The Sound of the Underground
Eco-acoustics in the Silk Wood Community woodland
If you’re old enough to remember the hit song by Girls Aloud, then you’re forgiven for thinking this article may be about hair blowing in the wind and pleather outfits worn in a dystopian future. Apologies if it doesn’t live up to expectations, but it features a subject can be equally captivating – eco-acoustics.
Eco-acoustics, or acoustic ecology, is the study of sound in the environment – including human generated noise (anthrophony), sounds generated by nature (biophony), or even sounds of the earth such as wind, waves, and geological movements (geophony).
It’s an exciting, relatively new field of study, and is used to monitor bat species, or track whale vocalisation, judge the health of a coral reef, or even see what effect high frequency noises generated by nature have on our wellbeing. You may use the bird-song ID app Merlin – this uses sounds and algorithms to identify what birds are singing nearby – and is a great example of how we can use sound to reveal what is nearby without being able to see it.
One place we have little knowledge of is the amazing world beneath our feet – fungi, worms and invertebrates, moles, and billions of microbes all make a home in our soil, and this is where the next generation of acoustic ecologists are pointing their microphones. Some studies have revealed that soils are incredibly noisy places, and much has been insinuated about these noises being the sound of the vast ecosystem that resides in it.
I was lucky enough to be joined by Robbie Sidhu from the University of the West of England (and FoWA member) at the Silk Wood community woodland. Robbie’s studying a masters in eco-acoustics and is specifically interested in soils. “There’s been a lot of press recently about initial findings of soil acoustics” Robbie explained, “but we’re still not sure if the sounds we’re hearing are of what lives there – that’s why I want to see what soil sounds like on its own, so we can have a baseline dataset to start with”.
His work includes recording the sounds of completely sterilised soils in sound-proofed chambers – this way he can record just the noise of soil. He then tests what it sounds like with water added, or how different soil types or structures might make different noises. This will hopefully allow scientists who record soils to remove the background noise, and be left with the sounds of what is living there.
“It’s such early days” he explains “that any studies we do will generate useful data – it’s exciting to be contributing new research that might assist in what comes next”.
This could be algorithms that know the difference between earthworm species, tell you what fungi are present and how they’re connected, or could tell you the health of the soil by simply listening to it. This may help vastly improve the productivity of soils in agriculture, or help inform how to restore degraded soils - and understanding the fungal species that make up woodland soils could help in tree planting schemes to increase survival rates.
Beyond his lab tests, Robbie has also been sampling sounds from various habitats to try to distinguish if he can use this data to tell us about a soil or ecosystem's health. This included samples from a rewilding site in Berkshire, a RSPB owned bog, and the summit of Cadair Idris in the Eryri National Park.
“What’s surprised me the most” he explains, “is how far sounds from vegetation permeate the soil - if you were to place a microphone in the ground a meter from a tree, you would be able to hear someone tapping on its trunk as well as the branches moving in the wind.”
One habitat we were keen to test was at the Silk Wood Community Planting Project - a landmark woodland creation scheme here at Westonbirt Arboretum, where 9,000 trees are being planted by local community groups in an area devastated by Ash Dieback. We headed into the sea of saplings with his equipment - a sound recorder, headphones, equaliser and contact microphones attached to two spatulas. “It’s not the most high-tech setup, but it works” he says as I look at the two metal prongs being pushed into the earth.
He wasn’t wrong, as soon as I put on the headphones I was immediately transformed into a world of chirps, clicks, swishes and taps – it’s incredibly noisy under there! It’s also amazing to be shut off from the sounds elsewhere in the woodland and have your focus solely on what’s going on beneath your feet – trying (in vain) to attribute these sounds to what you imagine is in the depths of the soil.
We ended testing a few different locations – one which was unplanted and still quite bare of ground vegetation – which was rather quiet. Then a fresh molehill where I was sure I could hear scratching, as if the sound of these elusive mammals was travelling along their lengthy network of tunnels.
We tried within the planting site itself where there seemed to be plenty of activity, as well as right next to the base of a mature oak. You could indeed hear the branches swaying, but also a gushing sound of what could be water being transported from the earth, up into the roots and throughout the tree’s capillaries – bringing life to the fresh spring leaves.
An interesting spot we listened to was directly near an area left unmanaged where there is an established badger sett – we could certainly hear knocking and scratching sounds, and what sounded like thudding against the microphone – my mind sprung straight to a family of badgers jostling for space in their den, or two young cubs playfighting.
Lastly, we sampled right next to the future planting site, where the grass and brambles had grown and become established – this was the noisiest spot, and Robbie put this down to the many blades of grass being blow in in the wind, and transferring that noise through their stems and roots into the ground.
It was an incredibly interesting morning’s listening, which gave me a unique insight into the noisy world of our soil. There’s also potential to use the sounds of Silk Wood in other ways, be it music or art – and to monitor whether over time, the soil gets louder as the trees grow, the fungi spreads, and more life moves in.
This is the true sound of the underground – sorry Girls Aloud fans.
Read our other blogs about the Silk Wood Community Planting Project here...
