Benefits of Fitting One Versus Two Hearing Aids

The consensus among audiologists is that fitting two hearing aids offers greater benefit than fitting one only. Most forms of hearing impairment affect both ears, so fitting only one hearing aid seems not to make any sense in the great majority of cases. The exception would be where fitting two aids adds to someone's hearing difficulties in the form, for example, of binaural interference-where the input to one ear distorts the combined binaural signal. But such cases are rare. The presumed benefits of bilateral fitting lie in improved directional hearing and related forms of binaural interaction, such as more audible signal (binaural summation), and improved hearing for speech in spatially separated noise. The latter is understood to be largely due to more effective functioning of the so-called head shadow effect. If a signal of interest (a target) is on one side of the head and an interfering noise is on the other side, a person listening with two hearing aids can take advantage of the fact that the aided ear on the same side as the target will be exposed to a better signal-to-noise ratio, because the head itself acts to diffract the higher frequency components of the contralateral noise. Most laboratory evidence bears out these expectations about improved performance with two hearing aids, thus supporting the consensus for fitting two. This evidence is summarized in Dillon (2001). However, a recent report (Arlinger et al., 2003) concludes that there is no robust clinical field evidence supporting a claim for greater benefit from two hearing aids versus one.