some frequencies below 20 Hz (say, 16 Hz) can be perceived as audible sound by about 6% of men and 18% of women, up to distances of 40 km in certain atmospheric conditions (a stable layer of air next to the ground). As a result, the lady in question in the memoir must switch the radio on in order to be able to sleep. http://www.epaw.org/documents.php?article=ns41 You can hear very low-frequency sounds at levels above 88–100 dB down to a few cycles per second, but you can't get any tonal information out of it below about 20Hz—it mostly just feels like beating pressure waves. And like any other sound, if presented at levels above 140 dB, it is going to cause pain. But the primary effects of infrasound are not on your ears but on the rest of your body. http://www.popsci.com/technology/article/2012-11/acoustic-weapons-book-excerpt