a) Schematic of an line excitation in x-direction, which excites phonons in water and hydrogel, which after some time will reach the probe beam. When the water phonons transmit through the interface, they will propagate through the probe beam. b) Schematic of the volumetric effect, where due to the angle φ the probe beam is reflected both in water and hydrogel, even though the center of the probe beam is in hydrogel.