In psychoacoustics, a pure tone is a sound with a sinusoidal waveform ; that is, a sine wave of constant frequency, phase-shift, and amplitude. By extension, in signal processing a single-frequency tone or pure tone is a purely sinusoidal signal (e.g., a voltage). A pure tone has the property – unique among real-valued wave shapes – that its wave shape is unchanged by linear time-invariant systems ; that is, only the phase and amplitude change between such a system's pure-tone input and its output.