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This paper introduces an experimental study on sub-harmonic friction excitations and rotor squeals. Experiments confirm that low-frequency friction excitation can trigger a stationary rotor to squeal at much higher frequency (either at or near a rotor natural frequency). The resulting squeal frequency is always a harmonic of the low-frequency excitation. Multiple squeal frequencies can also be excited at the same time, by a single sub-harmonic friction excitation. Those frequencies, when co-existing, also satisfy harmonic relationships (either harmonic or ultra-subharmonic) amongst themselves.