How does masking noise prevent the wrong ear from hearing a test signal?

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In pure-tone audiometry, masking noise is used to ensure that test results accurately reflect the hearing threshold of the specific ear being tested.

When a test signal is presented at a high intensity, it can cross the skull and be detected by the opposite, nontest ear—a phenomenon known as cross-hearing. This is particularly problematic if the nontest ear has better hearing than the ear currently under evaluation. By presenting a masking noise to the nontest ear, the clinician effectively occupies that ear, preventing it from detecting the signal intended for the test ear.1

It is essential to use the correct amount of masking; insufficient masking may allow the nontest ear to continue hearing the signal, while excessive masking can lead to cross-masking, which may artificially elevate the measured threshold of the test ear.1