The Science of Sound and How You Can Hear

By Dr. Mercola

In the video above, medical illustrator Brandon Pletsch gives a concise and straightforward explanation of a very complex topic: how you can hear. At the most basic level, you hear through a process known as auditory transduction.

Your ear converts sound waves in the air into electrical impulses that can be interpreted by your brain. Your ears capture sound traveling through the air as vibrations in air pressure. The outer part of your ear, or pinna, catches the sound waves first.

Once sound waves pass your outer ear, they enter your ear canal and trigger vibrations to your eardrum, a thin piece of skin that sits between the ear canal and your middle ear.

Your eardrum vibrates faster from high-pitch sounds while loud sounds move your eardrum back-and-forth a greater distance. Once sound waves vibrate your eardrum, it moves a group of tiny bones in your middle ear called the malleus, the incus, and the stapes.

Collectively known as the ossicles, these are the smallest bones in your body, but they have a very important job – amplifying the force from your eardrum so the sound information can be passed on to your inner ear.

The Role of Your Inner Ear and Brain

The amplified vibrations from your eardrum travel to the cochlea in your inner ear, which conducts sound through fluid (instead of through air, as is done in your outer ear). It's here that the sounds are translated into nerve impulses that your brain can recognize and understand as distinct sounds. According to How Stuff Works:¹

"The cochlea structure consists of three adjacent tubes separated from each other by sensitive membranes… The stapes moves back and forth, creating pressure waves in the entire cochlea.

The round window membrane separating the cochlea from the middle ear gives the fluid somewhere to go. It moves out when the stapes pushes in and moves in when the stapes pulls out.

The middle membrane, the basilar membrane, is a rigid surface that extends across the length of the cochlea. When the stapes moves in and out, it pushes and pulls on the part of the basilar membrane just below the oval window.

This force starts a wave moving along the surface of the membrane. The wave travels something like ripples along the surface of a pond, moving from the oval window down to the other end of the cochlea.

The basilar membrane has a peculiar structure. It's made of 20,000 to 30,000 reed-like fibers that extend across the width of the cochlea. Near the oval window, the fibers are short and stiff. As you move toward the other end of the tubes, the fibers get longer and more limber.

This gives the fibers different resonant frequencies… Because of the increasing length and decreasing rigidity of the fibers, higher-frequency waves vibrate the fibers closer to the oval window, and lower frequency waves vibrate the fibers at the other end of the membrane."

When a wave reaches fibers with a resonant frequency, it releases a burst of energy that then moves tiny hair cells found in the organ of corti, a structure that stretches across the length of the cochlea.

The movement from the hair cells sends an electrical impulse through the cochlear nerve, which in turn transmits the information to the cerebral cortex in your brain for interpretation.

Everyday Noise Could Cause You to Lose Your Hearing

It used to be thought that hearing loss was typically the result of hair cells in your ear degrading with age. However, excessive noise will also kill cells in your inner ear. The longer you're exposed to loud noise, the more hair cells in your ears will be destroyed.

Your hearing worsens as the number of hair cells in your ears decreases, and once they're gone, there's currently no way of restoring your hearing to what it once was.²

Most people are aware that exposure to loud noises, such as at a rock concert, can damage your hearing over time, but many may be surprised at the many everyday noises that may contribute to hearing loss.

According to the World Health Organization (WHO), more than 1.1 billion teens and young adults may be at risk of losing their hearing due to unsafe levels of recreational noise.³

WHO analyzed data from middle- and high-income countries and revealed nearly half of those aged 12 to 35 years are exposed to unsafe levels of sound from personal audio devices (such as smartphones).

Further, about 40 percent are exposed to potentially damaging levels of noise at entertainment venues, such as nightclubs, bars, and sporting events. WHO estimates that over 43 million young people worldwide are living with disabling hearing loss due to recreational exposure to loud sound.

The highest permissible level of noise exposure in the workplace, according to WHO recommendations, is 85 decibels (dB) for a maximum of eight hours. They note that many are exposed to levels of 100 dB or more while in entertainment venues, but this level of noise is only considered safe for a maximum of 15 minutes.

