The photoacoustic effect and Doppler effect involve two independent physical principles. The photoacoustic effect, the process of energy transformation from intensity-modulated optical radiation to acoustic waves, was first described in 1880 by Bell in his photophone research [1]. The Doppler effect, frequency shift of a wave when the wave source is moving with respect to an observer, was discovered in 1840 by Doppler while he was analyzing the colored light of stars [2]. The photoacoustic Doppler (PAD) effect is the combination of these two effects. By means of the photoacoustic effect, an acoustic wave is produced from a light-absorbing medium. From the Doppler effect, the acoustic wave undergoes a Doppler shift when the photoacoustic source, the part of the light-absorbing medium generating the acoustic wave at a given moment, has motion relative to a detector. While both the photoacoustic effect and Doppler effect are well understood, the combined photoacoustic Doppler effect and its applications have not been studied extensively.