Being relatively young and having a childhood history of asthma could get them hearing a little higher than 20 khZ as well, so it's not out of the question that

Being relatively young and having a childhood history of asthma could get them hearing a little higher than 20 khZ as well, so it's not out of the question that they are hearing the sound itself. But the most likely thing is that they are hearing some sort of undertone or artifact created in the process of the signal generation.