Digital input class-D amplifiers will be the predominant amplifier technology enabling consumer audio systems in the future. The traditional Power Supply Rejection Ratio (PSRR) measurement method cancels supply ripple in Bridge-Tied-Load (BTL) amplifiers, thus is unable to measure audio distortion induced. The proposed method employs innovative measurement of both Intermodulation Distortion (IMD) and Total Harmonic Distortion Plus Noise (THD+N) to more accurately represent audio quality. A new term known as Power Supply Ripple Distortion Factor (PSRDF) is introduced as a figure of merit for audio quality. Examples of how the proposed method effectively characterizes different levels of distortion induced by power supply ripple in closed-loop and open-loop BTL Class-D amplifiers are also presented. The proposed method is also applicable in characterizing all audio power amplifiers.