more slowly. The exact method of performing the alterations is dependent upon the synthesizer. The sound producer generates an alternating current that is analogous to the fluctuating values in the waveform of the sound - high frequency sounds have fast alternations, low frequencies have slow alternations. The loudspeaker pushes and pulls air particles which are, in turn, perceived by the listener as sound. In digital audio, sound is represented as a series of numerical values.