Hearing offers a richness to our daily lives that is easily taken for granted. It provides us with warnings about impending danger, it provides us with joy as we listen to music, or conversation with a friend or loved one. Its intricately designed system may surprise you as sound is navigated from external vibrations to the auditory nerve and directly to the brain for processing.
How Does Hearing Work?
Tonotopic organization is the system or map of how we hear. We hear sounds through a wave of pressure that is produced from an object that vibrates or from a release of compressed air. These vibrations and movements trigger nerve impulses that are perceived as sound.
The Mechanics of the Ear
Divided into three anatomical parts, external, middle and inner ear, sound waves travel from the external part of the ear to the auditory canal which then causes eardrum vibrations to travel to the middle ear along tiny bones. These bones called ossicles intensifies the vibrations as it travels to the basilar membrane of the inner ear. These three bones called ‘the hammer or malleus’, ‘the anvil or incus’ and ‘the stapes or stirrup’ are the smallest bones within the human body. The middle ear is filled with air, while the inner ear is filled with fluid and it is with this increase and decrease in pressure between the two that causes the tympanic membrane to vibrate. These vibrations travel to the inner ear by the amplified vibrations from the ossicles. These ossicles have another function as well, and that is to offer some protection from potentially damaging sounds. The vibrations then reach the receptor cells within the organ of corti, which layer the cochlea (a shell shaped structure), and continues to send nerve impulses from chemical neurotransmitters from the cilia through the auditory nerve into the brain. Scientists have found that the right ear cilia are more receptive to speech while the left ear cilia are more receptive to music. So if you have difficulty understanding someone, perhaps turn your right side toward their voice.
The snail shaped structure called the cochlea contains fluid that flows against the basilar membrane. The wave like motion of the membrane now causes the cilia to flow back and forth brushing up gently against the tectorial membrane which then causes the cilia to release a chemical neurotransmitter signal along to the auditory nerve and onto the brain for processing.
There are three characteristics of sound.
- Loudness – dependent upon the intensity of the wave pressure which also relates to the amplitude (or height) of the wave.
- Pitch – dependent upon the frequency of the wave, which one cycle equaling one Hertz. The frequency to which humans can hear is ranged between 20-20,000 Hz.
- Timbre – dependent upon the complexity of a sound wave. A single frequency produces a very pure tone such as the sound of a flute. Several waves with different frequencies produce far more common sounds. Each combination of frequencies is what produces a timbre. The combination of frequencies gives each timbre its distinguishability.
Several frequencies in no particular order produce what can be considered as noise. When all frequencies across the spectrum occur, this produces white noise, such as the background sound of a fan.
How we Determine Different Pitches
The movement of the basilar membrane determines the neural firings to the brain. High pitched sounds are determined by the location of where the activity happens along the basilar membrane. Neural firings depend on the location, and different pitches depend on vibrations.
Complete deafness is rare and occurs in only 1% of individuals with hearing impairment. One factor relative to the low prevalence is the ability of alternative auditory pathways to take over for any damaged pathways, much like neuroplasticity.
A large factor in hearing loss is damage to the cilia. This damage is classified as nerve deafness, along with damage to the cochlea or auditory nerve.
A second classification of hearing impairment is (conductive deafness), or damage to the ossicles.
It is very important to receive the proper treatment for hearing loss in a timely manner because the longer the auditory neural pathway is unused the higher probability of permanent degeneration.
There are several various hearing disorders;
Most of them fall into
- Conductive hearing loss
- Problems with inner ear, ossciles, ear drum or ear canal.
- Sensorineural hearing loss
- Inner ear and nerve related
- Mixed hearing loss
- Combination of both
Conductive Hearing Loss
This can include ear malformation, infection or fluid that interferes with the wave movements.
Conductive hearing loss can also be caused by allergies, tumors, otosclerosis (deformity to the stapes which prevents the sound from entering the middle ear).
Sensorineuronal Hearing Loss
This can include, but not limited to; loud noise, head trauma, disease, genetics and tumors.
This is a condition that causes a ringing, buzzing or pulsing within the ear. Tinnitus is unique and varies greatly between individuals and causes such as age, injury or a circulatory disorder. Specialized aids can be used to ‘fill sound voids’ and eliminate tinnitus symptoms.
Cardiovascular Disease and Hearing Loss
Harvard University Researchers discovered that hearing loss was found in 54% more often in those who suffered from heart disease. According to researchers, hearing loss could actually be one of the first signs of heart disease. As cardiovascular disease affects blood flow, when this is affected it further affects nerves within the inner ear.
Aging and Hearing Loss
As we age, our hearing capabilities tend to age as well. Some may even begin to experience a decline in hearing in their mid-30’s. The higher tones become more difficult to hear, and the loudness scale tends to become compromised.
Unfortunately, at this time, hearing loss does not have a cure; however, there are many devices available to aid those with hearing disorders. Hearing aids are available in ranges from basic to highly intricate. Basic hearing aids consist of a microphone, amplifier, and volume control that is battery operated. A complex hearing aid can be digital offering more flexibility within the different noise ranges and within different environments.
This is a small and very complex device that is used for those that have a severe hearing disorder. It consists of a microphone, and a speech processor which converts this to electrical impulses to be sent to the auditory nerve. The cochlear implant will have signals bypass the injured area of the ear and transmit directly to the auditory nerve. This however does take time to learn as these are actually sound ‘signals’ that require learning.
Middle Ear Implants
This implant consists of two parts, one external processing device, and one internal implant (just under the skin which is the receiver. The internal receiver produces a vibrating motion on the middle ear bones and then relies on the cochlea and auditory nerve to be in working condition for processing the sound.
Interestingly, the decibel (dB) derives from a measurement of telecommunication from the ‘Bell System’ during the 20th century, and named after Alexander Graham Bell. ‘Bel’ is a measurement of sound intensity, one decibel equals .01 bel.
An example of typical decibel (dB) levels:
- 30 dB = quiet whisper
- 60 dB = sewing machine
- 90 dB = lawn mower
- 140 dB = gunshot blast, or jet plane @ 15metres away
According to a dB loudness comparison chart, 140 dB is not recommended to listen to without proper sound protection to avoid severe damage
- 180 dB = kills heavy tissue
- 194 dB = loudest sound possible
The Danger of Loud Sounds!
It is incredible how the cilia within the cochlea can provide such detailed messages for processing, yet one thing that some may not realize is how delicate cilia really is. Repeated loud noise such as excessively loud music from headphones, or a car stereo can cause permanent damage to the tiny hair-like receptor by bending them, or causing them to lay flat. Once these receptors are broken, laid flat, or damaged, they are incapable of being repaired. Even repeated close in proximity exposure to loud traffic, or sirens can cause permanent damage to your hearing.
A good rule of thumb to always keep in mind, is that hearing is not guaranteed to last, taking precautions to avoid unnecessarily loud sounds can save your hearing, and hopefully enable you to continue hearing all the great things in the world worth hearing!
- Wade, Carole; Tavris, Carol; Saucier, Deborah; Elias, Lorin; (2007). Psychology, Second Canadian Edition. Pearson
- Pinel, J. P. (2011). Biopsychology (8th ed.). Boston, MA: Pearson.
- Bjorklund, Barbara R.,(2011).The Journey of Adulthood.(7th ed.).New Jersey. Pearson