A form of electrical filter which is designed to mimic the relative sensitivity of the human ear to different frequencies at high sound pressure levels (notionally 100 Phons or about 87dBA SPL).
Essentially, the filter rolls-off the low frequencies below about 20Hz and the highs above about 10kHz. This filtering is often used when making measurements of high-level sounds, such as when calibrating loudspeaker reference levels (shown in the YouTube video below).