  The ability to generate, amplify, mix, and modulate sound with no harmonic
distortion in a passive opto-acoustic device would revolutionize the field of
acoustics. The photo-thermo-acoustic (PTA) effect allows to transduce light
into sound without any bulk electro-mechanically moving parts and electrical
connections, as for conventional loudspeakers. Also, PTA devices can be
integrated with standard silicon complementary metal-oxide semiconductor (CMOS)
fabrication techniques. Here, we demonstrate that the ultimate PTA efficiency
of graphene aerogels, depending on their particular thermal and optical
properties, can be experimentally achieved by reducing their mass density.
Furthermore, we illustrate that the aerogels behave as an omnidirectional
point-source throughout the audible range with no harmonic distortion. This
research represents a breakthrough for audio-visual consumer technologies and
it could pave the way to novel opto-acoustic sensing devices.
