It is an incredible thing to think that Steve Reich's discovering phase-shifting was more or less an accident: upon playing copies of the same loop of sound on two different machines, he noticed that the playback speeds differed slightly, resulting in the loops slowly going in and out of synch. This Late Night Prom showcased what is widely regarded as Reich's 'opus one', It's Gonna Rain, which uses this pioneering phasing technique – one that would feature in much of Reich's output – and a much later work, The Desert Music, which, like It's Gonna Rain, uses vocal snippets and repeated phrases, but instead employs live musicians and createst a vastly different effect. Both works demonstrate the power of emotive language, whether in and of itself or as a basis for musical composition.

The journey of It's Gonna Rain began in 1964 at Union Square, San Francisco, with Reich's recording the words of Brother Walter, an African American Pentecostal preacher, who was railing about how Noah had been derided as foolish for building the ark, and relating the Biblical story to the recent Cuban Missile Crisis. In the first part, Reich extracts the words "it's gonna rain" from the speech and layers and repeats them, gradually fragmenting the soundbites and looping them over and over at different speeds. The effect is mesmerising, sounding like something fresh out of Ibiza, and was particularly effective in the silent 360 degrees of the Royal Albert Hall. A work solely for magnetic tape, there were no musicians on stage; all we had was the sound reverberating from the giant speakers high above us.

The words and fragments of words seemed to evolve, almost imperceptibly, into new words: "it's gonna rain" became, to my ears, "leave the car", "the car lane", and – perhaps because I subconsciously had work on the brain – "lay vicar".  The second half of It's Gonna Rain is more frantic; the temperature of Brother Walter's preaching has increased, and Reich uses the phrase "Noah couldn't open the door – they cried – it had been sealed – but sure 'nough – Hallelujah!" as the lynchpin of this part. Mimicking Brother Walter's crescendo in tone and drama, the loops build up into a frenzied cacophony not unlike speaking in tongues; a cacophony that commanded the listener to maintain focus, and to try and pick out any remaining recognisable elements of speech. Intense is not a strong enough word to describe the experience. 20 years after this seminal work, Reich created The Desert Music, taking words from the poems of William Carlos Williams (one of whose poetry collections provided the title for this piece) and fashioning a five-movement work in arch form. This piece, too, is not without its politics: it is from Williams' later works, written after the bombings of Hiroshima and Nagasaki, that Reich selected the texts.

This particular arrangement was written for chamber orchestra and amplified voices, provided by Endymion and the BBC Singers respectively.  Pulsing chords, with gently undulating volume levels, underline each movement, providing a simple, yet ever-mutating aural experience. As with many other of Reich's works, this process begins with the percussion instruments, whose players' endurance skills were richly tested over the 45 minutes of the piece. The strings and woodwind added to the texture and timbre by playing fragments of music, over which the singers layered the poetry. The music was by turns poignant and humorous – poignant because, with lines such as "Begin... for you cannot... take your song, which drives all things out of mind, with you to the other world", and "Man has survived hitherto because he was too ignorant to know how to realise his wishes... Now that he can... he must either change them or perish", Reich acknowledges the profound effect that the nuclear bombs had on Williams and the wider world; humorous because so much of the poetry seems to reflect Reich's compositionional style, and vice versa ("it is the relation of a flute note to a drum...The mind is listening"; and, best of all, "It is a principle of music to repeat the theme. Repeat and repeat again, as the pace mounts" as the music did exactly that).

Although repetition is a key element in Reich's compositions, The Desert Music is not in any sense easy to perform. It is extremely demanding, both rhythmically and, especially for the unflappable percussionists, physically. The ensemble was tight throughout, thanks to an on-the-ball, super-focused David Hill, and whilst the men of the BBC Singers sounded a little less delicate than might have been desired, it was an extremely satisfying live encounter with Reich's music.