The guy never shows the fog produced with only the ultrasonic generators running. He always has his regular fogger running as well.
I've never seen thick fog like that from an ultrasonic generator, even when you have a bunch of them running. Also, the fog from an ultrasonic generator tends to dissipate rather quickly compared to the fog from a fog machine.
Brian used a bunch of ultrasonic generators (like 24 emitters) for his
laminar-flow fog screen video display, and the reason he used ultrasonic generators was precisely because it dissipated so quickly. If you watch the videos he posted, you can see how fast the fog disappears. That's not what you want for ground-fog though...
I think if you wanted a cold fog generator to give you some ground-hugging fog for your Halloween display, you'd be better off using a standard fogger, a fan, 20 ft of flexible dryer hose, and a big trash can full of ice. Just coil the dryer hose around the inside wall of the can, fill the center with ice, and run the dryer hose out through a hole that you cut in the side of the can near the bottom. Put the fan at the top, blowing into the dryer hose, and put the fog machine right behind the fan so the fog goes into the fan, into the dryer hose, down into the coin in the trash can (where it is cooled by the ice) and out the bottom in a thick, ground-hugging cloud of cold fog.
Adam