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Q&A: Galaxies, Galaxy Clusters, AGN, and Quasars

Q:
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?

A:
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.

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