An experimental study of acoustic modes of resonant photo-acoustic detectors (PAD) of various types has been carried out. It is shown that ring longitudinal modes are formed in resonant differential PADs (Miklos's and Kapitanov's schemes) with two parallel acoustic resonators that protrude from the acoustic resonators of the detector into the buffer cavities for a short distance (∼1–2 mm) and do not reach the flanges (windows) of the PAD. This property of the ring longitudinal acoustic modes of differential PADs significantly reduces the sensitivity of the detectors to the parasitic effect of background absorption by the detector windows. The optimal length of the buffer cavities of the differential PAD is comparable to the diameter of the acoustic resonators of this detector (∼10 mm). It was also experimentally shown that in the longitudinal resonant PAD (Harren's scheme) only longitudinal acoustic modes are formed along the entire length of the detector, in which the antinodes of pressure oscillations of the standing sound wave in resonance are located on the flanges (windows) of the buffer cavities, which reduces the noise immunity of this detector to the effect of background absorption in windows as compared to the ring modes of the differential resonant PAD.