Listened to on a conventional two-speaker set-up, the first track with the slowest speed lasts 15 minutes and provides a seemingly clean, deep extended tone. Like a bed sheet under a microscope, however, concentration reveals a range of impurities – some, like the gilt-edged tinnitus that rides the drone, caused by the dragging down of very high frequencies by the reduced speed of playback, are immediately obvious, while others like the natural rhythms enjoyed by the colliding overtones amidst the two stereo channels can take a bit of focussing to perceive strongly. The ensuing three tracks have similar effects but at twice the speed and, consequently, being an octave or two higher than each preceding track, the experience of the impurities alters (the tinnitus and the pulsations become less separate but without totally reincorporating themselves into the tone).

However, simultaneous playback of the forwards and backwards versions on two stereo systems is where these impurities really begin to sing. Radigue’s original intention was to provide a sound that could be extended “ad libitum”, and Important’s reissue certainly allows the listener a rare opportunity to improvise combinations for playback, any of which will form its own patterns in a vector of tonal feedback. And, by persuading the listener to rearrange a room for the sake of exploring a monotonous tone, it becomes a highly effective sharpener of the auditory system – just a shame the recordings aren’t a little longer for a more immersive journey.