The phenomenon of rf sidebands in Mössbauer spectra is used to measure the relaxation time τ of ultrasonic vibrations in Fe foils. A Co57 in Fe source, collimator, metallic-Fe absorber foil, and NaI detector are rigidly mounted as a single unit. When a pulse of rf magnetic field is applied to the Fe foil, the modification of the absorption pattern allows more γ rays to be transmitted to the detector. A plot of the number of γ rays versus time thus provides a measurement of the intensity of ultrasonic vibrations versus time. The results at 16 MHz indicate that for freely suspended foils thicker than 5 μm, τ is 11.4 μsec. τ is independent of annealing, foil purity, magnetic field, and varies with frequency as 1ω. This value for τ corresponds to an acoustic Q of about 580, comparable to bulk iron. this large value for Q eliminates the difficulty of the smallness of the magnetostrictive constant in the case of magnetically generated rf sidebands in Mössbauer spectra. The experimental procedures employed can be generalized to any source-absorber pair, to scattering geometry, and to any method for modifying the absorption pattern. © 1973 The American Physical Society.