The encroaching silence

Listening to the health of an ecosystem

THE FIRST PLACE I recorded a forest soundscape was deep in Papua New Guinea’s Adelbert Mountains. Three days’ walk from the nearest road, this is what used to be called ‘virgin forest’, but is now referred to less evocatively, if more accurately, as ‘primary forest’. As a train-commuting, city-dwelling outsider, I don’t so much walk through the Adelbert forests as weave, scramble and slide, trying, and mostly failing, to keep up with Iwarame community rangers as we pepper their forest with microphones strapped to trees.

These forests are noisy. You feel enveloped in sound. Buzzes, clicks, whistles, screeches and whoops. Sound peaks at the day’s margins: the dawn and dusk choruses.

‘Noisy’ is not the adjective most people associate with untrammelled nature. In surveys asking people what they value most about nature, respondents commonly cite a quietness, stillness, or similar.

Part of the reason we associate nature with quietness is the time we’re typically out in nature: the middle portion of the day, generally the quietest time. Part of it is how we hear. Humans are experts at filtering out sound we don’t perceive as relevant: sound that is not about us communicating with each other and would not alert us to danger. This filtering is heightened when we’re in a foreign environment like a dense forest. When I listen to these forest recordings from the safety of my office, headphones on, eyes closed, there is so much more sound than I remember.

But part of the association between nature and quietness is because we often experience environments that have been heavily impacted and degraded, even if it’s hard for us to tell by looking.


ENVIRONMENTALISTS AND WRITERS have a history of associating absence of sound with environmental degradation, none more famous or impactful than Rachel Carson. Most people know Carson as the author of Silent Spring (Houghton Mifflin, 1962), her exposition on the effect of agricultural pesticides on the reproduction of songbirds and the consequent loss of sound in the environment. There was no shortage of science supporting this thesis, but it was Rachel Caron’s prose and the compelling notion that the world would be quieter – and that this would be a loss – that really made an impact. The creation of the United States Environmental Protection Agency and all the nature protection it has provided since can be traced to the impact of Carson’s bestseller – an incredible example of what writing about nature can achieve.

By the time Rachel Carson published Silent Spring in 1962, she was a full-time nature writer. Perhaps fewer people remember that before her writing career, Carson was a marine biologist. Her second published book, also a bestseller at the time, was the Sea Around Us (OUP, 1951). Following its publication, Carson was frequently asked to give talks, but she was such a reluctant public speaker that her agent suggested she turn down speaking events and focus on writing. Against this counsel, she agreed to give a luncheon talk in front of 1,500 people at the Hotel Astor in New York. At the talk, she played some recordings from under the sea, complete with shrimps cracking, fish feeding, dolphins squeaking and whales singing. I imagine the effect must have been very much like a modern TED talk – it was almost certainly the first time most of the audience had heard these sounds.

That was in 1951, and we’re just now learning how much the presence or absence of sound in the environment can tell us about its health.


THE HUMAN EAR can hear sounds from roughly twenty hertz to twenty kilohertz (20,000 hertz). But most of what we hear is between one and four kilohertz. That’s because these are the frequencies humans talk at. Our ears are very sensitive to sound in this frequency range; in that sweet spot of around three kilohertz we can hear even very quiet sounds. Outside this range, sounds effectively need to be louder for us to hear them clearly.

Most of us have found it hard to hear our dining partners in a busy restaurant. Maybe you’ve experienced the awful challenge of listening to a speaker at an event when people around you are talking. These difficulties occur largely because we’re all speaking at roughly the same frequency, competing for acoustic bandwidth.

The same thing happens with animals in the forest. This is one theory as to why animals vocalise at so many different frequencies. Imagine it like this: these animals are using different parts of the acoustic frequency spectrum so they can communicate with each other clearly through the din of other creatures making noise. This observation gives us a way to link sound to the health of the environment.

We can think of the sum of all the animals that are vocalising – the birds, the insects, the amphibians, mammals like bats or primates – as the soundscape of a place: the acoustic equivalent of a landscape. The soundscape changes throughout the day in a regular fashion. There tends to be lots of animals making noise in the morning and evening, fewer in the middle of the day, and then a different cohort of insects, frogs and bats at night. We can record this sound, and then at any point of the day can see whether a particular frequency band – for example, the five to six kilohertz range – has sound in it.

A healthy environment with its full complement of species will have a rich and saturated soundscape. Tropical forests contain more species than any other environment on the planet; they cover just 12 per cent of the land’s surface, but are estimated to hold more than half of all species on Earth. In a tropical forest, a saturated soundscape includes sound in most of the acoustic frequency bands, especially around dawn and dusk. As landscapes are disturbed (trees cleared, creeks drying or invasive species taking over) some species will be lost from the landscape, and with them goes sound at their particular frequency. A soundscape in a degraded environment is conspicuously less saturated with sound. Instead, the soundscape is punctuated with the gaps of missing species.

When we analyse forest soundscapes in this way – from Papua New Guinea, to Borneo, to Myanmar – we see the signature of anthropogenic impacts in sound. In echoes of Rachel Carson’s warning, these places are becoming quieter, especially in that morning period of peak acoustic activity, the dawn chorus. The silent dawn, if you like.


SOUNDSCAPES AREN’T JUST a way to diligently record the defaunation of our natural world; they are a tool in our quest to do something about the loss of species. The recordings we’ve made in the forests of Borneo are helping to determine which approaches to extracting timber have the least impact on biodiversity. The raucous soundscapes we’ve recorded in Papua New Guinea demonstrate the effectiveness of community-based forest protection happening there. And it’s gratifying to see sound returning to those places we’re helping to restore, like Burmese forests ravaged in the quest for teak.

I can trace a small, but direct, connection to Rachel Carson, as she generously donated royalties from Silent Spring to the organisation I work for: The Nature Conservancy. She also helped establish and oversee our work in the US state of Maine. I like to think that she would have been pleased to see sound becoming such a powerful and effective way of understanding the impacts – good and bad – that we are having on the environment.



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