You’re walking across a field. There are no streets, trees or people in sight. It should be completely silent and yet the earth crunches beneath your feet, the wind whistles around your ears, you can hear your own breathing. Those who don’t suffer from a hearing impairment always perceive some sound. True silence is practically unknown.
But what happens every time we perceive a snap, scream or melody? What are the physical underpinnings of auditory perception? What we hear is the result of a complex and fascinating interaction between the environment, ear and brain. Read on to learn more!
Transforming sound waves into sound
What exactly happens when a soprano opens her mouth and the audience hears her voice? At first, the singer causes her vocal chords to vibrate. These excite the air molecules they come into contact with which then radiate outwards as waves through the air. The principle of energy dispersing in a wave form can be observed with a towel. If you wave it very quickly, it won’t go up and down, instead the energy travels through the fabric in wave form.
On the way to the ear, sound waves bounce off surfaces creating reflections. Reflected sound hits the ear slightly later than direct sound and is dependent on the size and shape of the room. Both of these sounds – direct and reflected – reach the ear where they are enhanced by the outer shape of the organ. The pinna, or outer ear, is designed receive and amplify sound. Interestingly, the shape of the pinna differs from person to person and is as individual as a fingerprint. All pinnas, however, share an irregular shape resembling a miniature landscape of mountains and valleys. This rugged terrain acts like an acoustic filter, helping the listener localize sounds. Depending on how sound reflects off of the irregular surface of the pinna, your brain will be able to process from where a sound originates – handy if a bear is sneaking up on you in the forest and just in general.
The waves then travel onward through the ear canal to the eardrum. This fine membrane begins to vibrate, transforming soundwaves into mechanical energy which moves the three bones of the middle ear: The malleus (hammer), incus (anvil) and stapes (stirrup). The arrangement of the bones amplifies the sound. The bone furthest from the outer ear, the stirrup, sets another membrane in motion transferring the mechanical energy to the cochlea in the inner ear.
The cochlea is a small organ looks like a snail and, indeed, cochlea means “snail” in Greek. The cochlea is filled with fluid. When a vibration reaches it, the liquid moves and microscopic hair-like cells within it move as well. It is the movement of these small hairs, known as stereocilia, that triggers electrical signals which are then sent to the brain via the auditory nerve. Finally, the brain is responsible for making sense of these impulses: Is what I heard rain? Was it a baby crying? The brain does a good job of assigning sounds to their likely origins. Interestingly, the brain is capable of perceiving sounds even without an auditory stimulus. When the perception of sound is decoupled from any actual source, we speak of auditory hallucinations.
To make things even more complicated, there are different types of stereocilia in the cochlea responsible for four essential areas of auditory perception:
- Pitch
- Volume
- Location
- Timbre
Pitch
A friend’s voice, the bark of a dog, the sound of your sister’s car – we’re able to distinguish thousands of sounds from one another and pinpoint their origin. One of sound’s characteristics that helps us do this is frequency. Soundwaves that we experience as having a high-pitched sound vibrate at a higher frequency, lower tones have a lower frequency. This means higher frequency sound waves complete more cycles per second than lower frequency waves. Our perception of frequency is known as pitch. Within the cochlea there are different stereocilia responsible for determining different pitches. The stereocilia responsible for higher frequencies are located at the entrance to the cochlea, the stereocilia for lower tones are located deeper inside the organ.
Volume
Fireworks, explosive guitar riffs on a good pair of speakers and the roar of football fans in a stadium — all are sounds that can reach very high volumes. The volume level we register for a sound is dependent on two aspects: Sound pressure and frequency. The higher the sound pressure levels that reach the ear, the more the stereocilia will be set in motion for a correspondingly large number of nerve impulses. The quantity of nerve impulses has a big influence on how we perceive volume.
The frequency of a sound wave also has a large influence on the perception of volume. The human ear is most sensitive to frequencies in the midrange, at about 4,000 Hertz. When it comes to very low and very high frequencies, our ears’ ability to perceive sound decreases. We therefore perceive the volume of these sounds as being lower. In the following low frequency hearing test, note how low the volume of the very low frequencies seems to be:
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Our perception of sound volume is also subjective. Age and levels of stress can all have a big influence on how we perceive volume.
Location
Our auditory perception allows us to determine the direction from which a car is approaching or where someone who calls our name might be standing. When it comes to determining where a sound is coming from, it’s necessary to differentiate between the horizontal plane (left, center, right) and median plane (up, down, back, front), as the ear employs different strategies for each.
Differences in propagation time and volume between the two ears are essential for localizing sound along a horizontal plane. A sound coming from our left side will reach the left ear slightly sooner than the right. This time difference is minimal but is enough for our brain to process and localize the sound. Along the median plane, our ears primarily use frequency response in order to narrow down the source of a sound.
Timbre
No piano sounds just like another and the difference is more than just a matter of pitch. When it comes to music, we use words like “warm,” “edgy,” “lush” and “tinny” to describe a sound. Such auditory attributes fall under the sonic attribute timbre. Timbre primarily describes the way different tones combine. Instruments and even our own voices are comprised of bass notes and overtones which combine to produce complex harmonics. Think of the deeply resonant voice of the late Alan Rickman, the British singer Stuart Staples or Kathleen Turner.
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Coda: The basics of auditory perception
- Sound waves are changes in sound pressure that are transmitted through an elastic medium such as air or water
- Sound waves arrive at the ear and travel through the ear canal, striking the ear drum and converting into an electrical nerve impulse in the cochlea
- Different types of microscopic hair-like cells known as stereocilia within the cochlea are responsible for conveying different aspects of sound
- The inner ear passes a range of complex information on to the brain for processing, resulting in the identification and localization of sound
We’ve attempted to explain the complex and wonderful way in which our body transforms vibrations into a range of sound in honor of World Hearing Day on March 3rd. The World Health Organization using this day each year to advocate for better understanding of human hearing and the sources of hearing damage.
