2) the film-air "back" interface, and 3), the air-receiving transducer interface. Although the electrode-current collector interface can be considered, the individual response of the electrode and current collector can be homogenized and considered as a single film as the acoustic impedance of solids are much higher than of air, thus showing negligible difference between the "front" and "back" interfaces. Experimental justification is given in the Supporting Information (Figure ).