"No melodies, no harmonies, no rhythms, no bullshit."
Tom Johnson on Phill Niblock, 1979

If you stand at the window of Phill Niblock's loft space on New York's Centre Street, a huge dusty lumber room with Chinese sweatshops on all sides, and look down at the shoppers in the busy little supermarket across the way, you have to wonder: are they hearing it? Can they hear this bruising, cruising clangour behind you? Up here it easily drowns the traffic. Does a whisper of it reach them, a distant drone to puzzle at as they pass by? Tom Johnson, chronicler of downtown New York sounds in the 70s and 80s, once described stopping a more 'conventional' musician friend as he was walking past a Niblock rehearsal, and teasing him by asking him what the sound he heard was. Engine hum? A subway train passing underneath? Music? Music? His friend looked at him aghast, and hurried away.

You step away from the window, and back into the space defined by the four massive, old fashioned speakers, and the full, physical weight of this music once again assails you; the high volume microtones catfighting in mid-air, a turbulence of piercing or caressing notes. As you move inside the sound, hearing it shift and alter with your position, time feels suspended - a 20 minute piece can seem less than five. "Like walking around," as Tom Verlaine once sang, "in the ring of a bell."

"The easiest one to describe," says Niblock of his compositions, "is the first of the trombone pieces, where I worked with three octaves. I used As and sharp As: so the lower octave was 55, 57, 59 and 61 Hertz, so there were two Hertz differences at that octave. The next octave up was 110, 113, 116, and 119, so there were three Hertz difference at that octave. The octave above that was 220, 224, etcetera, so it was four Hertz. So when you combine 57 and the octave above would be one Hertz on, you get a lot of very strange combinations. What happens in the middle of the piece is it becomes - at its most complex - virtually pure distortion. It comes apart."

Comes apart? Well, yes. It sounds like a concerto for jet engine; it sounds like a music the Futurists would have sold their grandmothers to have made; it sounds (very quickly) nothing at all like the trombone, and just as far removed from the careful pages of scored numbers that describe the beginning of each piece.

How does Niblock go about producing these extraordinary soundscapes? Carefully chosen and precisely played long tones are elicited from musicians of his acquaintances, and recorded. They are then built into predetermined sonic sculptures, so that - when played at volume - harmonic interaction and acoustic phenomena occur. With or without live musicians taking part, his striking, gripping music objects are liable to produce effects that would have been unpredictable when the original scores were drawn up, and which are also quite different from performance to performance. And yet, if the end result is purely sensual, indeterminate, and almost beyond analysis, the beginning is - as Susan Stenger, flautist, Band Of Susans guitarist, and a favoured performer of Niblock's music puts it - "pretty anal". For a man whose mentors are John Cage and Morton Feldman, Niblock's processes of music making are astoundingly painstaking: when People's Heroes Of Tape Splicing finally receive their retroactive due, he'll get his medal.

The tangled story of downtown New York will probably never be clear: too many things were going on for all the facts to be put in place, for everyone to be rightfully cited for their contributions. Niblock, who is also a film maker and photographer, was put on track less by music than by the paintings of Mark Rothko, plus the early minimalist artists: Carl Andre, Sol LeWitt, Donald Judd, Robert Morris. In all his work - film, music, whatever - the idea is to remove the rhythm of conventional form. Once he had an idea of the sound he had to make - his sculpted dance of microtones - he never looked back. From 1968, it took him nearly six years to sort out the recording techniques, and get the first piece he still listens to made, but thereafter - work in mixed media aside - he's worked this single seam (though sampling and sequencing technology have speeded up the process). Before this awakening, the closest he'd come to a career in music was as a collector of jazz 78s (that's if you don't count his work, for a while, in the late 50s as court photographer to Duke Ellington).
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He's remained absent from the usual tellings of the minimalist story, maybe because his music doesn't really work on record - few systems are hi-fi enough, and when they are, their owners are frightened of the volumes he's asking to be played at. He's nonetheless pretty central to the scene, simply because this very loft space has, over 20 odd years, been host to more than 800 performance events from New York's outer edgers; ever since the night when the notorious performance artist Hermann Nitsch left the Kitchen (a more hallowed NYC art space) all covered in blood and stink and unfit for human habitation. Niblock was due to perform there the following night, but rather than cancel the show, he bussed the audience back to his loft, and realised what a great place it was to play in.

"Music gets more interesting the better the reproduction of music gets," Niblock tells me. "The CD is the beginning of what's possible in music." But if his music is a precursor of Ambient (the music that arrived to fill out the new CD format, as jazz once did the 78), and a music to be played alongside his films - beautiful, clear, unedited long-take videos of people of all cultures working with their hands - it matters most that it's loud. Eno and Aphex Twin may be outspoken fans, but this is a music that pushes itself to the forefront of your attention; music in a charged space to steal your sense of time from you; music to break down the unitary notion of the 'note' to harmonic splinters and build it all up again.

Though there is a solid tradition of New York drone-guitar, Verlaine himself never really heard the ring of the bell; certainly not to walk around in. By inference, there's always been Metal Machine Music, Lou Reed's single, fabulous stab at proving John Cale wasn't the only Velvet who knew where La Monte Young was at. In plainer, present sight, there's Band Of Susans: they know their avant garde history; they've participated in some of it. And of course there's the Youth: Niblock begat Branca begat Sonic Youth. So perhaps you could say he invented both Grunge and Ambient. He doesn't say so. He hardly says anything that isn't cheerful self-deprecation.

For example, he insists he's no expert in acoustic phenomena. "I have such a lousy ear: I like the sound of my own music very much, but I don't think it's an intellectualised hearing process." Asked if he could do a blindfold test on his own pieces, he gives a sudden, sly, Buddha-like grin, and says no: everything from speaker system to volume to where you are in the room, produces too many variables in each piece. This may be essentially recorded music, but what transpires once you set it running can be new every time.

If it's too loud, you're too old. Niblock is 60 this year. "If you play the flute piece too loud, you'd go crazy. It would really get into your head. And of course, in concert, women generally have more trouble than men with the high frequency stuff. And rock 'n' roll musicians have less trouble because they're all deaf anyway."