Every acoustic environment can be decomposed into three fundamental source classes: biophony(sounds produced by living organisms), geophony (sounds produced by non-biological natural sources), and anthrophony (sounds produced by human activity). This trichotomy, formalised by Bernie Krause in the 1990s, is the workhorse organising principle of modern ecoacoustics: an ecosystem's "sonic signature" can be characterised by the relative proportions and temporal patterns of these three classes.
The Three Pillars of the Soundscape
Every acoustic environment can be decomposed into three fundamental source classes, a framework formalized by Krause and widely adopted in the discipline.
Biophony in Detail
All sounds produced by living organisms constitute the biophony. Insects use stridulation, tymbal resonance (cicadas), and wing-beat aerodynamics, typically 2–10 kHz. In tropical forests, insect biophony dominates total acoustic energy. Anurans produce advertisement calls primarily through laryngeal mechanisms. Birds use the syrinx — a bifurcated structure unique to birds — enabling independent control of two sound sources. Marine mammals generate sounds ranging from infrasound (blue whale calls at 10–40 Hz, detectable at 1,000 km) to ultrasound (dolphin echolocation at 40–150 kHz).
Geophony as Acoustic Baseline
The geophonic soundscape carries rich ecological information. The spectral signature of wind through vegetation is diagnostic of leaf morphology and canopy density. Streamflow acoustics reveal hydrological regime.
Anthrophony: The Geological Force of Sound
Human-generated noise now permeates virtually every habitat on Earth. Low-frequency shipping noise in the oceans has increased by roughly 32 dB since the mid-20th century. On land, traffic noise is detectable in 83% of the contiguous United States, with only 12% of land area remaining more than 1 km from a road.
Why the trichotomy matters
The three pillars are not just descriptive: they map onto distinct frequency bands, distinct temporal signatures, and distinct ecological functions. A degraded ecosystem is often signalled by a shift in the biophony/anthrophony ratio months before visible biodiversity loss. Acoustic indices (Module 5) quantify this shift automatically across millions of recordings.