How the Ear Works
As a premier specialty hospital in New York City dedicated to hearing disorders, the Ear Institute offers the latest medical and surgical treatments and support services to children, adults, and seniors affected by hearing and balance disorders in a safe and comfortable environment.
How Do We Hear?
There are three parts to the ear: the outer ear, the middle ear, and the inner ear. Each part has special functions and can be affected by specific diseases that cause different symptoms and problems.
The Outer Ear
The outer ear has two major parts: the pinna and the external auditory canal. The pinna is that part of the ear that protrudes from the side of the head and is clearly visible when we look at someone. The pinna is the part of the ear with which we are all familiar. The function of the pinna is to collect sounds and funnel them towards the organ of hearing deep within the head. The external auditory canal is the channel that carries the sound collected by the pinna to the ear drum (tympanic membrane).
The Middle Ear
The middle ear is bounded on one side by the ear drum and on the other by the inner ear. The function of the middle ear is to take the sound collected by the outer ear and concentrate and direct that sound to the inner ear. As the sounds collected by the outer ear strike the ear drum, the ear drum vibrates (like the top of a musical drum). There are three little bones in the middle ear that act together as a lever mechanism and direct the sound energy from the vibrating ear drum to the inner ear. The first bone of hearing, attached to the ear drum, is called the malleus. The middle bone of hearing is called the incus. The smallest bone in the human body, and the one that presents sound to the inner ear, is called the stapes. These three bones act as a lever mechanism and transmit sound from the ear drum to the inner ear.
The Inner Ear
The inner ear has two key components: the organ of hearing and the organ of balance. The organ of hearing is called the cochlea. The cochlea contains fluid and little cells, which are called “hair cells” because they are topped with tiny “hairs” that float in the fluid of the inner ear. Each of the hair cells connects to a branch of the nerve of hearing that goes from the inner ear to the brain.
The stapes bone sits in a window that separates the middle ear from the inner ear, called the oval window. When the stapes bone moves, the fluid of the inner ear moves, and the little hairs on top of the hair cells bend. When these hairs bend the mechanical energy coming from the outer ear and middle ear is converted into an electrical signal that travels up the nerve of hearing to the brain, where we perceive sound and speech.
The organ of balance is called the labyrinth. There are hair cells in the labyrinth that are similar to those in the organ of hearing. When we move these hairs bend and we perceive motion. When we turn, we are aware that we are turning because the inner ear is being stimulated. When we go up in an elevator, we sense that we are going up because the inner ear is being stimulated. Vertigo is defined as the false sensation of motion that often happens when the inner ear malfunctions.
Types of Hearing Loss
Hearing loss may be unilateral (occurring in only one ear), or it may be bilateral (occurring in both ears). Bilateral hearing loss may be symmetrical (the same in both ears) or asymmetrical (the degree, severity, and/or configuration of the hearing loss may be different in each ear).
There are the following three types of hearing loss:
- Conductive hearing loss is a mechanical hearing loss due to a problem with the outer or middle ear. The mechanism that conducts sound from our environment to the inner ear, where it is processed into an electrical signal, is not working properly. Examples of conductive hearing loss include blockage of the ear canal by ear wax, a hole in the ear drum, an infection in the middle ear, or a problem with any of the three tiny bones of hearing.
- Sensorineural hearing loss is a term used interchangeably with neurosensory hearing loss and nerve hearing loss. Nerve hearing loss is a misnomer because the nerve of hearing is rarely affected. Sensorineural hearing loss almost always happens when the hair cells within the inner ear are not working properly. Examples of sensorineural hearing loss include the genetic hearing loss that most commonly occurs in newborn children and the hearing loss common as we all age.
- Mixed hearing loss means some conductive and some sensorineural.
There are two key aspects to hearing, regardless of the type of hearing loss. One aspect is the ability to hear a sound (pure tones) and the second aspect is the ability to understand words (speech understanding or discrimination). Hearing is useful only if you can understand what you are hearing. Usually, as sound is made louder it also becomes clearer. However, there are some inner ear diseases where the ability to hear a sound is good but speech understanding is poor.
