This paper focuses on experimentally verifying the physics of damping in 1 GHz laterally-vibrating composite piezoelectric resonators. This work confutes a previously developed theory of interfacial dissipation, a slip phenomenon occurring at the interface between dissimilar materials, which associated damping to a stress jump (or difference in Young's moduli (ΔE)) of the materials forming the interface. This work finds that damping in 1 GHz laterally-vibrating AlN resonators could be attributed to either interfacial dissipation due to an acoustic velocity jump (Δv) or thermoelastic dissipation (TED) in the electrodes.