The middle of everything
500 Hz (just above B4) sits in the heart of the speech band: vowel energy, the melody range of most singing, the core of guitar, piano, and horns. Human hearing is comfortable and accurate here - no equal-loudness tricks, no hardware excuses - which makes 500 Hz a fair reference when comparing devices: any speaker that struggles at 500 Hz struggles everywhere.
In mixing vocabulary, excess 500 Hz reads as "boxy" - the sound of singing into a cardboard box - most notorious on drums and electric guitar. Small cuts here open up a crowded midrange; the skill is hearing the boxiness in the first place, which is exactly what listening to the frequency in isolation trains.
Practical checks at 500 Hz
On an audiogram, 500 Hz is the second-lowest standard frequency and part of the pure-tone average that defines hearing-loss grades - it matters directly for understanding speech. As a self-check it should be clearly audible at low volume on any device.
For rooms, 500 Hz marks the rough boundary where bass problems (standing waves) hand over to treatable mid/high problems (reflections and reverb). If a room sounds bad above 500 Hz, absorption panels at reflection points help; below it, think bass traps and positioning instead - the studio monitor setup guide walks the split.