Noise and Hearing Protection
The majority of people begin experiencing a decline in their hearing as they get older. If they are healthy and were not exposed to extreme noise, the symptoms of hearing loss may begin to show after the age of 60. However, excessive noise exposure is dangerous and can result in permanent hearing loss at an early age. For example, by age 25, the average carpenter has the same hearing as someone who is 50 years old and has worked in a quiet job.
You cannot train your ears to accept loud noises. Once the delicate inner structure of the ear is damaged by noise, no medical or surgical treatment will restore your hearing to “normal.” Even hearing aids are only able to improve your hearing.
How Does “Normal” Hearing Work?
Hearing depends on a series of events that change sound waves in the air into electrical signals. Our auditory nerve then carries these signals to the brain through the following complex series of steps:
- Sound waves enter the outer ear, travel down the ear canal, and make the ear drum vibrate.
- This action moves the tiny chain of bones in the middle ear. The last bone in this chain ‘knocks’ on the membrane window of the cochlea and makes the fluid in the cochlea move.
- The fluid movement then triggers a response in the hearing nerve located in the inner ear. Our brain interprets the nerve impulses as sounds.
How Does Noise Damage Hearing?
Since the nerve endings in the inner ear send impulses to the brain, which are then interpreted as sound, a decrease in the number of nerve endings will impact a person’s ability to hear. Noise can destroy the nerve endings in the inner ear. When the nerve endings are killed, sensorineural hearing loss or nerve deafness result. Hearing loss will grow progressively worse as the number of destroyed nerve ends grows. At this point, the hearing loss is permanent because the damaged nerve endings cannot be restored.
What Does it Mean to be Hard of Hearing?
“Hard of hearing” denotes a mild to severe hearing deficit. People who are hard of hearing may benefit from the use of hearing aids or other assistive listening devices. However, it should be noted that hearing aids can only improve a person’s hearing, they cannot restore it to “normal.”
What is Noise-Induced Hearing Loss (NIHL)?
Every day we experience sounds from televisions, lawnmowers, household appliances, construction equipment, and traffic. Normally, these sounds are at safe levels that don’t damage our hearing. But sounds can be harmful when they are too loud or both loud and long-lasting. In addition, noise exposure is cumulative. So the noise at home, at work, or at play must be counted in the total exposure during the day. The damage to the nerve endings of the inner ear as a result of all the noise exposure can cause noise-induced hearing loss (NIHL).
NIHL can be immediate or it can take a long time to be noticeable. It can be temporary or permanent, and it can affect one ear or both ears. The good news is noise-induced hearing loss is something you can prevent by using hearing protection devices and avoiding unprotected exposure to dangerous noise levels.
If you must work in an excessively noisy environment, you should wear protectors. You should also wear them when using power tools, noisy yard equipment, or firearms, or riding a motorcycle or snowmobile.
Loud noise exposure can also cause tinnitus—a ringing, buzzing, or roaring in the ears or head. For some, the symptoms of tinnitus may subside over time. But others can experience tinnitus constantly or sporadically throughout a their lifetime. Hearing loss and tinnitus can occur in one or both ears.
What is a Decibel, and How Does It Impact Hearing?
A decibel (dB) is a unit that measures the intensity or loudness of sound. Sounds of less than 75 decibels, even after long exposure, are unlikely to cause hearing loss. However, long or repeated exposure to sounds at or above 85 decibels can cause hearing loss. The louder the sound, the shorter the amount of time it takes for NIHL to happen.
Here are the average decibel ratings of some familiar sounds:
- Whisper, quiet library: 30 decibels
- The humming of a refrigerator: 45 decibels
- Normal conversation: 60 decibels
- Noise from heavy city traffic: 85 decibels
- Motorcycles: 95 decibels
- An MP3 player at maximum volume: 105 decibels
- Sirens: 120 decibels
- Firecrackers and firearms: 150 decibels
While people have different tolerance levels to noise, two simple rules can help determine when noise is dangerous to your health:
- First, if you have to raise your voice to talk to someone who is an arm's length away, then the noise is likely to be hazardous.
- Second, if your ears are ringing, or if sounds seem dull or flat after leaving a noisy place, then you probably were exposed to hazardous noise.
