Ultrasonic waves are attenuated at a rate of 3-180 dB/m[1] (100-1000 kHz waves). So at 10 m a 150 kHz wave will have decreased in amplitude by over 300x. There's no amount of transmitters that can make up for the attenuation at high frequencies. It's not even possible for devices operating above a couple hundred kHz or farther than 10-20 m. A 500 kHz wave will decrease 200 dB over 5 meters, taking it from a shockwave (ie the pressure between pulses is vacuum) to undetectable. You'd need a transmitter strong enough to disintegrate organic material[2] but even the best detectors on earth won't hear it from across a road.

[1]: http://www.ktu.lt/ultra/journal/pdf_50_1/50-2004-Vol.1_09-A....

[2]: https://en.wikipedia.org/wiki/Sonication

That makes more sense to me, but still, the energy requirements would be huge. Given Cuba's electricity infrastructure (and probably even without that) I think you would be looking at 3 or more big pieces of equipment that require very large, very heavy, batteries. It seems like that would have been very conspicuous.

It'd be awfully difficult to precisely position and aim multiple transmitters discreetly.