A person speaks, a musician plays, a bird sings, an inanimate object is struck. This creates pressure waves which travel through the air to the outer ear or pinna.
The outer ear comes in a wide range of shapes and sizes, but its purpose is simple: to efficiently channel these pressure waves along the ear canal until they hit the eardrum.
These pressure waves cause the eardrum to vibrate. On the other side of this small vibrating membrane is the middle ear. A miracle of bioengineering, the middle ear is an air-filled chamber containing the ossicles, the smallest bones in the human body. These three bones, known individually as the malleus (hammer) incus (anvil) and stapes (stirrup) focus the vibration of the eardrum, and, through a system of levers amplify it by around twenty times.
These vibrations are then transmitted from the air-filled middle ear to the fluid filled inner ear. This transmission takes place through a flexible membrane called the oval window, setting the fluid in motion. (This process creates pressure in the fluid of the inner ear which is relieved by an adjacent flexible membrane known as the round window).
The inner ear fluid is contained within a spiral shaped structure known as the cochlea. Within the cochlea are microscopic hairs, each attached to the basilar membrane which is sensitive throughout its length to different frequencies. When an individual hair is stimulated by the frequency it is “tuned” to it sends an electrical signal to the brain via the auditory nerve. This signal is then processed by the brain, creating the experience of sound. The brain then filters this sound (or component parts of it) out or focusses on it (or more likely some component parts of it) having concluded through past experience which sounds are worth focussing on.
Here is an animated summary of this process: https://blausen.com/en/video/hearing/#
Locating Sounds
Humans (and indeed most creatures) with a pair of ears can locate the source of sound with tremendous accuracy, usually within two degrees of space. Here is the process vividly in action in the natural world:
This accuracy is a result of the ability to compare the information received by both ears. The Italian scientist Venturi first investigated this in the 1790’s. He played a flute in different parts of a room and asked a blindfolded group to pinpoint his exact location. As a result of this experiment, Venturi’s theory was that people were detecting a subtle difference in volume experienced by their left and right ears and were using this difference to subconsciously calculate his exact position. A different theory was proposed in 1908 when Malloch suggested that it was the difference between the precise times that the sound hit the left and right ears that determined the position of its source. Neuroscientists now believe that both theories are correct having discovered parts of the auditory processing centres of the brain attuned to both cues.
If sound is coming from directly above or directly below however, these systems alone will not work, and clearly we are able to locate sounds in three dimensions, rather than on a single horizontal plane. This is where the acoustic properties of the outer ears (pinnae) come into play. The pinnae are shaped to capture vibration in three dimensions and then amplify frequencies depending on the source of the sound:
Tracking moving sounds
When a sound is moving (e.g. a fire engine) we track the source of the sound and its speed of motion through the Doppler effect. First described in 1842 by the Austrian physicist Christian Doppler, the Doppler effect is experienced as a rise in pitch as a sound source approaches and a corresponding lowering of pitch as the sound source moves away. The rise in pitch is caused by the crests of the sound waves becoming compressed together by the motion of the sound source relative to the observer and the drop in pitch by the process happening in reverse:
Filtering Sounds
The ability to filter information is essential for our survival, but also works against us when we seek to experience everything sound has to offer. There is a vast amount of information coming at us through our five senses. We are filtering this information continually, only paying attention to information our subconscious tells us is most essential.
Consciously observing this filtering process as it applies to the sound is the first step on the road to Deep Listening.
Here is an effective exercise to initiate conscious listening:
- Sit quietly with the head balanced on the spine and the ears poised and alert.
- Experience the soundscape you are experiencing as if it were a piece of music with many layers – experience them all simultaneously
- List each individual sound you can hear, and then ask yourself the following questions:
- Which sound has the highest pitch?
- Which sound has the lowest pitch?
- Which sound is the most rhythmic?
- Which sound is the most chaotic?
- Which sound is closest?
- Which sound is the furthest away?
- Which sound do I enjoy the most?
- Which sound do I like the least?
Focus on combinations of the above, e.g.
- the highest pitch sound combined with the sound that is furthest away.
- the sound that I like the least combined with the sound closest to me.
This exercise contrasts well with the following echolocation exercise.
Hearing spaces
Move around a building, through corridors and stairwells, arriving in each new space, clap your hands with your eyes closed. Notice how much information your ears give you about the individual characteristics of the space – how high is the ceiling? Is the floor carpeted? Where are the windows? If you stop to think about what is happening it is miraculous – your ears are locating the sources of the individual echoes of your clapping and your mind is building a 3D map of the space from this information.