## Acoustic Sensor Architecture  
Surface acoustic wave transducers are classical analog radiofrequency processing elements widely described in the literature [24; 25; 26]. Their use as wireless passive sensors goes back more than 30 years [27; 28; 29; 30], and yet, wide acceptance remains elusive [31; 32], in the context of wireless, passive sensors, possibly due to the challenges of deploying the wireless RADAR system needed to probe their response. Targeting an audience of RADAR users already familiar with the challenges of radiofrequency wave propagation and antenna deployment might leverage this limitation. Such an audience is however hardly familiar with the details of acoustic transducers, which will be described here. Acoustic transducers are based on the principle of converting an electromagnetic wave into an acoustic wave through the inverse piezoelectric effect: interdigitated electrodes are patterned on a piezoelectric substrate so that the incoming electromagnetic wave induces an alternating electric field in the substrate. This alternating electric field induces a periodic motion of the crystalline lattice propagating in the piezoelectric substrate: an acoustic wave. Amongst the various wave polarizations meeting boundary conditions, surface waves are confined to the piezoelectric substrate surface and are prevented from propagating towards the bulk of the substrate. One classical polarization exhibiting an out-of plane component is the Rayleigh wave, a strongly coupled wave in lithium niobate orientation YXI/128\({}^{\circ}\), exhibiting little propagation losses at sub-GHz frequencies: such a wave is hence well suited for wireless measurement. The reflective delay line sensor architecture is based on two elements: first, an interdigitated transducer is connected to the antenna converting the electromagnetic wave into an acoustic wave by polarizing the piezoelectric substrate. The second element is a set of acoustic reflectors: the acoustic wave propagates on the surface of the piezoelectric substrate until it meets varying boundary conditions inducing acoustic velocity variations and hence reflection. In our case, electrodes patterned on the substrate induce free space to metalized boundary conditions, velocity variations and, hence, reflection of a fraction of the acoustic energy. By patterning multiple electrodes at a half-wavelength period, a Bragg reflector is designed to coherently reflect energy, hence providing a global reflection coefficient tuned by selecting the number of electrodes in the reflector (Figure 1). The reflected wave reachesthe interdigitated transducer to be converted back to an electromagnetic wave through the direct piezoelectric effect. From the user perspective, the surface acoustic wave transducer acts as an electric dipole delaying the incoming wave by a few hundreds of nanoseconds to a few microseconds, with sub-centimeter dimensions thanks to the slow acoustic wave propagation velocity. Acoustic wave propagation is sensitive to the environmental properties of the piezoelectric substrate. Temperature affects the elastic constant and the density of the substrate through dilatation: acoustic velocity being the square root of the ratio of these two quantities, it exhibits some dependence on temperature and so does the reflected echo time of flight. Similarly, stress induces variations in the elastic properties and the propagation path length, again inducing variations in the reflected echoes' time of flight. Enhancing one measurement property and rejecting unwanted interferences from other parameters is achieved by designing the packaging, selecting the proper piezoelectric substrate, as well as the proper crystalline orientation. The piezoelectric substrate is selected for its compatibility with the environmental conditions (e.g., Curie and twinning temperatures), while the propagation direction is selected for the wave polarization to comply with boundary conditions (e.g., liquid loading).