That's not how they're describing it. The "paper" emits sound whether or not it's applied to a wall.
"The hand-sized loudspeaker the team demonstrated ... can generate high-quality sound no matter what surface the film is bonded to."
It is, as I understand it from the article, a bunch of 3-4nm sized piezoelectric elements in an array, all powered concurrently.
"... their design relies on tiny domes on a thin layer of piezoelectric material which each vibrate individually. These domes, each only a few hair-widths across, are surrounded by spacer layers on the top and bottom of the film that protect them from the mounting surface while still enabling them to vibrate freely."
If the vibrating elements are decoupled from the surface, the surface won't act as a resonator for it (at least, not as the primary resonator, as it would with a transducer). The vibrating elements are moving the air directly.
"The hand-sized loudspeaker the team demonstrated ... can generate high-quality sound no matter what surface the film is bonded to."
It is, as I understand it from the article, a bunch of 3-4nm sized piezoelectric elements in an array, all powered concurrently.
"... their design relies on tiny domes on a thin layer of piezoelectric material which each vibrate individually. These domes, each only a few hair-widths across, are surrounded by spacer layers on the top and bottom of the film that protect them from the mounting surface while still enabling them to vibrate freely."
If the vibrating elements are decoupled from the surface, the surface won't act as a resonator for it (at least, not as the primary resonator, as it would with a transducer). The vibrating elements are moving the air directly.