Headphone measurement variability is greatest below 500 Hz and above 3 kHz

Evidence

The positioning and coupling of the headphone on the ears can produce significant variations in the measured response, particularly at frequencies below 500 Hz and above 3 kHz. Closed-back headphone designs typically require the cushions to properly seal around the ear to reproduce good bass, which can be a challenge for many test fixtures and humans depending on the size and shape of their heads/ears and how well the cushions fit and seal around the ear. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1717