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We hear sounds over a huge frequency range of about 20Hz to 20,000Hz, although this range diminishes as we age.  It has long been recognised that identifying the direction of a sound source (or 'localizing' it) requires a vast amount of neural processing.

For accurate localization, we require the neural computation of a variety of cues.  When a sound comes from one side of us, say the right, it arrives first at the right ear and is also louder in that ear. At low frequencies, the brain processes differences in the time of arrival of the sound at each ear. At higher frequencies, the important cue is the difference in loudness perceived by each ear.

The final main piece of information is the way that our ears modify sounds - amplifying some frequencies and diminishing others - as they pass over the convolutions of the pinna. This is called the head-related transfer function (HRTF) and the HRTF of any one person is unique to that individual. The role of the HRTF is particularly important when we are trying to determine whether a sound comes from directly behind or in front of us.  In such a case, the differences in timing and loudness at each ear are negligible and there is consequently very little information on which we can base a decision.

Each type of sound localization cue operates over a different and relatively narrow frequency range.  Information from all cues is combined by our brains to provide us with a sense of where a sound is coming from.

Only certain types of sound are easy to localize and the crucial component is that they contain a large spectrum of frequencies    i.e. broadband noise. (It can most easily be described as the Sound of a rushing river or waterfall.)  With broadband noise, the brain has the maximum number of cues available to process. Pure tones, simple tone combinations or narrowband noise cannot be localized.

In one example of research at Leeds University it was shown that the sound of conventional police, fire and ambulance sirens is particularly difficult to localize.  In laboratory tests involving some 200 drivers, 56 per cent (worse than a random guess) were unable to tell whether the sound of an approaching siren was directly behind or in front of them - a potentially dangerous situation if they were at the wheel of a real car.




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