(a) Sketch of a SAW with amplitudes along the propagation direction (horizontal) and out of the plane of the crystal surface (Rayleigh mode). The amplitude along the orthogonal in-plane direction of the crystal (into the drawing plane) is zero and the wavefronts are straight. The colour shading of the background symbolizes the piezoelectric surface potential. (b) Scheme of the stroboscopic imaging concept: the SAWs are synchronized by an electronic phase-lock loop (PLL) to the X-ray pulses of the synchrotron, providing a perfect long-term stability with a finite single-cycle jitter. Many partial images of the sample in the same state are averaged on the `slow' detector (on top, not shown). (c) Image of the sample as seen through the central hole in the cap of the sample holder. Four pairs of IDT electrodes are visible (left and right), where each pair appears as a single rectangular pattern. Additional T-shaped grounded electrodes arrive from top and bottom to reduce sample charging. (d) Composed image of the sample holder used for the experiment [reproduced with permission from Foerster et al. (2016), copyright (2016) Elsevier]. It includes a printed circuit board for wire bond contacts to the sample and a raised cap, which does not touch the sample surface to avoid short-circuiting these wires.