12 Sound localisation
12.1 Localisation of sound in the horizontal plane
While information about frequency and intensity is essential for interpreting sounds in our environment, sound localisation can be of critical importance for survival. For example, if you carelessly cross the street, your localisation of a car's horn may be all that saves you. Our current understanding of the mechanisms underlying sound localisation suggests that we use different techniques for locating sources in the horizontal plane and vertical plane.
Imagine a sound source that is directly in front of you. All else being equal, will the sound reach each ear at the same time?
Yes, if it is directly in front of you, since the distance it must travel to each ear is the same.
Will the sound be equal in loudness at your two ears given that as sound travels over distance, it decreases in intensity?
Yes, because it travels the same distance and therefore attenuates to the same extent before reaching each ear.
Now imagine a sound source that is directly to one side of your head – say the left side. Which ear will receive the sound first?
The left ear, because the distance the sound must travel to the left ear is shorter than to the right (it must travel over your head).
At which ear will the sound be louder?
At the left ear since it travels less distance and therefore attenuates less compared to the sound arriving at your right ear.
These two kinds of information, differences in intensity of sound at the two ears (interaural intensity differences) and differences in the time of arrival of sound at the two ears (interaural time delay) enable our auditory system to localise a sound source in the horizontal plane.