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A healthy place sounds complex
Three kinds of sound
This is a core principle of ecoacoustics, a field established in 2014 to gauge ecosystem health through soundscapes. It listens to three kinds of sound: biophony, the sounds of animals; geophony, the sounds of wind, rain, and waves; and anthropophony, the sounds of humans and their activities.
A Bornean dawn chorus
What marks a healthy habitat is the diversity of sound, not just the volume. The dawn chorus in an intact Bornean tropical forest layers birdsong with the whoops of a troop of gibbons swinging through overhead.
Cities versus forests
Cities are incredibly noisy, but the noise is the harsh, more monotonous sound of car engines and other machines, with little sound from living things, and cities are generally not ecologically healthy. Soundscapes dominated by human-made sounds have been linked to poor mental health in people, and the opposite has been found for natural sounds like birdsong or forest soundscapes.
How do scientists score a soundscape's complexity?
Researchers summarize recordings with acoustic indices, which turn hours of audio into numbers. The Acoustic Complexity Index, proposed by Almo Farina and colleagues in 2011, measures how much sound intensity varies over time within each frequency band. Birdsong and insect calls make that variation high, while a steady engine hum makes it low.
Why do forest animals call at different pitches?
Bernie Krause, the musician and recordist who coined the terms biophony and geophony, proposed the acoustic niche hypothesis. It says species settle into their own slices of frequency and time, much as instruments in an orchestra occupy different ranges. That spacing keeps their calls from drowning each other out. Dawn choruses in old, intact habitats are often cited as an example.
How does city noise change birdsong?
A 2003 study by Hans Slabbekoorn and Margriet Peet in Nature found that great tits in noisy parts of Leiden sang at a higher pitch than great tits in quieter areas. Traffic noise sits at low frequencies, so higher notes are easier to hear over it. Later studies have found changes in other bird species living in cities as well.
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