Sound, as perceived by a human being, is variation in the instantaneous air pressure above and below the barometric air pressure (ambient pressure). Similar to waves in a pond; sound pressure variations tend to be cyclical in nature, so they are commonly referred to as “waves” or “waveforms.” Like the surface of an ocean in a storm, the waveform of audio containing a mixture of sounds is not like the smooth ripples on a pond because many smaller waves are adding to and subtracting from each other to produce complex patterns. Complex audio signal appear more similar to a jagged mountain range than the smooth rounded sine wave; so the term “waveform” is more descriptive.
To graph the waveform; two axes are required: vertical and horizontal. The horizontal axis typically represents the passage of time (the “timeline”). The vertical axis typically represents amplitude; or how high or low the instantaneous air pressure varies in relation to ambient pressure. It is related to the Volume of the sound in a long-term manner. If the average amplitude is higher, the volume of the audio is louder. But at the waveform level; the amplitude at a given instant in time can be zero; even in a very loud signal! So although there is a relationship between amplitude and Volume, they are not exactly the same.
Most analog audio equipment works with an electrical voltage waveform that is analogous to the sound pressure waveform. A microphone acts as a transducer to change the sound pressure variations into voltage variations. A higher voltage represents a higher pressure, and a lower voltage represents a lower pressure. Because the air pressure varies above and below ambient pressure, positive and negative voltages are typically used to represent positive and negative pressure, with ground (no voltage) representing ambient pressure in the absence of sound.
Once in this voltage wave form, the signal can be transmitted on cables, processed (Volume, Bass, treble, etc), then amplified again to drive speakers. The speaker then transduces the voltage waveform back into pressure variations that make up sound we can hear.
Most analog distortion occurs in the amplitude domain because most analog audio is “real-time.” Time domain distortions tend to occur during recording and playback of audio; particularly in digital audio encoding and decoding. Jitter in clocking of conversion is a major source of time domain distortion. Please see time domain for more details on that form of distortion.