The noise didn't wake me up: I just woke up, at about 3 a.m., and heard it. It was one of those noises that are felt as well as just heard: a very low-pitched hum. I couldn't make out where it was coming from so, researcher by nature, I got up to investigate.
After walking all round the house I drew a complete blank: there was no obvious direction, and no obvious source. Just the same level of hum no matter where I stood, except it was slightly louder near the windows.
But when I stepped outside, it was gone.
Thinking it might be something electrical but invisible (there's quite a lot of that in our house, all installed by the previous owners and roundly cursed by me), I hit the mains. Still there. And next door was uninhabited at the time, so it couldn't have been anything there either.
I had to admit defeat.
Until I saw an article about a car in the USA which used noise cancellation technology to make the low frequency of its idling engine bearable for the driver. It turns out that this frequency (a few hundred rpm) is not too far away from the resonant frequency of crucial parts of the human body. Like the main artery, or even our brainwaves. Which probably explains why the sound of idling cars is so irritating.
But cars are not usually idling in traffic-jams at three in the morning: they are far more likely to be on some or other mission of mercy on our city's ring road, their engines turning over at some 2000 rpm: just at the point where "vibrations" become "sound".
Those of us who can remember offhand the speed of sound in air can work out the size of a "lump" of air that will resonate with these car engines. Resonance is a strange thing: it takes very little wave power to get something resonating, if it happens to be the right size and shape, and the results can be dramatic. Outdoor air will not resonate, but will quietly carry the energy to the air inside something of the right size, that will oblige.
Something, it turns out in this case, of four and a half metres across. About the size of a typical living-room.
As if that's not enough, windows will also resonate at similar frequencies.