The lowest note in the known universe is caused by the rumbling of a black hole in the Perseus galaxy. It's 57 octaves below middle C.

Middle C has a frequency of about 261.6 Hz.

Lowering by one octave halves the frequency.

261.6 Hz / 257 ~= 1.815*10-15 Hz

That’s right, 1.815 fHz (femtohertz)

Since Hz is frequency in cycles per second, this is corresponds to a period of about 550899036987216 seconds per cycle, or nearly 17.5 million years.

(via)