Answer

Linear phase means that all frequencies are delayed by the same amount when passing from the input to the output of a system. The image below depicts a square wave, a waveform that consists of fundamental and odd harmonics (only two of which are shown). When the square wave passes through a linear phase system, the time relationship between fundamental and harmonics is preserved and the waveform that emerges maintains its square wave character.

In a system with non-linear phase, the time relationship between fundamental and harmonics is altered and something other than a square wave emerges at the output. Another term for this altered waveform is ‘phase distortion’.

Phase non-linearity can be introduced by the electronics as well as by the transducers, waveguides, and other elements of a loudspeaker system.