that sort of transducer only works at resonance,
changing the frequency would just make it not work very well.
the driving board likely seeks resonance to drive the part at
so, if you want to change the frequency, you likely have to make mechanical changes to the ultrasonic transducer

edit:
also, without looking at any of the details on this one,
the 42.8Khz was likely just what his 40khz ultrasonic transducer happened to resonate at.

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