Millihertz to Microhertz

After a magnitude-9 earthquake the entire planet vibrates like a struck gong, with its deepest mode at about 0.3 mHz — one oscillation every 54 minutes. The surface rises and falls by fractions of a millimeter. You cannot hear it (human hearing starts at 20 Hz), but gravimeters and seismometers worldwide pick it up. The 2004 Sumatra quake kept Earth ringing measurably for weeks.

Ocean swells, tidal constituents, and seiches (standing waves in harbours or lakes) all oscillate in the millihertz band. A 10-second ocean swell is 100 mHz; a harbour seiche with a 10-minute period is about 1.7 mHz. Monitoring these frequencies helps coastal engineers predict resonance in ports and design breakwaters that don't amplify destructive wave energy.

Not directly — our senses are far too fast. But some physiological rhythms operate here: the Mayer wave, a ~0.1 Hz oscillation in blood pressure, sits at the high end of the millihertz scale, and slower vasomotion (tiny blood vessel contractions) can dip below 10 mHz. You don't feel them as vibrations, but they show up clearly on a continuous blood-pressure monitor.

Infrasound is sound below the ~20 Hz threshold of human hearing. The lowest infrasound blends into the millihertz range — the International Monitoring System for nuclear-test detection listens down to about 20 mHz. Sources include volcanic eruptions, meteor airbursts, severe storms, and ocean microbaroms (standing pressure waves between ocean swells and the atmosphere).

Instruments record a time series (pressure, acceleration, displacement) over hours or days, then apply a Fourier transform to extract frequency content. Superconducting gravimeters can resolve Earth's free oscillations below 1 mHz by measuring gravity changes of 10⁻¹² g. The trick is not a fast sensor but a patient, ultra-stable one and enough data to separate signal from drift.

Solar oscillation modes with periods of hours to days, slow tidal harmonics, and long-period stellar variability all live in the microhertz band. Earth's free-core nutation — a wobble of the liquid outer core relative to the mantle — oscillates near 1 μHz. These are real physical processes, just far too slow for any wristwatch to track.

Ground-based detectors like LIGO are deafened below about 10 Hz by seismic noise. LISA will float three spacecraft in a triangle 2.5 million kilometers across, far from terrestrial vibrations, making it sensitive from ~0.1 mHz down into the microhertz regime. That band contains signals from massive black-hole mergers and thousands of compact binary stars in our own galaxy.

You need at least one full cycle to confirm a periodic signal, and preferably several. At 1 μHz (period ~11.6 days), a few months of data suffices. At 0.01 μHz (period ~3.2 years), you need a decade or more. This is why long-baseline observational campaigns — decades of pulsar timing or stellar photometry — are essential for low-frequency science.

Helioseismology studies sound waves trapped inside the Sun. The Sun rings like a bell with millions of overlapping oscillation modes. Most solar p-modes peak around 3 mHz (5-minute period), but gravity modes (g-modes) deep in the solar core are predicted at microhertz frequencies. Detecting those elusive g-modes would let scientists probe conditions at the Sun's very center.

A microhertz is a million times slower than one hertz. If middle C on a piano (262 Hz) were slowed to 1 μHz, a single wave cycle would take about 30 years. You would hear the first peak of the note in your twenties and the first trough around your fiftieth birthday. It puts cosmic patience into perspective.