Importance of the Law of Interaural Attenuation (IA) for Air-conduction

Abstract

Since the interaural attenuation (IA) value for bone conduction is almost 0dB, the bone-conduction component of the air-conduction signal transmitted to the opposite inner ear through IA for air-conduction must be transmitted to the simulated ear on the almost same level.
Accordingly, an air-conduction signal is transmitted to the stimulated ear by two routes: the ordinary air-conduction route through air-bone gap of that ear and the bone-conduction route through IA for air-conduction.
Comparison of these two transmission routes led to the conclusion that the value of the air-bone gap never exceeds the IA value for air-conduction, and this phenomenon will be referred to as “the law of IA for air-conduction”. The following relationships can be deduced from this law of IA.
1. The difference between the unmasked poorer air-conduction threshold and the unmasked better bone-conduction threshold never exceeds the IA value for air-conduction on condition that the air-conduction receivers are placed on both ears.
2. Shadow hearing (cross-hearing, transcranial hearing) of air-conduction occurs only when the above difference is equal to the IA value for air-conduction.
3. If the unmasked poorer air-conduction threshold of the test ear (tentatively referred to as “the initial threshold”) is the result of shadow-hearing by the opposite inner ear, the bone-conduction threshold of the test ear must be poorer than that of the opposite ear by at least the difference in air-conduction threshold between the initial threshold and the “true” threshold of the test ear.