Mark, I know that you were looking for a specific answer to the question of how low that we can actually hear, but as Alan implied, unfortunately you ain't gonna get it. You should look at
these equal loudness graphs which don't appear to be Fletcher-Munson(the term is often applied to them generically)but rather from the work that Robinson and Dadson did in the 1950s. Note for example that to get the same subjective loudness as say 80 phons(80dB at 1,000Hz, a comfortably loud level)would require about 112dB at 20Hz. Even more recent investigations indicate that the previous results at very low frequencies may be optimistic. For example, the
paper by Suzuki and Takashima summarizes much of this. Note in fig.1 on p. 919 that the latest results indicate even higher levels are needed at very low frequencies for audibility. I suppose that the bottom line may be that your walls will collapse before you'd actually hear below 20Hz.
