A childhood friend of my sister's lost most of her hearing in a riding accident when she was in her twenties. She now lives a couple states away, but when her first child was about 2 1/2, they were visiting my sister one day when I was there. At one point, her son decided he didn't want to be ignored, and let out the most high-pitched and piercing noise I have ever heard come from a human being. As we all simultaneously jumped, flinched and covered our ears, the sound abruptly stopped as the mom reached down and clapped her hand over her son's mouth.
"Mommy can HEAR that noise" was her simple explanation! Thankfully, he learned not long afterwards to never use that noise outside of his own home again!