Oh. Time for countermeasures then. If possible, from commonly accessible components, capable of being rigged in minutes or less.
Try out if speaking while listening to loud music that overpower this would help. Or with just those earplug headphones, unplugged, that attenuate the sound.
Or a throat mic, or sufficiently close-to-mouth mic, feeding the headphones with one's own amplified speech. Maybe the same could be achieved with a phone handsfree and a suitable app that takes sound from the mic and sends it to the headphones (and filters eventual feedbacks with an adaptive notch filter)?
The test can be done with a conventional speaker, loud enough. No need to maintain the directionality nor distance, so the cost/availability of the ultrasound rig can be avoided for testing purposes.