There is a lot of noise in the world. Even in a quiet room on a winter's day, a quick census of extraneous noise produces the following: car motors, occasional distant honks, plane overhead, wind swishing through trees, light-toned hum of computer (with clacking of keyboard and mouse optional), creaking chair, whispers of air pushing up partially blocked nostrils, and faint, discontented growls reaching up from my abdomen.

With all this rowdiness going on, it seems odd that radio should be based on silence. That probably needs re-phrasing: radio is based on an assumption of silence - if a noise occurs, it will have some purpose or meaning. In effect, this means two sorts of noise are permissible, speech and music; everything else, all that background clutter, is rigorously excluded. Trying to get ordinary, uncensored noise on to the radio is like trying to get past a nightclub bouncer in torn jeans and muddy DMs.

Just how much that noise means to us was glimpsed in a short, sharp feature on Radio 3 last night. In The Acoustics of Everyday Life, Derek Sugden, an acoustic engineer whose credits include Glyndebourne and Snape Maltings, talked about the sound peculiar to a number of sites: a glassed-in shopping arcade, a London street, a Hawksmoor church, a tube station, a concert hall, a wood, the seashore. His point was that uncorrupted silence, like absolute zero, never happens - noise, like water, leaks in through the tiniest cracks. The silence behind what we hear on the radio is thoroughly artificial.

Most of the time, that's all to the good - you don't want to hear Sue MacGregor, say, rustling crisp packets over the eight o'clock news. Where it is most annoying, though, is in drama. Take the new serialisation of PD James's Devices and Desires (R4, Thursday). Admittedly Dalgleish, the poet-detective, is an improbable Plod, and infects the whole affair with a plodding improbability. But James's stiffness is exacerbated by the wooden unreality of the radio world - a dinner party where nobody scrapes chairs, clinks glasses, slurps coffee; a whistling serial killer who's pitch-perfect and studio-recorded. That headless corpse in the library? It's the listener's sense of involvement.