The top of the spectrum: pure sine tones from 5 kHz to 22 kHz with presets at the landmarks - tweeter checks, hearing-limit exploration, and the frequencies most adults can no longer hear. Keep the volume moderate; up here you cannot always judge loudness by ear.
Volume warning: if you cannot hear a tone you cannot judge its level - keep system volume moderate for the sake of tweeters, young ears, and pets in the room.
8 kHz is sibilance and cymbal sizzle - nearly everyone hears it. 15 kHz marks the age line (the old CRT television whine - full story on the 15 kHz page). 17.4 kHz is the famous Mosquito deterrent frequency, mostly under-25 territory - explored on the mosquito tone and 17 kHz pages. Find your personal ceiling with the hearing age test, and for dog training frequencies the dog whistle adds the right presets.
Tweeter and driver testing (a healthy tweeter renders a clean sine smoothly, damage adds crackle), hearing-ceiling checks, pest and pet experiments, finding hardware limits (many speakers and Bluetooth codecs roll off before 20 kHz), and demonstrating age-related hearing differences.
Either you crossed your hearing ceiling - which drops with age from about 20 kHz in childhood to 12-15 kHz in middle age - or your equipment quit first: many speakers, earbuds, and low-bitrate Bluetooth connections stop reproducing before your ears stop hearing. Test with wired quality headphones to separate the two.
Loud sound at any frequency damages hearing, and high frequencies are sneaky: if the tone is above your ceiling you cannot gauge how loud it is. Keep volume moderate always; the tone is present at full energy whether or not you perceive it.
Barely. Consumer speakers roll off steeply above 20 kHz - within the range this tool covers (up to 22 kHz) you get real output on decent hardware, but true ultrasound beyond that is out of reach for browsers and consumer speakers alike.