  Sound is usually generated in a medium by an electromechanical vibrating
structure. The geometrical size and inertia of the structure set the frequency
cutoff in the sound-transduction mechanism and, often, different vibrating
structures are necessary to cover the whole range from infrasound to
ultrasound. An alternative mechanism without any physical movement of the
emitter is the thermoacoustic effect, where sound is produced by Joule heating
in a conductive material. Here we show that a single thermoacoustic transducer
based on a graphene aerogel can emit ultrabroadband sound from infrasound (1
Hz) to ultrasound (20 MHz), with no harmonic distortion. Since conventional
acoustic transducers are frequency band limited due to their transduction
mechanism, ultrabroadband graphene aerogels may offer a valid alternative to
conventional hi-fi loudspeakers, and infrasound and ultrasound transducers.
