Tone generator

A single tone at whatever frequency you set. A440 is the tuning reference; sweeping upward is a rough way to find where your hearing stops, and a square wave is the honest test of whether a speaker rattles.

Two cautions worth taking seriously. First, sustained pure tones damage hearing at volumes that do not feel dangerous, because there is none of the variation that normally makes loudness obvious — start quiet and stay there. Second, what you hear at the extremes is your speakers as much as your ears: most laptop speakers produce essentially nothing below 150 Hz and phone speakers even less, so silence at 40 Hz usually means the hardware, not you. Hearing above 16 kHz declines steadily with age and most adults over forty hear nothing above about 15 kHz, which is normal rather than a problem.