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The reflection at a stress-free boundary of the third harmonic of an ultrasonic wave has been observed. A maximum in the amplitude of the third harmonic is observed after four reflections from the stress-free boundary, contrary to theoretical expectations. Measurements under vacuum indicate that this is not due to a loading effect of the air in the capacitance detector used to measure the harmonic amplitude. It is noted that the harmonic generation is due almost entirely to dislocations in the crystal.

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