Binaural loudness is measured by matching against a monaural comparison tone in filtered noise

Evidence

250-Hz tones with the same level in each ear have been used as binaural signals, the loudness of which was measured by adjusting the level of a monaurally presented 250-Hz tone to sound equally loud. The background noise was low-pass filtered with a steep cut-off at 840 Hz and an SPL of 43 dB in each critical band, corresponding to a density level of 23 dB. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1727