External real-world sounds show loudness constancy across one and two ears

Evidence

Cox and Gray ( 2001 ) collected loudness ratings (7 categories from “very soft” to “uncomfortable”) for speech at different sound levels when listening monaurally and binaurally. These two modes of listening were compared by using either one or two earphones and, using a loudspeaker, by occluding one ear with a plug and ear muff. They found that results with one or two earphones produced a conventional loudness summation effect, whereby the mean rating was substantially higher at each sound level for binaural presentation. However, when listening to an external source in the environment (the loudspeaker) there was much less summation effect: occlusion of one ear had little effect on the loudness ratings compared to listening binaurally. This experiment showed for the first time that the loudness of external sounds may display constancy across monaural and binaural listening modes. — Dijk, Pim van, Physiology, Psychoacoustics and…, 1528

Categorization

• QUESTION: Why would headphone presentation reproduce summation while free-field listening suppresses it? The mechanism behind this modality difference is worth chasing.