HERD — The Science: how infrasound works. Research library

01 · Physics

An earthquake and a tsunami create a sound that outruns the wave

An undersea earthquake shakes the seafloor and the coast, and the moving wave presses on the atmosphere. That is how infrasound is born — sound waves below 20 Hz that humans cannot hear. In air they travel at roughly 340 m/s, while a tsunami in the open ocean moves at around 200 m/s and slows sharply near the shore. That difference in speed is exactly the "window" of minutes.

Diagram: infrasound from an undersea earthquake outruns the tsunami wave and reaches the coast first — Simplified diagram: a shock on the seafloor (left) generates infrasound waves in the air that reach the shore before the slow tsunami wave in the water.

This is not hand-waving theory. After the 2004 tsunami, the team of Alexis Le Pichon showed, using data from the Diego Garcia infrasound station, that the tsunami itself was a source of infrasound waves — the first time in history that their radiation zone, about 1,500 km long, could be reconstructed.

"These results provide unique evidence that the tsunami generated infrasonic waves." — Le Pichon et al., Geophysical Research Letters, 2005

02 · Planetary-scale confirmation

The Hunga Tonga eruption (2022): infrasound circled the Earth

If you need proof that geophysical events literally "sound out" across the whole world — here it is. The eruption of the Hunga volcano (Tonga) on 15 January 2022 produced an atmospheric wave that, according to Science, circled the planet several times over six days. The infrasound from the blast was recorded by all 53 infrasound stations of the international IMS network.

That same atmospheric wave, according to another paper in the same issue of Science, helped tsunami waves reach distant shores earlier than classical models predicted — precisely the "sound outruns the water" effect that HERD is built on.

The Hunga blast was comparable in power to the 1883 eruption of Krakatoa. Its infrasound (0.01–20 Hz) was detected worldwide. — Matoza et al., Science, 2022

03 · Elephants

Elephants really do communicate and listen through infrasound

This is not a legend: elephant "rumbles" have a fundamental frequency of 14–24 Hz in Asian elephants, and these sounds travel both through the air and through the ground. Research by Katy Payne, Caitlin O'Connell-Rodwell and Simon Klemperer has shown that low-frequency elephant vocalisations generate seismic Rayleigh waves that propagate up to ~2 km, and that elephants pick them up with receptors in their feet and trunk.

In field experiments in Namibia, herds of wild elephants were "played" seismic copies of alarm calls — and the elephants reliably changed their behaviour. This was the first demonstration that a large mammal recognises signals transmitted through the ground.

An elephant is, in effect, a walking low-frequency antenna. That is exactly why elephants are our scientific reference for calibrating the network.

04 · Honestly: where science argues with us

Did elephants really escape in 2004 thanks to a "sixth sense"? It's not that simple.

The papers wrote at the time that in Yala National Park (Sri Lanka) not a single dead elephant was found — the animals supposedly sensed the danger and fled to the hills. A beautiful story. But honest science demands verification.

A Smithsonian Institution study of two elephants wearing satellite GPS collars, which on the day of the tsunami were near the impact zone in Yala, showed that neither of them behaved as if it had sensed the wave in advance and run away from the shore. The authors state plainly that the data do not support the "sixth sense" version.

Why do we put this on our own site? Because this is the very essence of HERD: isolated observations of animals are a hypothesis, not proof. The physics of infrasound is firmly established; "animals always sense everything in advance" is not. That is exactly why we are building not a faith in a sixth sense, but a measurable network of instruments that will either catch the signal or honestly show that it did not.

"These data indicate that neither elephant behaved in a manner consistent with a 'sixth sense' that would allow early detection of an approaching tsunami." — Smithsonian / Gajah, 2008

05 · Why a network of cheap sensors makes sense

Professional networks already catch infrasound. We want to make them a thousand times denser.

