“Dr. Andrew Fabian and his colleagues at the Institute of Astronomy in Cambridge, England made their discovery using NASA’s Chandra X-ray Observatory, an orbiting X-ray telescope that sees the Universe in X-ray light just as the Hubble Space Telescope sees it in visible light.
Using the piano keyboard’s middle C note as a reference point for the middle of the piano key music range, Fabian’s team determined the note is a B -flat. On a piano, the B-flat nearest middle C is located midway between 1/8th and 2/8th of an octave away. In musical terminology, this B flat is 1-1/2 steps from middle C.”
Source : https://www.nasa.gov/centers/goddard/universe/black_hole_sound.html