Ultrasonic Absorption at Microwave Frequencies and at Low Temperatures in MgO andAl2O3

Measurements are presented of the ultrasonic absorption in MgO and ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ for transverse and longitudinal waves in the low-temperature region. In contrast to some theoretical expectations, the absorption of longitudinal waves is not negligible but even stronger than for transverse waves. In the theoretical discussion it is shown that this strong longitudinal absorption probably is caused by three-phonon processes.