[doh]

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.

[cloudripper]

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.

[doh]

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