If you run across the concept of "towed array", you'll be in the correct neighborhood, where the US and other navies use a towed array of transducers to beamform their sonar signals. I'm not that familiar with the physical configuration of the array, but it always sounds (no pun) like a fairly large structure...
I'm thinking that an ultrasound with an aperture of about 3 inches for MHz operation would translate into something with and aperture of about 300 inches for something in the tens of kHz range.
TTFN