A picosecond study of ultrahigh‐frequency acoustic phonons in specifically engineered GaAs/AlxGa1-xAs periodic multilayer structures is presented. The lattice‐matched boundary conditions for photothermal acoustic generation and optical properties of these materials make these structures ideal for sound‐wave generation in the 100 GHz to THz range. The acoustics are generated using ultrashort‐laser‐pulse excitation and detected in real time by measuring the strain‐induced change in reflectivity with the pump‐probe technique. By using 12 nJ, 90 fs pulses from a Ti:sapphire laser source, the generation and detection of ∼50 GHz acoustics in a 6‐bilayer, ‐oriented GaAs/Al0.4Ga0.6As structure, 500 Å thickness per layer, on a GaAs substrate, are successfully demonstrated. The structure was specifically designed to give the maximum sensitivity to the acoustics through étalon‐induced modulations in the reflectivity spectrum. With similarly designed multilayer structures, the upper frequency limit can be achieved for the thermoelastic generation of coherent acoustic phonons, that is, ∼300 GHz in GaAs for ∼1 eV above band‐gap‐energy photons.