How does our auditory system localise the position of a sound source?

The fundamental mechanism is based on having two ears and not just one (similar to vision). Through binaural hearing, i.e. with both ears, our perceptual system can compare the physical characteristics of sounds arriving at both ears and, from that, obtain information on the position of the source that generated it. Let's see how.

Identification of the direction of origin of sound

To explain the strategy that our perceptual system uses to identify the direction of origin of sound, let's imagine that we have a loudspeaker that generates a sound and a listener who turns their head to position their right ear closer to the speaker than their left one.

In this situation, two effects come into play:

• the right ear, being closer than the left, collects the sound first with its pinna. The difference between the two arrival times of the sound is called the Interaural Time Difference (ITD). We can obtain an estimate of this property by dividing the longer path that the sound has to travel to get to the ear that is further away (the "width of the head") by the velocity of sound in air. Assuming that the head's width is 0.25 m and the speed of sound is 340 m/s, we obtain a ITD of:

${\displaystyle ITD={\frac {0,25}{340}}\simeq 7\cdot 10^{-4}s}$

i.e. about 0.7 thousandths of a second. This time seems extraordinarily short also considering the fact that it is the maximum ITD possible (if the two ears and the loudspeaker are not aligned, the ITD is obviously shorter seeing that the difference in the path diminishes). However, in optimal conditions, our perceptual system can detect an ITD of 0.1 millionths of a second and, therefore, is quite able to evaluate delay times that occur in typical situations. Turning the head will cancel out the ITD (or at least bring it under the minimum detectable value). In this way, the line that indicates the direction of the source rests in a plane perpendicular to the segment connecting the two ears and crossing its midpoint. Obviously, this plane is the locus of the points equidistant from the two pinnae.

• The ear that is further away is located in the "shadow zone" created by the head and receives the sound with less intensity. This difference in intensity is called the Interaural Intensity Difference (IID). By processing the IID, the auditory system receives further information on the direction of origin of the sound. The smallest value of IID that our auditory system can discern is approximately 1 dB.

The two "strategies" described above have a maximum efficacy in two different frequency ranges:

• the first strategy is very effective for low frequency (and long wavelength) waves, for which the obstacle represented by the listener's head is easily circumvented (for more on this, see the page on sound diffraction);
• the second strategy is very effective for high frequency (and short wavelength) waves, for which the obstacle represented by the listener's head is almost insurmountable and causes a significant decrease in the sound energy (intensity) that reaches the listener's ear that is further away.

One last note: the two effects described above are effective if the sound wave coming from the loudspeaker has a very precise direction (i.e. its path can be represented by a straight line). As explained on the page about sound diffraction, this condition is less likely with low frequency waves and the source of these waves is, therefore, less easily locatable.

Identification of the distance of the source

Measuring the ITD and the 'IID (unless the sound source is not very close to the head) does not allow for localisation of the distance of the source but only for the direction of origin of the sound. Our auditory system uses other strategies to evaluate the distance of the source:

• in closed environments, it can evaluate how much of the sound energy captured by the pinnae arrives directly from the source and how much from the phenomena of reflection off the walls. From the ratio of these two contributions, the auditory system can estimate the distance of the source;

• in open environments (if the source is far away), it can evaluate, through experience, the changes in timbre of a sound as the distance varies (the classic example of this is thunder, which "sounds" quite different depending on the distance from which it is perceived).

Identification of the elevation of the source

Another type of information that our auditory system uses to localise a source is its height (elevation) with respect to the pinnae. Recent experiments have shown that the pinnae play a decisive role in "capturing" this information. You can experience this personally:

• press your pinnae against your head and hold them there. Close your eyes and get someone to generate a sound in front of you (e.g. jangle a key chain) and you will discover how difficult it is to identify the height of the source;

• fill all the folds of your pinnae with plasticine until your outer ears are a flat surface (while this experiment has been done before, we actually do not recommend it). You will find that, also in this case, the auditory system has difficulty detecting the height of the source. This experiment shows that the particular conformation of the pinnae plays a decisive role in determining the elevation of the source. Evidently, the many folds of the pinnae introduce small phase shifts in sound reflection, which can be used by the central nervous system to obtain directional information.

In-depth study and links

Along with allowing us to localise the origin of a sound source, incoming information on intensity, phase delay, spectral differences, etc. is also processed by the brain to create a sound map of the environment in which we move. This fact is of utmost importance.

The page dedicated to architectural acoustics presents several sound parameters that are characteristic of closed environments from an acoustic point of view. You will also find several sound examples that recreate the acoustics of various types of environments.

---