A model of hair cell excitation that involves mechanical impact between the hair cells and the tectorial membrane has been proposed by Crane [J. Acoust. Soc. Am. 40, 1147–1154 (1966); J. Acoust. Soc. Am. 51, 508–514 (1972)]. The model predicts that if the impact is unilateral, biasing of the basilar membrane toward or away from the tectorial membrane will significantly affect the generation of nonlinear combination tones. Anf2−f1envelope tone synchronized with thef1andf2primaries was used to achieve such biasing. The experiment reported here investigates the effect of the phase of thef2−f1tone on the prominence of the beating of a 2f1−f2combination tone (generated by input primariesf1andf2) and a probe tone that differs in frequency by a few hertz from the frequency of the combination tone. The input thus consists of four explicit tones:f1,f2,f2−f1, and (2f1−f2)+δ. It was found that thef2−f1tone can have a large effect on the level of the 2f1−f2combination tone. The phases of thef2−f1tone that lead to a maximum and minimum level of combination tone are approximately 180° apart, and there can be almost total discrimination between those two phase conditions.Subject Classification: [43]65.56, [43]65.28, [43]65.60.