Acoustically thin capping structures and acoustic droplet ejectors having fluid wells and which use such capping structures to create fluid cells. The inventive capping structures permit the accurate positioning of the free surface of a fluid, permit acoustically induced fluid droplet ejection, and prevent fluid from spilling from the fluid wells. "Acoustically thin" means that the thickness of the capping structure is small enough that the acoustic energy that is lost passing through the capping structure is less than 50% of the incident acoustic energy.