Binaural loudness summation shrinks dramatically when one ear is occluded in a sound field.

Evidence

The lack of summation in the Loudspeaker condition supports the observations made by Cox and Gray ( 2001 ) and by Epstein and Florentine ( 2009 , 2012 ), that, when monaural presentation is achieved by occluding one ear in a sound field, binaural summation can be dramatically reduced. — Dijk, Pim van, Physiology, Psychoacoustics and…, 1525