Brian Greene on The Music Instinct (PBS) says there is a black hole that exhibits a frequency 57 octaves below what humans can hear. Hmmm. One octave is a power of two, so if the lower limit of human hearing is 20 Hz, the frequency he describes is 2^-56 Hz. This is 10^-17 Hz. So one cycle lasts 10¹⁷ seconds, or 3 x 10¹⁰ years, which is roughly the age of the universe. Go figure.