Human ears function as efficient, uncredited editors, discarding the vast majority of environmental sound waves to leave behind a neat, rounded soundtrack known as silence. While human hearing generally ranges from 20 hertz to 20,000 hertz, sound waves exist far beyond these boundaries. Frequencies below 20 hertz are classified as infrasound, while those above 20,000 hertz are ultrasound. Three remarkable animal groups demonstrate what exists outside this narrow human window.

Nearly everything in acoustics comes down to a simple mathematical relationship with wave length. Sound travels through air at roughly 343 meters per second, meaning that lower frequencies produce exceptionally long waves while higher frequencies create very short waves. Long waves slide past obstacles and travel vast distances with minimal energy loss, whereas short waves bounce off tiny objects and dissipate quickly. Elephants and bats occupy opposite ends of this acoustic spectrum.

In the 1980s, researcher Katy Payne and her team discovered that elephants communicate using low-register rumbles that humans can barely register, typically measuring between 14 and 24 hertz. These powerful calls can travel substantial distances, though environmental barriers like trees and undergrowth absorb sound faster than open grasslands. Researchers studying Asian elephants note that transmission ranges vary by habitat, and humans standing close to an elephant can sometimes feel the onset of a rumble as a physical hum in the chest.

Elephant rumbles are complex vocalizations used for contact, greetings, social interactions, and alarms, with their duration and structural makeup shifting depending on the context. Recent studies on wild African elephants even indicate that individuals use name-like calls to address one another, responding more strongly when played recordings intended specifically for them. Additionally, research suggests some elephant species may sense seismic vibrations through their feet, though scientists emphasize that empirical testing is still required to confirm whether Asian elephants utilize this same ground-based communication.

At the opposite extreme, bats utilize ultra-short waves operating in the ultrasonic range, spanning from about 11 kHz to above 200 kHz across various species. Because higher frequencies have much shorter wavelengths, their calls can bounce off tiny prey items like mosquitoes that would be entirely invisible to longer waves. While cruising, a bat emits 10 to 20 calls per second, accelerating this rate into a rapid terminal buzz of up to 200 calls per second when locking onto a moving target.