What do trees sound like - from the inside? Pascal Wysemeets the artist whose high-tech 'ear trumpet' lets us listen in on their secret soundtrack

THE TREE IS bearing some very strange fruit: pairs of headphones have sprouted from its branches and are dangling invitingly at head height. Put on a pair and you'll hear something surprising - the secret soundtrack of trees.

The inner life of trees has been a lifelong obsession for the artist Alex Metcalf. "I just knew there had to be something going on in there," he says. In the course of a two-year MA in design at the Royal College of Art, in London, he found a way to indulge his curiosity, designing a device that can listen "inside" a tree. His first installation - in Kensington Gardens in London, as part of the 2007 RCA degree show - made it clear he had captured something valuable.

"It was amazing. All kinds of people would just stop by and listen, including people who said they were afraid to go into the exhibition because it was overpowering, but loved this quirky thing going on outdoors. And that was completely what I wanted it to be, as open as possible."

So what does a tree sound like? When children hear it for the first time, says Metcalf, they tend to think it is thunder, or a motorbike. This is the structural sound of a tree in motion. "But then, as you keep listening, you can pick out this clicking sound, which is slightly hidden. Once you hear it, you can't ignore it."

The clicking sound is the tree drinking. As water moves up a tree trunk and enters a cell, the air gets displaced, causing a "pop".

Metcalf's eavesdropping is not entirely new, but it is a very specialised area. Scientists have listened to trees to gauge how stressed or water-deprived they are. (The popping indicates air where there should be water.) Metcalf's approach, however, is original. "The technology for this is usually invasive," he explains. "You bore into the tree and take away a section, then seal in a listening device. The thing about my device is that you don't have to cause any damage, and you can listen to any tree, anywhere, any time - plus you can do it long term. Cutting a hole in a tree means you are wounding and infecting it, which will affect the recording." What Metcalf refers to as "the listening device" is a beautiful object he designed and built - a polished metal cone, based on an old-fashioned ear trumpet but packed with technology that amplifies tiny sounds by up to 33 times, then filters out unwanted ambient noise. Depending on the nature of his installation, you might be listening to a tree "live", or to recordings Metcalf has made earlier. As the weather changes, so the water uptake changes - and the sound.

"What I love is that two- to five-year-olds get really addicted, running from headphone to headphone. This is just a wild guess but perhaps it has similar frequencies to what they heard in the womb. They're not relating it to the tree or anything, just pure sound. Then you get slightly older kids, seven to 10, dancing around to it and finding the rhythms in there, and then amazingly, 15- to 17-year-old dudes, coming along with their bling, and really appreciating the connection between nature and technology."

Metcalf was a sailing teacher in Cornwall, south-west England, for 10 years before attending the RCA, and his parents both worked as lecturers. He uses the word "educational" a lot, and it is an aspiration he has for the project.

"Most people don't look at a tree and think about the water," he says. "You might listen to the wind through the leaves or look at the amazing structure, the age, the grandness, but you miss how fluid it is. The key thing is for it not to be forced on people - that's why I kept it very simple, without too many preconceptions. I don't have an ecological message. I just want the general public to access something only a handful of scientists have heard."

Metcalf's hanging headphones were part of Atmospheres 2, a season of performances and events recently exploring the sounds of the natural world and the second collaboration between arts organisation Touch and the Museum of Garden History in London.

Metcalf has set up an installation for Kew Gardens in London, which will run until September, and was asked to present his work to delegates at the forthcoming Canadian urban forestry conference in Alberta. Like the work of Touch artist Chris Watson, Metcalf is taking a microphone to the places we wouldn't normally get to hear.

"My sense of the world, in terms of sound has changed through doing this," he says. "I am busy all day long in my workshop, planning and making these installations. I'm full-time listening to trees. What an unbelievably privileged position to be in." - Guardian service

• The Tree Listening Installation is open to the public until Sept 30 at Kew Gardens, London.  www.alexmetcalf.co.uk