"killer octave:

[Newton Hunt]

The Killer Octave is most often in the area of the fifth octave and
sometimes a little high, depending upon the piano.  This octave is
often weaker than the others and often also lacks a tone color of it's
neighbor octaves.

Look in the actives at http://www.ptg.org/archive/pianotech.php/
 and search for "impedance".

		Newton