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It seems that the most common sources of infrasound are earthquake, tsunami, volcano eruption, etc, which are all large scale behaviors.

Why is it difficult for a human to generate infrasound?

Is it because that the infrasound has long wavelength, which is much bigger than the size of a human? Presumably, any action can generate some infrasound. The problem is just that the low frequency component is negligibly small, right?

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    $\begingroup$ It depends of course on what frequencies (wavelengths) you are talking about, but wind is a common source and corridors can be good resonator cavities for that. $\endgroup$ – Pieter Dec 24 '18 at 1:12
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To efficiently radiate sound waves from a vibrating source in air, the physical dimensions of the radiator should be approximately the same length as the wavelength of the sound wave. So, for example, a 1000Hz source (having a wavelength of about one foot) is well-coupled to air if the source is about a foot across, as it would be for a loudspeaker of 12" diameter.

A 100Hz sound source should ideally be ten feet across, and a 10Hz source would need to be 100 feet across to radiate well. You can see that ultralow sound frequencies require gigantic radiators, which is why it's hard to build radiators that function with reasonable efficiency in that range.

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