Amplitude simply means power. In an analog transmission system, it refers to the height of a wave of energy relative to the horizontal axis. If the system is electrical, it refers to the specific voltage at a moment in time. For an electromagnetic system (wireless, it refers to the strength of the electromagnetic pulse at a moment in time. In an optical system it refers to the strength or power of the optical system, typically measured in terms of the number of photons detected per unit of time. In an acoustical system, it refers to the air pressure density at a particular moment in time.
We can create two waves of acoustical energy with different amplitudes by plucking one guitar string lightly and another more forcefully. Both strings vibrate back and forth, but the one plucked harder moves a greater distance in either direction. To the human ear, amplitude is associated with volume, so, the string we plucked harder creates a louder sound. Lightly touching a piano key, versus pounding on a key with your fist, creates the same phenomenon, since a hammer strikes a string inside the piano, with force relative to how hard the keys are pressed. In the visual, the graph on the left shows waves with two different amplitudes.
Some transmission systems use the amplitude of a transmitted signal to convey information. For example, on a POTS phone line, the amplitude of the electrical signal is varied in direct proportion to the analog pressure waves of air impacting the speaker. If a modem is attached to the same phone line, an electronic carrier tone with a constant amplitude and frequency is established and then the amplitude of the carrier, as well as the phase, is modulated to carry digital information. Another transmission system that uses the amplitude of the signal to convey information is your AM (amplitude modulation) radio station.
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