I was down at the snow-covered beach photographing the sea ice for another of my blogs. Out across the sea it sounded as if several large manufacturing workshops were working away. Probably an inversion layer but what could be making so much "machinery" noise out of doors to carry for so many miles across the sea? The nearest land is about 10 miles away. With no guarantee that several hundred people are beavering away out of doors in sub zero conditions with heavy machinery. Absolutely no sign of any ships or boats. Very odd.
Later, I was descending the stairs in a four story office block. Halfway down I was subjected to a VLF vibration of immense proportions. A very uncomfortable experience indeed! As I exited the building I spotted a large defence helicopter some way off. Could this have excited the incredibly heavy air vibrations I experienced? I can't think of anything else which would have enough acoustic energy. There was no sign of any building work in the neighbourhood. The sound was clearly located in just one section of the middle run of stairs. Above and below it was quite inaudible.
Case three is ongoing. A neighbour, who lives two hundred yards away, brings his lorry/truck home with him. The house shakes to the sound of the diesel motor even at this considerable distance. He used to start the engine at 4am and leave it running for 20 minutes to warm up. Lately he has been leaving at more sociable hours. Though the effect is always the same. Even in the closed bathroom the engine sounds incredibly loud. Or rather the frequency with which the room vibrates in sympathy is very loud. It doesn't actually sound like an engine. More of a heavy beating in the upper infrasonics. More felt than heard.