A major goal of the basic audiologic evaluation is assessment of auditory function of each ear. There are situations during both pure-tone and speech audiometry, however, when the nontest ear can contribute to the observed response from the test ear. Whenever it is suspected that the nontest ear is responsive during evaluation of the test ear, a masking stimulus must be applied to the nontest (i.e., contralateral) ear in order to eliminate its participation.

Cross-hearing occurs when a stimulus presented to the test ear “crosses over” and is perceived in the nontest ear. It is the result of limited interaural attenuation during both air-and bone-conduction testing. Interaural attenuation refers to the reduction of energy between ears. Generally, it represents the amount of separation between ears during testing. Specifically, it is the decibel difference between the hearing level of the signal at the test ear and the hearing level reaching the nontest ear cochlea.

A major factor that affects interaural attenuation is the transducer type: air-conduction versus bone-conduction. Two types of earphones are commonly used during air-conduction audiometry. Supra-aural earphones use cushions that press against the pinna, while insert earphones are coupled to the ear by insertion into the ear canal.

Interaural attenuation for supra-aural earphones varies across frequency and subject, ranging from about 40 dB to 80 dB (e.g., Coles and Priede, 1970; Snyder, 1973; Killion, Wilber, and Gudmundsen, 1985; Sklare and Denenberg, 1987). The smallest reported value of interaural attenuation for speech is 48 dB (e.g., Snyder, 1973; Martin and Blythe, 1977). When making a decision about the need for contralateral masking during clinical practice, a single value defining the lower limit of interaural attenuation is most useful (Studebaker, 1967). The majority of audiologists use an interaural attenuation value of 40 dB for all air-conduction measurements, both pure-tone and speech, when making a decision about the need for contralateral masking (Martin, Champlin, and Chambers, 1998).

Commonly used insert earphones are the ER-3A (Etymotic Research, 1991) and the E-A-RTONE 3A (E-A-R Auditory Systems, 1997). A major advantage of the 3A insert earphone is increased interaural attenuation for air-conducted sound, particularly in the lower frequencies. Consequently, the need for contralateral masking is significantly reduced during air-conduction audiometry. Based on currently available data, conservative estimates of interaural attenuation for 3A insert earphones with deeply inserted foam eartips are 75 dB at 1000 Hz and below and 50 dB at frequencies above 1000 Hz (Killion, Wilber, and Gudmundsen, 1985; Sklare and Denenberg, 1987). The smallest reported value of interaural attenuation for speech is 20 dB greater when using 3A insert earphones with deeply inserted foam eartips (Sklare and Denenberg, 1987) than when using a supra-aural arrangement (Snyder, 1973; Martin and Blythe, 1977). Consequently, a value of 60 dB represents a conservative estimate of interaural attenuation for speech when using 3A insert earphones.

Interaural attenuation is greatly reduced during bone-conduction audiometry. Regardless of the placement of a bone vibrator (i.e., mastoid versus forehead), it is generally agreed that interaural attenuation for bone-conducted sound is negligible and should be considered 0 dB (e.g., Hood, 1960; Sanders and Rintelmann, 1964; Studebaker, 1967; Dirks, 1994).