As Tonyr1084 learned by sticking his finger in one, there is only a relatively small actively ultrasonic area and a whole lot of resonant/canceled sound waves. Which makes larger units less efficient looking at Watts/Area even with adding multiple transducers to improve cleaning. Enough power will overcome weakness but not necessarily be efficient in gently cleaning ultrasonically. Too much power causes bubble erosion. FWIW In our QC and Development labs 2-4 gal. ultrasonic was used for cleaning very nasty labware. The big stuff went into a steam chest instead after being soaked in caustic. Normal messes went into a labware industrial dishwasher. This was the poor "new-guy's job".

 

Well, I've got a bunch of reading on the subject to do. Don't forget my questions here, the original Q has been answered in the most basic way: https://forum.allaboutcircuits.com/...before-tutorials-articles.166050/post-1464626