An MRI scan is never a pleasant occasion – even if you aren’t worried about the outcome, lying still in a confined, noisy space for long periods of time is at best an irksome experience. For hearing protection and to ameliorate boredom or claustrophobia, the patient wears headphones. Since magnets and wires can’t be used inside an MRI machine, the headphones have to literally pipe the sound in through tubes, which gives them poor sound quality and reduces the amount of noise they can block. [SomethingAboutScience], however, thinks that photoacoustic speakers could improve on these, and built some to demonstrate.

These speakers use the photoacoustic effect, which is mostly caused by surface heating when exposed to an intense light, then transferring the heat to the surrounding air, which expands. If the surface can transfer heat to the air quickly enough, and if the light source is modulated quickly, the rapid expansions and contractions in the surrounding air create sound waves. As a test, [SomethingAboutScience] shone a modulated 5-Watt laser on a piece of gold leaf, which produced recognizable music.

Gold leaf works because it absorbs blue light well and is thin enough to transfer heat to the air quickly. To cut out the absorbing surface, [SomethingAboutScience] also shone the laser directly into orange nitrogen dioxide gas, which produced a somewhat cleaner sound (in a purely auditory sense; nitrogen dioxide is quite dangerous, and calling it “a little toxic” is an understatement).