Optimizing Sound Localization with Hearing Aids

At a grant application interview the authors were greeted by the interview panel's spokesman who said: “I'm an eye doctor. Since reading your application, I've been asking my patients: ‘Does it matter whether you can tell where sounds are coming from’? and they say it doesn't. ” This attitude may epitomize how many people regard sound localization. It is such a natural part of living that this ability tends to be taken for granted. Even when problems arise from impaired localization, the affected people may not recognize the source of their difficulties. Before discussing localization with hearing aids, it is appropriate to briefly review the significance and mechanisms of sound localization and how this ability is affected by hearing impairment. This general discussion will be followed by more specific information about the effects on localization of type of hearing aid, unilateral versus bilateral fitting, choice of earmold type, and possible effects of directional microphones and compression. The issue will conclude with a set of recommendations for how clinical audiologists should consider sound localization and how hearing aids should be fitted to optimize this ability. Localizing Sounds Does Matter In the animal world, the ability to localize sounds has obvious survival value. It is vitally important to many animals for avoiding danger and/or finding food. Masterton et al (1969) ascribe a central role to localization in the evolution of hearing systems in animals and humans. They present a good argument for the view that the evolution of auditory systems has been driven primarily by the need to provide effective sound localization for animals with varying head sizes and living in different environments (air or water). Mammals with small heads and those living in water (e.g. dolphins) receive little interaural time difference information but they hear much higher frequencies than man which provides greater opportunities for localization on the basis of interaural intensity differences. The cues for localization are discussed below. Human survival in urban environments continues to depend on sound localization. There will still be occasions when we need to jump the right way when we hear an approaching car or when we hear a tree or wall starting to fall over. Localization is important for hunting which is still part of survival for some civilizations. If we extend the concept of “survival” to include all forms of responding effectively to other people and our environment, then there are many situations where localization is essential. If we hear someone call, we may need to locate the person quickly. It may be important for their well-being or even survival. If we are awakened by a noise, we may want to know which neighbor to complain to. If we are looking for someone or something, the sounds we hear may be helpful if we can tell where they are coming from. For example, sounds of movement in another room suggest that may be a good place to look for a family member who is wanted on the phone. Knowing where a sound is coming from may help us to identify the sound, just as the reverse is true. In another context (in relation to experiencing loudness variations), Killion (1997) has aptly pointed out that auditory sensations are essential to aesthetic experience. Localization is a vital part of that experience. We like to locate the bird that is singing in the tree or the animal that is rustling in the bush, even though it is not a danger to us and we have no intention of eating it. Another auditory experience, almost totally ignored, is the externalization of sounds and a natural feeling of spatial orientation. If we wear a pair of earplugs, or tightly fitting earmolds, a common sensation is that somehow sounds are not “out there” in their proper places. This can bring a disconcerting feeling of being isolated from the environment and this is not attributable simply to the attenuation of sound. Localization is part of experiencing the environment in a natural way and this may be very important subjectively, in addition to the more obvious utilitarian purposes of localization. Under difficult listening conditions, or for a hearing-impaired person, localization can play a vital part in understanding group conversations. When the conversation switches from one person to another, the listeners need to locate the new speaker instantly or they will miss the first part of each segment of the conversation, which may seriously reduce understanding. This is especially so for people with substantial hearing loss who typically rely on a lot of help from speechreading. Impaired localization probably plays a more significant role in speech communication difficulties than is usually appreciated. It has been suggested (e.g. Hirsh, 1950) that there is a direct connection between the ability to localize sounds and the ability to hear speech in noise. The basis of this argument is that interaural phase differences are cues for both localization and release from masking (i.e. masking level differences). However, no strong link between these abilities has ever been demonstrated and a recent study showed only weak correlations between localization and the ability to separate speech from noise (Noble et al, 1997). This issue requires further investigation and, at present, it seems an open question whether there is any direct connection between impaired localization and difficulty in recognizing speech.