We present here precise measurements of the resonance frequency, lifetime, and shape of confined acoustic modes in the tens of GHz regime in GaAs/AlAs superlattice planar and micropillar cavities at low temperatures (∼ 20 K). The subharmonic resonant pump-probe technique, where the repetition rate of the pump laser is tuned to a subharmonic of the cavity resonance to maximize the amplitude of the acoustic resonance, has been used in combination with a Sagnac interferometer technique for high sensitivity (∼ 10 fm) to the surface displacement. The cavity fundamental mode at ∼ 20 GHz and the higher order cavity harmonics up to ∼ 180 GHz have been clearly resolved. Mechanical Q-values up to 2.7 × 104 have been measured in a planar superlattice, and the direct spatial mapping of confined acoustic modes in a superlattice micropillar cavity has been demonstrated. The Q-frequency product obtained is ∼ 5 × 1014, demonstrating the suitability of these superlattice cavities for optomechanical application