Phononic crystals

Sound waves propagate through air pretty much independently of frequency: if you listen to a sound from a distance, its amplitude is smaller (the sound is softer) but its frequency spectrum is unchanged (the tone is the same).  Everyone who has lived in a dorm knows that certain materials do selectively transmit some frequencies better than others.  That’s why you are woken up by the throbbing bass from your neighbor’s stereo.  This frequency selectivity of normal building materials is pretty crude, however.

Recently, people have built (or actually, discovered) phononic crystals: structures that selectively reflect certain frequencies of sound waves.  Perhaps one day objects will be designed with a particular “acoustic color” as well as particular optical colors.

You can choose a somewhat dry introduction here or a more friendly and colorful summary here; the latter site is labeled ‘vulgarisation’, which I can only guess reveals the author’s ambivalence at making such lofty material accessible to you and me.

If you want to make your own phononic crystal, follow the directions here.

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