The masking dilemma and its solution. Fusion at the inferred threshold (FIT) and sensorineural acuity level (SAL)...

It is difficult or impossible to obtain air or bone conduction threshold levels in some cases of moderately severe conductive involvement because it may be impossible to mask the nontest ear without also masking the test ear. This has been termed the "masking dilemma." In most of these "masking dilemma" cases the fusion at the inferred threshold (FIT) test can determine the air conduction hearing levels, and in some cases the sensorineural acuity level (SAL) test can help determine the sensorineural function.