Equal-loudness contours were first measured using 1 kHz reference tones and phons

Evidence

Fletcher and Munson (1933) were among the first to evaluate the sound levels at which we judged different frequencies to be equal in loudness. Starting with a reference pure tone at 1 kHz, other tones at different frequencies were adjusted until they appeared to be equally loud. With enough of these measurements it was possible to draw a contour of equal loudness identified by the sound pressure level of the reference tone, but called phons rather than decibels, to denote its subjective basis. For each SPL of the 1 kHz reference tone, curves were derived, collectively called the equal-loudness contours. — Toole, Floyd E, Sound reproduction- the acoustics and…, 904