We report on a study of the resonance trapping of monochromatically generated 29 cm-1 phonons in strongly excited ruby crystals. By use of a pulsed far infrared laser we generated, in homogeneously excited crystals, monochromatic phonon pulses and studied the trapping and decay of these phonons. We found that the trapping time increased with the square root of the concentration of excited Cr3+ ions and that it depended on the crystal dimension. We show, furthermore, that the trapping time depends on crystal quality and that it can be strongly influenced by inhomogeneous stress and by X-ray irradiation. We used a model of spectral-spatial phonon diffusion for a description of the experimental results.