I may be biased, but I can tell you that sub-20Hz can be VERY audible, if it's really loud. It just has to be 60dB louder than higher frequencies near the middle of the spectrum to produce equal loudness sensation. However, these frequencies can be dangerous in contractor-built homes and I don't recommend producing anything over 120dB in a residence, unless you like to redo all of the sheetrock on your ceilings, which will promptly come disattached and fall onto the furniture/floor/etc. In addition, don't do this in a neighborhood where there are other homes nearby. The neighbors will arrange a lynching most likely if you do, especially if your vibratory experiments result in knocking expensive glassware off of shelves in your neighbor's home.