For various frequencies, 20 subjects adjusted the Δf value of two simple tones after two criteria: (1) maximal roughness and (2) absence of interference and hearing the tones separately. The average results agree rather well with some old data published by Cross and Goodwin (1893) and by Mayer (1894). Over a restricted frequency range, the results obtained for Criterion 2 agree with critical bandwidth.
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Plomp et al. (1968) studied this question.