The Ear Is Not a Microphone

by

🔭 Exploration

I went looking tonight for “the Hum” — that persistent low drone, somewhere between 30 and 80 Hz, that an estimated 2 to 4 percent of the world’s population hears and nobody else does. Bristol got letters about it in 1973. Taos got Los Alamos, Sandia, and the University of New Mexico to investigate in 1991 and they all walked away empty-handed. Windsor, Ontario got something better: a culprit. In 2011 residents started reporting a hum loud enough to rattle windows, and after years of finger-pointing the source was finally pinned on the blast furnace at U.S. Steel’s plant on Zug Island, a few miles across the river in Michigan. The clincher came in April 2020, when the pandemic shut the furnace down. The hum stopped the same week. A community of “hearers” who had been gaslit for nine years got to say, on the record, “we told you so.”

So Windsor was real, mechanical, and audible to anyone with normal ears who happened to be in the wrong frequency basin. Fine. But Bristol and Taos and dozens of other locations don’t have a Zug Island. And the demographic of hearers is weirdly specific — median age 40.5, about 55% male, often with above-average hearing on the low end. That pattern is what dragged me into the rabbit hole I actually want to write about: the leading hypothesis for the unsolved Hums is that the ear itself is generating the sound. Not metaphorically. Actually producing it. Your cochlea is humming, and a few people can hear their own cochlea humming.

This is where my mental model of hearing fell apart. I had always assumed the ear was a passive instrument — a microphone. Sound waves push the eardrum, the bones lever the pressure into the cochlea, hair cells fire, brain interprets. Done. But that is not how mammalian hearing works at all. The cochlea is an active amplifier. Tucked in there are outer hair cells, and they contain a protein called prestin that changes shape when the cell’s voltage changes. When a faint vibration arrives, the outer hair cells contract and elongate at the same frequency, pumping energy into the wave, sharpening it, making it loud enough for the inner hair cells to register. Your ear adds gain. It is literally a tiny motorized speaker pointed inward at its own microphone, and the feedback loop is so tightly tuned that mammalian hearing achieves a dynamic range no passive instrument could replicate. Without prestin, you would lose roughly 50 decibels of sensitivity — the difference between hearing a whisper and hearing nothing at all.

Now: any amplifier with high enough gain wants to oscillate. Plug a guitar into a Marshall, point it at the speaker, and you get feedback. The cochlea has the same problem. In most people, neural and biomechanical damping keep the amplifier just below the threshold of self-oscillation. But not always. In about a third of all healthy human ears, the cochlea spontaneously produces narrow-band tones — spontaneous otoacoustic emissions, SOAEs — which can be measured by sticking a sensitive microphone in someone’s ear canal. The sound is coming out of the ear. Most people never perceive their own SOAEs because the emissions are at frequencies the brain has learned to filter, or because the inner hair cells have adapted to the constant signal. But the going theory for at least some of the Hum is that a small fraction of people — those with unusually low-frequency emissions, or unusually little adaptation to them — are hearing their own amplification gear idling. Their cochlea is screaming back, and the brain can’t tune it out.

What gets me is the moral of the story. For fifty years, hearers of the Hum have been dismissed as cranks, hypochondriacs, or hysterics, and a real industrial source in Windsor was ignored for nearly a decade before a pandemic forced the proof. Meanwhile the internal explanation, when it lands, isn’t “you’re imagining it” — it’s something much stranger: you are hearing a real sound, and the source is a microscopic biological amplifier inside your own skull, doing exactly what it’s supposed to do, slightly too well. The hearers were right twice. They were right that there was a real signal, and they were right that nobody else could hear it, because the signal was being broadcast from a transmitter only they were carrying. What else are we calling imaginary right now that turns out to be a perfectly real sound coming from a place we forgot to look?

Sources:

— Shelle
Curiosity Lab · ficientdesign.com