Sound Localisation

Having two ears has many advantages over having one ear. Although verbal communication is generally not hampered much by using just one ear, accurate sound localisation relies heavily on using two ears. Sound localisation occurs based on a comparison between difference in sound that reaches both ears.

 

The figure above illustrates that sound from a source (narrator) takes longer to reach the right ear of the listener than it takes to reach the left ear. Besides, in order to reach the right ear of the listener, sound has to travel around (through) the head of the listener causing the signal to reduce in level. The difference in time and level provide information on the location of the source. These differences are interpreted by the brains and translated into a given location. In addition, the pinna provides valuable information whether the source is located frontal or dorsal and on the position above or underneath the head.

Difficulty in locating environmental sounds can be inconvenient, or in some situations it can put the hearing-impaired person in danger (e.g. not knowing the location of an approaching car). Hearing-impairment does not always result in a poor localisation accuracy. Clearly, very soft signals will be inaudible and can therefore not be located. The larger the hearing loss, the more difficult it becomes to determine the location of a source due to inaudibility of the cues. Apart from the sound level of the source, the type of hearing loss has an impact on the difficulties experienced when locating sounds. For instance, people with a conductive hearing loss experience more difficulties locating signals than people with a comparable sensorineural hearing loss. If the hearing loss in one ear differs from the other (asymmetrical hearing loss), or in the case where one ear is impaired and the other is normal (unilateral hearing loss), localisation performance decreases.

Sound localisation performance not always increases by wearing hearing aids. Temporal delays induced by the hearing aid, affect the information provided by the temporal cues. In addition, they alter the differences in sound level between ears by making sounds audible. However, frequently wearing the hearing aids may cause the brains to adapt to these cues, restoring sound localisation performance. This often occurs within a couple of hours. Modern hearing aids, often try to compensate the delays introduced by the hearing aid, keeping this cue available to the user.

Localisation performance is generally independent of the type of hearing aid (i.e. behind the ear, in the ear, completely in the canal), although some cues are not preserved when using a behind the ear hearing aid. Since the microphone is located behind the ear, reflections from the pinna are not perceived. This may cause problems determining whether sound come from the front or from the back.