Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars
If my calculations are good, the period for this wave in Perseus
is over 9 million years and of course we cannot hear it. So my question
is why you call it a sound as opposed to some specific astronomy-type that
would refer to such low frequency waves?
Note, this isn't exactly a B-flat - the frequency of the sound from
the black hole, when multiplied by 2**57 is close to 475 Hz, so B-flat
is the nearest note.
For more details see: http://chandra.harvard.edu/press/kits/perseus/blackhole_sound.html
Even though we can't hear it because the frequencies are too low, this
is still sound. Sound is defined as a pressure wave and this definition
isn't constrained by the limitations of human hearing. For example dogs
and bats can hear sound at much higher frequencies than we can, and elephants
much lower frequencies.