> I won't be surprised if single cell organisms, even prokaryotes can detect and react to sound
It would have to be pretty insanely high in the ultrasound. A 30 kHz (above the top of usual audible sound range) wave is around 10 mm (10000 µm) in wavelength; a typical prokaryote is of order 1 µm. While there are some uses for MHz-scale ultrasound, the attenuation starts getting pretty insane -- there's just not much energy at the scale of gigahertz sound to detect.
Why would you assume that the order of magnitude of the animal is similar to the wavelength? Elephants can communicate with as low as 1hz. The wavelength of that is over 300m.
If the amplitude is high enough, human too can comfortably “feel” infrasonic.
Imagine an elevator that moves up and down its shaft with a sinusoidal elevation over time. Riding in that elevator, one could detect frequencies with quite long wavelengths compared to the size of a person.
It would have to be pretty insanely high in the ultrasound. A 30 kHz (above the top of usual audible sound range) wave is around 10 mm (10000 µm) in wavelength; a typical prokaryote is of order 1 µm. While there are some uses for MHz-scale ultrasound, the attenuation starts getting pretty insane -- there's just not much energy at the scale of gigahertz sound to detect.