Nonlinear interaction of quasiharmonic longitudinal waves, which propagate in solid porous material, was investigated theoretically. It was shown that as a result of such interaction between low-frequency waves (vibration field) and high-frequency waves (ultrasound) an ultrasound wave of summarized frequency would be generated. This newly generated wave can be in a phase-group synchronism with the vibration field. Analytical analysis qualitatively agrees with experimental data of ultrasound generation through seismic load.