The deepest know note in the universe is B-flat, 57 octaves below middle C; so say astronomers observing pressure waves moving through gaseous clouds surrounding a black hole 250 million light years away. The waves show up as concentric rings of x-rays emitted by the colliding particles. A new pressure wave is emitted every 10 million years.
The 53 hours of Chandra observations revealed a note that is more than a million billiontimes deeper than what you can hear.