Our ears are extraordinary organs. They are good at recognizing sounds. They are sufficiently sensitive that they’re capable of differentiating between variations in sound equivalent to less than one-billionth of atmospheric pressure, and our brains can identify and memorize complex aural patterns. That ability probably evolved because our ancient ancestors had a better chance of survival if they could tell the difference between, say, the whistle of the wind and the hiss of a snack about to attack. One of the most remarkable things about this process is that it is completely mechanical. Our sense of smell, taste and vision all involve chemical reactions, but our hearing system is based solely on physical movement.
To understand how your ears hear sound, you first need to understand just what sound is.
An object produces sound when it vibrates in matter. This could be a solid, such as earth; a liquid, such as water; or a gas, such as air. Most of the time, we hear sounds traveling through the air in our atmosphere. When something vibrates in the atmosphere, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air.
To see how this works, let’s look at a simple vibrating object: a bell. When you hit a bell, the metal vibrates — flexes in and out. When it flexes out on one side, it pushes on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them, and so on. This is called compression. When the bell flexes away, it pulls in on the surrounding air particles. This creates a drop-in pressure, which pulls in more surrounding air particles, creating another drop-in pressure, which pulls in particles even farther out. This pressure decrease is called rarefaction. In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. We hear different sounds from different vibrating objects because of variations in the sound wave frequency. A higher wave frequency simply means that the air pressure fluctuation switches back and forth more quickly. We hear this as a higher pitch. When there are fewer fluctuations in a period, the pitch is lower. The level of air pressure in each fluctuation, the wave’s amplitude, determines how loud the sound is.
To hear sound, your ear must do three basic things:
- Direct the sound waves into the hearing part of the ear
- Sense the fluctuations in air pressure
- Translate these fluctuations into an electrical signal that your brain can understand
The following explains how this deceptively simple process works.
The Human Ear is constructed with three sections.
- The outer ear
- The middle ear
- The inner ear
The outer ear is the visible part, also called the pinna or auricle. It consists of skin and cartilage with muscles attached to the back. The pinna’s primary function is to collect and direct sounds down the ear canal. Constructed of twists and folds that enhance high frequency sounds and help determine the direction of the sound source, sounds coming from the front and to your side are slightly enhanced as they are directed into the ear canal, while sounds from behind you are somewhat less pronounced. This helps you hear whatever you are looking at while reducing distracting background noise. If your pinna is functioning normally, it is providing about 5 dB (DeciBels) of high frequency enhancement, as well.
The ear canal is also part of the outer ear. This letter S shaped passage is approximately one inch in length, and roughly the diameter of a pencil eraser. The outer portion of the ear canal is surrounded by cartilage and contains glands that produce cerumen (more commonly known as earwax) while the inner portion is encased by bone.
A word about earwax—although you may think of it as an annoyance to be removed, it provides the following three vital functions:
- Protects your skin and tympanic membrane from drying out
- Helps guard against intrusion by foreign bodies (e.g., dust, insects)
- Helps keep unhealthy bacteria from multiplying
The resonant tubes of the ear canal also increase gain (amplification) by 15–25 dB as sound is conducted toward the middle ear.
Your middle ear is separated from the outer ear by the tympanic membrane (eardrum). Directly behind the eardrum is a row of three tiny bones collectively known as the ossicles.
These consist of the following:
- Malleus (the hammer)
- Incus (the anvil)
- Stapes (the stirrup)
The ossicles vibrate in response to eardrum stimulation, amplifying and relaying sound to the inner ear through an oval window. They also boost the sound gain by an additional 27 dB.
The opening to the Eustachian tube also ventilates your middle ear, connecting it to the back of your throat. This tube opens when you swallow, yawn, blow your nose, or cough.
Your inner ear, which is shaped a lot like a snail’s shell, is divided into two functionally separate sections, the vestibular, or balance organ and the cochlea, or hearing organ. The cochlea conducts high frequencies at its base and low frequencies at its apex.
You lose hearing at the higher frequencies earlier because the sound wave always passes through the base first.
The sound wave causes the fluid in the inner ear to move, stimulating rows of thousands of tiny hair cells (hearing nerve cells) inside the cochlea for each frequency. These hairs, which assist in how you perceive the loudness of a sound among other functions, release neurotransmitters via the auditory nerve to the brain, which will interpret them as sounds. These hair cells are delicate and can be damaged or destroyed due to overexposure to loud noise, genetic predisposition, or the aging process. Once you lose a hair cell, it cannot be replaced with today’s technology. The more of these you lose, the less hearing you retain.