Turn off as many sources of local noise and listen to the city. Swirl of birdspeak, mulch of clamour, the groan and heavy squeak of the tram turning, various snippets of acceleration and braking, bursts of machine groan. And occasionally, the lonely, wearied sound of an aircraft ploughs the air distantly, sounding more like a tractor than a sleek jet, the aural experience of its passing on the ground nothing like the smooth rushroar of its flight when you are aboard. A plane’s passage is always solitary, it so rarely has a compatriot within the range of hearing. Very rarely are two jets above the city at the same time, sometimes a light plane can be detected in another sector of the sky and at another altitude, but the jet, even the helicopter is an occasional texture; the overwhelming mass of the everyday sound collage is made up by terrestrial vehicles, thousands of them, stopping and starting, hushing and sighing and screeching. The sky accounts for so little of the soundscape; this is what we have been used to for all time, and it is about to change.