"While the equipment we used in our Cuban re-creation is relatively bulky, ultrasonic emitters can be quite tiny, no larger than a piece of Rolo candy. Online, we found a manufacturer in Russia that sells a fashionable leather clutch that conceals an ultrasonic emitter, presumably to jam recording devices at cocktail parties. We also found electronics stores that carry high- power ultrasonic jammers that cause microphones to malfunction. One advertised jammer emits 120-dB ultrasonic interference at a distance of 1 meter. That’s like standing next to a chainsaw. If a signal from that caliber jammer were to combine with a second ultrasonic source, audible by-products could result."
Consider an countermeasure ultrasonic emitter intended to jam microphones in proximity to, say, a spy emitter using ultrasonic sound as a carrier wave for a microphone, as described in the article.