Adjusting ultrasonic horn frequencies

I recently reworked one of my ultrasonic horns and I now need to tune its frequency. The horn is a 20 kHz solid steel horn that has a round diameter in the rear and narrows to a flat "blade" in the front. I do have access to a Dukane horn analyzer.

Can you tell me where I need to machine the horn to increase and also decrease the frequency?

Thank you.

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