The story I am about to tell you is true; it had serious consequences, the cause was little understood by most, ridiculed by the know it all who wondered why he got wet if he stood in the rain.
Shortly after WW2, Highway 401 was built through the Fraser Valley farming belt. A farmer close beside the highway decided to have an implement shed built, using 8x8x16 concrete blocks, ideal for the building which was about 32 feet long with the ends about 20 feet; the front facing the highway was open, to be a series of sliding doors.
A crew of several bricklayers had the blocks laid almost to a height of 12 feet when suddenly the back wall collapsed killing one bricklayer. Nobody knew how such a tragic thing could happen. These bricklayers were qualified tradesmen; how could such an unconscionable accident happen? The mortar was properly mixed. I doubt if anyone came up with the right answer. I never heard.
Sound has tremendous energy: when I was playing around with radios, in the thirties when parts were cheap but I still had no money to buy any, I came across a very interesting article where General Electric built several large speakers into a wall, wired them all in phase, and then fed a low frequency sine wave to these speakers. At a very low frequency they could knock the opposite end out of the building. A big piece of pie no matter how you cut it.
First visualize the setting; the 2 west lanes of 401 were not over a hundred feet from the opening of the building. A semi would pull out and pass another; normal traffic, except when one passed a situation might be created.
Let's say for example the trucks were the same make; if they were both going 60 MPH the diesels theoretically would be running at the same speed where the sound of their exhaust would be identical, say 50 hertz per second.
Then the truck following pulls out to pass; the frequency of his exhaust will increase to, say 53-hertz/sec. Now we have a phenomenon that's an integral component of any two frequencies, the difference and the sum of the two. Using the hypothetical muffler sounds of 50 and 53, we would have 3 hertz and 103 hertz.
The 103 would do no harm to the building, but the 3-hertz could start that wall rocking or, it could create a standing wave where a reflected sound meets one coming in which can create enough pressure to break the "barely curing" mortar bond and topple the whole wall.
This is only my theory of why the building collapsed. The low frequency might have been anywhere in the very low range, well below what the human ear could detect.