For comparison, the American Hearing Research Foundation has compiled this chart of approximate decibel levels of common sounds:⁴

Approximate Decibel Level	Examples
0 dB	The quietest sound you can hear
30 dB	Whisper, quiet library
60 dB	Normal conversation, sewing machine, or typewriter
90 dB	Lawnmower, shop tools, or truck traffic; 8 hours per day is the maximum exposure (protects 90 percent of people)
100 dB	Chainsaw, pneumatic drill, or snowmobile; 2 hours per day is the maximum exposure without protection
115 dB	Sandblasting, loud rock concert, or auto horn; 15 minutes per day is the maximum exposure without protection
140 dB	Gun muzzle blast, jet engine; noise causes pain and even brief exposure injures unprotected ears; maximum allowed noise with hearing protector

Noise-Induced Hearing Loss May Be a Hidden Epidemic

The pure tone audiometry test is most often used to assess hearing, but it tests hearing in a quiet setting. While your hearing may test fine according to this test, you could still have trouble hearing in real life, when you must pick out sounds in the midst of background noise. Researchers believe this "hidden hearing loss" may be on the rise. As reported by The Daily Mail:⁵

"… [N]oise may actually cause irreversible damage to the connections (called synapses) between hair cells and nerve cells in the cochlea, so that they cannot send information to the brain. This is thought to be what's going on in hidden hearing loss, says Chris Plack, a professor of Audiology at Manchester University.

… The damage is thought to occur because over-stimulated hair cells produce an excess of the chemical glutamate, which is toxic in large amounts. Scientists hope to one day be able to reverse this damage by treating the surviving neurons with neurotrophins (molecules which promote healthy nerve growth), injected through the eardrum."

Signs you may have hidden hearing loss include:⁶

• Difficulty understanding speech when there is background noise (but not in quiet environments)
• Not being able to understand what people are saying on TV even when the volume is loud
• Struggling in social or work situations when there are lots of people talking
• Standard hearing tests say your hearing is fine, but you still struggle

There is, unfortunately, no widely available test to reveal hidden hearing loss. Dr. Etienne Krug, WHO Director for the Department for Management of Noncommunicable Diseases, Disability, Violence, and Injury Prevention, stated:⁷

"As they go about their daily lives doing what they enjoy, more and more young people are placing themselves at risk of hearing loss… They should be aware that once you lose your hearing, it won't come back. Taking simple preventive actions will allow people to continue to enjoy themselves without putting their hearing at risk."

How to Avoid Hidden Hearing Loss

Worldwide, 360 million people have moderate to severe hearing loss due to various causes, from noise or infectious disease to the use of certain drugs and aging. It's estimated that half of these cases of hearing loss are avoidable.⁸ This includes cases of hidden hearing loss or noise-induced hearing loss. WHO recommends teens and young people take the following steps to protect their hearing and avoid hearing loss:

Turn down the volume on personal audio devices	Try a decibel meter app for your smartphone, which will flash a warning if the volume is turned up to a potentially damaging level	Wear earplugs when you visit noisy venues
Use carefully fitted noise-cancelling earphones/headphones, which may allow you to listen comfortably at a lower volume	Limit the amount of time you spend engaged in noisy activities	Take regular listening breaks when using personal audio devices
Restrict the daily use of personal audio devices to less than one hour

On a larger scale, Konstantina Stankovic, an auditory neuroscientist and surgeon at Massachusetts Eye and Ear, and Harvard Medical School, believes loud noises should be banned in public spaces just as smoking is. Certain countries have established laws to monitor noise levels in bars and clubs in order to protect workers' health, while in 2014 the Minneapolis City Council passed a law requiring bars and clubs to offer free ear buds to customers.⁹New Scientist reported:¹⁰

"Stankovic thinks more will need to be done to change socially accepted norms around recreational noise. 'I think it will require a public health effort similar to the efforts for limiting smoking, because of the peer pressure associated with loud music and noisy environments,' she says. Many of us find events or venues uncomfortably loud, but we assume we are the only ones that do, she says, because everyone seems to be having such a good time."

Is Noise Pollution a Problem for You?

Recreational noise is just one type that can damage your hearing. In the US it's estimated that 100 million people are exposed to unhealthy levels of noise pollution, typically from automobile and aircraft traffic (although everything from leaf blowers and lawnmowers to loud music can also contribute).¹¹ Noise pollution may increase your risk of hearing loss, but it goes much further than that. Stress, sleep disturbances, diminished productivity, and even heart disease can also result.

One of the key ways excessive noise harms your heart is by elevating stress hormones such as cortisol, adrenaline, and noradrenaline, which, over time, can lead to high blood pressure, stroke, and heart failure. Research also suggests long-term exposure to noise pollution may have an affect on cognitive development in children and cognitive and psychological functions in adults.¹² If you live in a very noisy area, such as near a highway or airport, you may want to consider moving.

If that is not an option, consider adding acoustical tile to your ceiling and walls to buffer the noise. Double-paneled windows and insulation can also help. At the very least, you can sound-treat your home by adding heavy curtains to your windows, rugs to your floors, and sealing air leaks. If noise is only an issue occasionally, sound-blocking headphones can eliminate such disturbances.

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