The APU is a small gas turbine engine which is normally located in the tail cone of the aircraft but, in some cases, is located in an engine nacelle or in the wheel well. The APU can be started utilizing only the aircraft batteries and, once running, will provide electrical power to aircraft systems as well as bleed air for air conditioning and for engine start. Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircrat.
The sound track from an engine test was extracted from: YouTube video
A Fourier transform was applied to the sound time history. The Y-axis is unscaled sound pressure. The steady-state portion yielded a spectral line near 700 Hz which corresponds to the rotor speed of 42,000 RPM, Harmonics are also present at 3X & 6X.
– Tom Irvine