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comparative more ultrasonic, superlative most ultrasonic
(acoustics) Beyond (higher in frequency than) the range of sound perceptible to the human ear; with a frequency of 20 kilohertz or higher. quotations examples
The wheel shop also has equipment for ultrasonic testing of axles for flaws; and systematic checking of locomotive axles and crank pins is carried out as they pass through the shops.
1961 March, C. P. Boocock, “The organisation of Eastleigh Locomotive Works”, in Trains Illustrated, pages 159–160