Schumann resonances.
The Earth and its ionosphere form a spherical waveguide. Every lightning strike in the world rings this cavity, and certain frequencies — set by the planet's circumference and the speed of light — survive as standing waves. The fundamental sits at 7.83 Hz, just below human hearing. Multiply by four and the lowest mode lands on the bottom edge of the audible band.
A planet, ringing.
The diagram shows the Earth–ionosphere cavity in cross-section. Each mode has a different number of nodes around the equator — SR1 is a single global standing wave, SR7 has seven. Pick a mode to see its shape, hear its frequency, and read what it does in physics.
A waveguide the size of a planet.
The Earth's surface and the bottom of the ionosphere — about 80–100 km up — both reflect extremely-low-frequency radio waves. Together they make a closed shell, and any radio wave with a wavelength matching the cavity's geometry can bounce around it indefinitely.
Lightning is the energy source. Roughly fifty strikes happen worldwide every second, and each one dumps a broadband pulse of electromagnetic energy into the cavity. The mismatched frequencies cancel themselves out; the matched ones reinforce. What's left is a faint, persistent ringing at a handful of specific frequencies — the Schumann resonances.
You can measure them yourself with a magnetic-loop antenna and a long, quiet recording. The fundamental sits stubbornly near 7.83 Hz, below the lower limit of human hearing by a factor of about 2.5.
A natural tuning fork.
Because the cavity is enormous and stable, the Schumann frequencies barely move. They drift with the height of the ionosphere (day vs. night, solar weather), but the diurnal range is small enough that researchers use them as a continuous, planet-wide thermometer of the upper atmosphere.
The ratios between modes — 7.83 : 14.3 : 20.8 : 27.3 … — aren't octaves. They follow √(n(n+1)), so the spacing tightens up as you climb. The intervals don't line up with any standard tuning system; they're whatever the geometry of the cavity dictates.
Why these specific frequencies.
For a thin spherical cavity of radius R, the resonant frequencies of the lowest electromagnetic modes are approximately:
fn ≈ c / (2πR) · √(n(n+1))
Plug in the speed of light c and Earth's mean radius R = 6371 km: the c/2πR prefactor is ≈ 7.49 Hz. The √n(n+1) factor gives 1.41, 2.45, 3.46… so the predicted modes come out near 10.6, 18.3, 25.9, … Hz.
Observed values are a touch lower (7.83, 14.3, 20.8 …) because the real ionosphere is lossy and a bit fatter than the textbook thin-shell approximation. Eigentone uses the measured frequencies, not the predicted ones — same physics, less idealization.
SR1 is below human hearing — barely. Two octaves up (×4) puts the fundamental at 31.32 Hz, the bottom of a bass guitar's range. The ratios between modes are preserved exactly.
SR1 ≈ 7.83 Hz → 31.32 Hz
Constants: c = 299 792 458 m/s, R⊕ = 6371 km. Observed mode frequencies after Balser & Wagner (1960), Nickolaenko & Hayakawa (2002).
What each white key plays.
A linear ladder again — seven keys, seven cavity modes. There are no canonical observed frequencies above SR7, so higher keys stay silent.
Hear the planet ring.
Schumann is in the free tier. Every white key from C4 up plays a real cavity mode at four times its natural frequency.