When I told people that I was working on a book about sound and noise I was quite often asked if a tree makes a sound when it falls in a forest but there is no one there to hear it? The short answer to this is yes: a trunk crashing down sends vibrations through the air whether or not anyone is listening. That''s what sound is. But there is also a way in which the short answer is no, because sound as we usually think of it is an experience of a sentient being (and we tend to assume that trees and rocks are not sentient, or at least not in that way). If that''s all you wanted to know, then you can put this book down now. But while these short answers may be true they are also unsatisfactory because there is, I think, often something else lurking behind the question concerning the listener''s relation to the universe as represented for them by the forest. That unspoken (and perhaps unconscious) thought, I''d suggest, is something like, will the world really go on without me? It can be hard to get one''s head around the idea that the world will continue without the awareness to which we as individuals so often cling. As Alexander von Humboldt wrote in 1800, ''This aspect of animated nature, in which man is nothing, has something in it strange and sad.'' Some sounds can be a kind of revelation to those who hear them, and sometimes the experience can be deeply unsettling.

In Don DeLillo''s novel White Noise, an air-raid siren in a residential neighbourhood that has been mute for a decade or more shrieks back into life, like a sonic monster, ''a territorial squawk from out of the Mesozoic. A parrot carnivore with a DC- 9 wingspan.'' And when, in his exploration of the world of those preparing for apocalypse, the writer Mark O''Connell visits a former US Air Force bunker that is being repurposed for end- of-the-world preppers, the sound of its great doors closing is like nothing he has ever heard: ''an overwhelmingly loud and deep detonation, the obliteration of the possibility of any sound but itself''. In a poem by W. S. Merwin, a foghorn becomes a ''throat'' that ''does not call to anything human / But to something men had forgotten / That stirs under fog''. And in Apichatpong Weerasethakul''s film Memoria, an extremely loud noise heard only by the protagonist foretells (and maybe causes) a descent, or possibly an ascent, into a strange dimension of existence - or annihilation. But revelations in sound can also be comforting and life-expanding, bringing reassurance and beauty in the wide view.

This is expressed in comic form by Roald Dahl''s Big Friendly Giant, who ''is hearing the little ants chittering to each other as they scuddle around in the soil [and] is sometimes hearing faraway music coming from the stars in the sky''. It takes a mysterious, transcendental form in Jorge Luis Borges''s short story ''The Aleph'', where the faithful who gather at the great mosque of Amr in Cairo know that the hum of the entire universe can be heard by placing one''s ear against one of the stone pillars in its central courtyard. The physician and essayist Lewis Thomas took pleasure in imagining all the non-human sounds of the Earth together: ''If we could listen to them all at once, fully orchestrated, in their immense ensemble,'' he writes in ''The Music of This Sphere'', ''we might become aware of the counterpoint, the balance of tones and timbres and harmonics, the sonorities.'' And in one of his ''Love Letters to the Earth'', the Zen monk Thích Nha^ t Hanh writes that ''humanity has great composers, but how can our music compare to your celestial harmony with the sun and planets - or to the sound of the rising tide?''. We live in times in which more is being destroyed than is being created. (Extinction rates of non-human forms of life, for example, are much higher now than at any time in Earth''s history, including during mass extinction events millions of years ago.) ''Modernity stands at risk of no longer hearing the world and, for this very reason, losing its sense of itself,'' writes the sociologist Hartmut Rosa. ''Our greatest fear should perhaps be that we have forgotten how to listen to the living Earth,'' adds the biologist David George Haskell, who documents a catastrophic loss of sonic diversity and richness worldwide.

And it is precisely because of this that it has never been more important to pay attention. Building on pioneering work a generation ago by the composer R. Murray Schafer and others, ecologists today are increasingly recording ''soundscapes'' on land and in the ocean over the seasons and years as a means of assessing the vibrancy and health of ecosystems. By enabling us to listen more carefully and deeply, new technology can help us to limit and even reverse some of the damage that has been done.