For ultrasonic you want speakers that have minimum low frequency range of 20 kHz and stereo is much preferable for maximum results.
Download and play the free frequency test. If you don’t hear crackling and the last few seconds or so are silent (or you hear a super-high pitched whine, if you have exceptional hearing), then they are properly reproducing the ultrasonic audio. If not, then use the masked track until you can get something better.
Posted in: Hardware Questions (Speakers & Headphones)