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Seems like the challenge would be heating the right places. How would you know what food to heat more or less?

Also, ultrasonic transducers today, as far as I know, require essentially direct contact to impart energy. You probably don't want to dip something into your leftovers just to heat it.

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Rather than heating food more or less, I care about equally. Couldn’t combination of thermal camera and ultrasonic transducer achieve the goal?

If they indeed need a contact with the object, then obviously that wouldn’t work. But I am not sure why the air molecules couldn’t carry it.

Impedance mismatch and attenuation. The impedance difference between air and the piezoelectric elements of the transducer is too great - this will cause soundwaves to reflect on contact (and thus never leave the element). If some transducer innovation changed that variable, then attenuation would be the next hurdle.

In general a form of couplant (water, oil, some form of gel, etc) is necessary to eliminate air between the transducer and a material in order for soundwaves to pass.

Thanks for the context. That seems to invalidate the idea.

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