The International Monitoring System (IMS) of the CTBTO is a global network of 60 infrasound stations, originally built to detect nuclear tests. It also catches volcanic eruptions, meteors and explosions. After 2004 the CTBTO received a mandate to pass data directly to national tsunami warning centres.

But 60 stations for the whole planet is a very sparse grid. Our bet is exactly the same as Google's with its smartphone earthquake detector and the Raspberry Shake network: many cheap sensors, combined by correlation, can see what individual expensive instruments cannot. We do not replace professional systems — we densify the picture between them.

Where the honest line is. Today an official tsunami warning is built first and foremost on seismic and hydroacoustic data and DART buoys. The role of infrasound is to complement them. HERD is a research network and an open-data provider, not a certified alerting system. We hold that line strictly.

05b · Can such data even be trusted?

Trust is not in one sensor — it's in the agreement of the whole network.

A fair sceptic's question: how can you trust a cheap device on someone's balcony? The answer: we don't ask you to. What matters is not "a sensor said so", but dozens of independent nodes recording the same signature in agreement — with the right delays across geography and a bearing toward the source. Random noise or a slamming door cannot line up like that.

So that the data are useful not only to enthusiasts but to insurers and agencies, several layers of verification are built into the architecture itself:

Signature and time. Each node signs its measurements and synchronises its clock by GPS — the data cannot be faked or "shifted" in time.
Immutable log. Records are protected so that an event cannot be "drawn in" after the fact.
Calibration against a reference. Nodes are calibrated against a verified monitor — there is traceability of the readings.
Confidence, not yes/no. The system outputs a probability with a confidence interval, and a serious trigger requires confirmation from an independent source (seismic data, DART buoys).

And, honestly, once more. The system never starts an evacuation by itself — it gives information to the people and agencies who make the decision. The key trust metric — the false-alarm rate — we measure and publish openly.

07 · Video

See it and hear it

Popular-science material on the topic — about infrasound and about how elephants hear the ground.

Infrasound: What You Can't Hear CAN Hurt You

What infrasound is, how it propagates and why we cannot hear it · EN

Studying Elephant Communication

HHMI BioInteractive: O'Connell-Rodwell's experiment with elephant seismic signals · EN

Secrets of the Elephants (Nat Geo)

How elephants "talk" in infrasound and hear it through their feet · EN

These Illusions Fool Almost Everyone (Veritasium)

How hearing and low frequencies work — good intuition for the topic · EN

Videos are embedded from YouTube and belong to their authors; we include them as educational material on the topic, with no affiliation to HERD.

08 · In the field

A lab on wheels: we go where the Earth is listening

HERD — a mobile lab van with Starlink, solar panels and sensors on the coast of Thailand

The science of infrasound is field work. To calibrate sensors next to elephants in sanctuaries and to place stations along coastlines where there is neither a power socket nor a signal, we are building a mobile listening lab on a Toyota Hiace: two sleeping berths for researchers, a workbench with equipment, off-grid power from solar panels and Starlink satellite internet — data goes to the network straight from the wilderness, in real time.

This means reference measurements and node installation are no longer tied to cities and hotels. The lab drives to the source of the signal itself — to the elephants, to the volcano, to the shore.

The image is an artistic visualisation of the concept. The exact build of the van is chosen to fit the budget of the first sprint.

How infrasound works — and why we're not just talking.

An earthquake and a tsunami create a sound that outruns the wave

The Hunga Tonga eruption (2022): infrasound circled the Earth

Elephants really do communicate and listen through infrasound

Did elephants really escape in 2004 thanks to a "sixth sense"? It's not that simple.

Professional networks already catch infrasound. We want to make them a thousand times denser.

Trust is not in one sensor — it's in the agreement of the whole network.

Sources. Check us.

Infrasound of tsunamis and eruptions

Elephant infrasound communication

An honest counterweight: criticism of the "sixth sense"

Monitoring networks and citizen science

Cheap MEMS sensors hear infrasound

See it and hear it

A lab on wheels: we go where the Earth is listening

Convinced the physics is real?