The impact of noise adds up over a lifetime. If you are exposed to loud sounds on a regular basis, your risk for permanent damage increases over time. Even a single but long-lasting loud event can cause damage. Limit your exposure to sounds at or above 100 decibels to no more than 15 minutes, and limit sounds that are at or above 110 decibels to no more than one minute.
What Do I Need to Know About Occupational Noise Exposure?
Noise may be a problem in your workplace if:
- You hear ringing or humming in your ears when you leave work.
- You have to shout to be heard by a coworker an arm's length away.
- You experience temporary hearing loss when leaving work.
The Occupational Safety and Health Administration (OSHA) sets standards and legal limits on noise exposure in the workplace. In 1981, OSHA implemented new requirements to protect workers in general industry (e.g. the manufacturing and service sectors). Employers were required to implement a Hearing Conservation Program for workers who are exposed to a time weighted average noise level of 85 dB A or higher over an eight-hour work shift.
Hearing Conservation Programs require employers to measure noise levels, provide free annual hearing exams and free hearing protection, provide training, and evaluate the adequacy of the hearing protectors in use, unless changes are made to reduce worker noise exposure to less than 85 dB A. Workers should check with their employers for specific information.
What are Hearing Protectors? What Types are Available?
In order to decrease the intensity of sounds reaching the ear drum, proper hearing protection devices should be worn. Simply stuffing your ears with cotton balls or tissue paper will not offer sufficient protection since they only reduce noise by approximately seven decibels. There are many hearing protection options available, and choices vary depending on whether you’re mowing the lawn or working in a factory. Here are some common options:
- Expandable foam plugs: These plugs are made of a formable material designed to expand and conform to the shape of each person's ear canal. A few manufacturers now offer a small size expandable plug.
- Pre-molded, reusable plugs: Pre-molded plugs are made from silicone, plastic, or rubber and are manufactured as either “one-size-fits-most” or are available in several sizes. Many pre-molded plugs are available in sizes for small, medium, or large ear canals, Keep in mind you may need a different size plug for each ear.
- Canal caps: Canal caps often resemble earplugs on a flexible plastic or metal band. The earplug tips of a canal cap may be a formable or pre-molded material. Some have headbands that can be worn over the head.
- Earmuffs: Earmuffs come in many models designed to fit most people. They work to block out noise by completely covering the outer ear.
The best hearing protector is the one that is comfortable and convenient, and that you will wear every time you are in an environment with hazardous noise.
Do Hearing Protectors Work?
Properly fitted earplugs or muffs can reduce noise 15 - 30 decibels. When measuring the effectiveness of hearing protectors, studies have shown that one-half of the workers wearing hearing protectors receive one-half or less of the noise reduction potential of their protectors because these devices are not worn continuously while in noise, or because they do not fit properly. Even if ear protectors are worn continuously they do little good if there is an incomplete air seal between the hearing protector and the skin.
A useful sign that the hearing protectors are properly positioned and properly fit inside the ear canal is when the wearer’s own voice sounds louder and deeper.
Do Hearing Protectors Prevent a Person from Communicating with Others?
People with normal hearing will not have trouble understanding regular conversation while wearing hearing protection. If you use the sunglasses analogy, whereby sunglasses help vision in very bright light, hearing protectors help enhance speech understanding in very noisy places. However, people with compromised hearing or problems understanding speech will have difficulty understanding normal conversation while wearing hearing protection devices.
How Can I Tell if I Have Hearing Loss?
The progression of hearing loss is usually painless and gradual. But there is cause for concern if you are experiencing any of these symptoms:
- Trouble understanding what people say. They may seem to be mumbling, especially when you are in a noisy place, such as a party.
- Difficulty hearing high pitched sounds, like the voices of women and children
- Difficulty differentiating between certain words that sound alike, especially words that contain S, F, SH, CH, H, or soft C sounds
- Ringing or other sounds in your ear (called tinnitus)
If you suspect hearing loss, consult a physician with special training in ear care and hearing disorders. This doctor can diagnose your hearing problem and send for additional diagnostic exams, including hearing tests.