Surface acoustic waves (SAW) of µm wavelength on a monocrystal give diffraction satellites around each Bragg peak in an x-ray diffraction diagram. By using a four crystal monochromator, a secondary two-crystal analyzer and masks reducing the fingerprint to the part of the crystal containing the acoustic modulation, their observation on a GaAS (001) surface is possible on a laboratory diffractometer. The finite extension of the satellites diffraction rods and of the crystal truncation rod perpendicularly to the surface leads to geometrical correction factors when convoluted with the instrumental resolution function which had previously been ignored. The calculations of these geometrical correction factors in the framework of the kinematic approximation allows the determination of the surface acoustic wave amplitude and to study its attenua-tion and its dependence with RF power and duty cycle. The ability to perform such determinations with a laboratory diffractometer should prove useful to optimize SAWs which are presently used in a broad range of condensed matter physics studies.