Right now, tucked inside your smartphone, there are tiny drums made of crystal being struck two billion times per second. They're called thin-film bulk acoustic resonators (FBARs), and without them, the modern wireless world would collapse into a screeching mess of overlapping signals.

Here's the problem they solve: your phone's antenna receives everything — Wi-Fi, LTE, 5G, your neighbor's baby monitor, cosmic microwave background. To pick out just one channel, you need a filter that's absurdly precise: it must pass, say, 2.4 GHz while brutally rejecting 2.5 GHz. Traditional inductor-capacitor filters can't come close to that sharpness at those frequencies. Ceramic filters are too big. So engineers did something wonderful: they gave up on electronics entirely and turned the signal into sound.

An FBAR is a wafer of piezoelectric material — usually aluminum nitride — sandwiched between two electrodes, suspended in mid-air over a tiny etched cavity. When your incoming radio signal hits the electrodes, the piezoelectric effect makes the crystal physically vibrate. The wafer thickness is tuned so that only one specific frequency creates a standing acoustic wave resonating between the top and bottom surfaces. Every other frequency gets swallowed. The resonance is then converted back into an electrical signal via the same piezoelectric effect. Radio in, sound in the middle, radio out.

The numbers are staggering:

• The aluminum nitride layer is often less than a micrometer thick — thinner than a red blood cell.
• Acoustic waves travel about 100,000 times slower than electromagnetic waves, which is why acoustic filters can be so much smaller than their electronic equivalents at the same frequency.
• A single modern phone contains dozens of FBARs, one for each frequency band it needs to talk on.
• The global market for these things is measured in billions of dollars a year, and most people have never heard the word "FBAR."

The technology has an interesting lineage. Quartz crystal oscillators — the ticking heart of every wristwatch since the 1970s — use the same piezoelectric principle, but at kilohertz frequencies. FBARs are essentially quartz watches scaled down and sped up by a factor of a million, then manufactured with the same photolithography that makes microprocessors. Agilent (later Avago, now Broadcom) commercialized them in the early 2000s, and by the time the iPhone arrived, they were already indispensable.

There's a poetic loop here worth savoring. The piezoelectric effect was discovered by the Curie brothers in 1880 by squeezing quartz crystals. A century and a half later, that same fundamental physics — squeeze a crystal, get electricity; apply electricity, watch it deform — is what allows a device in your pocket to distinguish "this millimeter of the electromagnetic spectrum" from "that millimeter of the electromagnetic spectrum" while you scroll through cat videos.