Voyage au centre de l'audition : spécialistes

> Perception: overview

Drawings and animations: S. Blatrix; scientific contributors: A. Lorenzi, M. Camilleri; editing: S. Irving

Auditory perception is our brain’s interpretation and understanding of our sound environment.
Because absolute silence exists only in a vacuum, we are constantly confronted with sound sources, either useful or unwanted, that can occur simultaneously or one after the other. How do we sort through all of these sounds?
Put simply, our ears code the different sound messages that reach us, and our brain carries out the task of reconstructing and sorting them using attention and memory (see also “How do I hear?”). This allows us, for example, to recognise different voices, to understand speech, to follow a conversation when there are other sounds around, or to distinguish between different elements of a complex sound and therefore enjoy music!

Auditory sensation
Foundamental characteristics of the auditory sensation are: the loudness, the pitch and the timbre. - Loudness is the sensation of intensity: a sound is loud or weak. - Pitch is the sensation of fréquency: a sound is high or low. - Timbre is the charcteristic, which allows us to differentiate between sounds of same pitch and loudness: everyone's voice, every music instruments has its specific and recognizable timbre (see "Music perception").
Binaural hearing
When both of our ears are stimulated, the difference between the intensity and the frequency at each ear, over time, has a major effect on sound perception: This is stereophonic hearing, which is very important, and we will come back to it when we talk about listening to music. Binaural hearing is essential for localizing sound sources.
Loudness elevation with binaural hearing When the same sound reaches the two ears at the same time (as when wearing headphones) the perceived loudness is greater (sound #2) than when it reaches only one ear (sounds #1 or 3).
Listen to the 3 sounds (preferably with headphones)
Sound localization in the horizontal plane With only one ear it would be very difficult to guess from where the sound is coming. Differences in loudness and time between the two ears allow us a pretty good sound localization in the horizontal plane (see animated drawing below). In the vertical and medial plane, localization is much less precised.
	When a sound source is placed to the side, the sound wave, which decreases with distance, arrives at the exposed ear first (here, the left ear), before working its way around the head to the other ear (here, the right ear). This means that the sound arrives at each ear at a slightly different time and with different intensity. The brain uses these differences to determine the location of sound in space. In this example, although the subject has his eyes closed, he is able to say that the loudspeaker is on the left. On the other hand, it’s much harder for him to tell how far away it is.

Analysis of an auditory scene
In reality, we are constantly bombarded with multiple sound sources. The analysis of an auditory scene depends upon the properties of binaural hearing that we have just described, as well as the frequency information that comes from different sound sources.
The animation below shows how we are able to analyse, organise and reconstitute the different sound elements of our environment.

When the man first enters the room, he perceives a cacophonic mixture of sound: the sum of all of the sound sources in the room … little by little, he recognises each of the sound sources (the radio, the fan, the purring cat, the clock and the telephone), and he goes towards the ringing telephone, which he considers a priority. During the following phone conversation, our man will temporarily ‘forgets’ the other sound sources

A similar process allows us to ignore the background chatter and music of a cocktail party, and focus our attention on the person that we are speaking to.

> Perception: overview

Auditory sensation

Binaural hearing