Contralateral masking for monaural speech intelligibility measurements with hearing aids in free-field speech conditions
AbstractPatients with asymmetrical hearing loss or unilateral hearing loss often suffer from bad hearing at the poor side, from localisation problems, and from poor speech understanding in noise. In many cases speech audiometry in free field can be an effective tool to decide whether speech understanding is equivalent for both aided ears, making binaural interaction possible, but only if the speech intelligibility is measured for each ear separately. However, it is difficult to evaluate the effectiveness of each (aided) ear individually. This is due to the fact that sound generated in free field can reach both ears, i.e., also the non-test ear. The sound can reach the non-test ear in three ways: directly from the loudspeaker, indirectly by transcranial transmission via the test ear (cross-hearing), or via the skull. In many clinics the non-test ear is “masked” by a foam plug and/or earmuffs. This method helps to minimise the effect of hearing direct sound at the non-test ear. However, transcranial transmission cannot be ruled out by this method. We suggest a new method of contralateral masking, while stimulating in free field. Theoretical considerations are outlined to determine the masking levels necessary to mask sufficiently, and to avoid too much masking (over-masking). For most asymmetric hearing losses a simple rule can be used.a
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