My husband says I have “bat ears”—that I can hear him whispering a mile away. While that may be an exaggeration, I do admit that I have sensitive hearing. That’s not always a good thing.  I have little tolerance for loud noises (at concerts or in movie theaters) and high-pitched sounds set my teeth on edge.  So imagine my surprise when I took an online hearing test (linked below) and discovered that my hearing isn’t as good as it could be for my age.  At 46, I am already experiencing age-related hearing loss. But I take care of my hearing; how did this happen?

To understand how hearing loss occurs, we need to review the science of hearing.

The Science of Hearing

What is Sound?

The science of sound is so interesting that it deserves a post all its own (coming soon). Simply, sound is a wave that travels through some sort of medium (like air or water). As sound travels, it causes the substance through which it travels to vibrate. Those vibrations are passed from one particle to another as the sound wave moves.

This video reviews the properties of waves including sound waves.

Like all waves, each sound wave has a wavelength (the distance between two sound waves), an amplitude (how “high” each wave reaches), and a frequency (how many waves pass a given point per second).

The properties of a wave:  wavelength, frequency, amplitude, and velocity of propagation
The Properties of a Wave

A sound wave’s amplitude correlates with how “loud” a wave is.  A sound wave’s frequency relates to pitch. Sound waves with high frequencies result in high-pitched sounds while sound waves with low frequencies produce low-pitched sounds. Because of the limits of the ear, humans are only capable of hearing sounds with frequencies ranging from 20-20,000 Hertz (abbreviated Hz). Not surprisingly, scientists have determined that certain animals can perceive sounds that humans can not.

The Human Ear and How We Hear Sound

The human auditory system, showing the location of the ear canal, eardrum, malleus, incus, and stapes, and the cochlea
The Human Auditory System

The human ear is a marvel of engineering.  

We hear when sound waves enter our ears through the ear canal. Within the middle ear exists a thin membrane called the tympanic membrane (more commonly known as the eardrum).   The sound waves cause the eardrum to vibrate, and these vibrations are transferred to a series of three bones attached to the eardrum: the malleus, incus, and stapes (also called the hammer, anvil, and stirrup, respectively.  Interestingly enough, the stapes is the smallest bone in the human body.) Vibrations pass from these bones to a region of the inner ear called the cochlea.

The cochlea resembles a snail shell, is filled with fluid, and is lined with special hair cells.  As sound vibrations are transferred to the cochlea, the sound waves cause this fluid to vibrate which in turn vibrates the hair cells.  These hair cells convert the vibrations into electrical signals which they send to the auditory nerves. These signals travel to the brain where they are recognized as sound. Different hairs within the cochlea respond to different frequencies of sound.

This video does a great job of reviewing the process of how our ears allow us to hear sound.

What Causes Hearing Loss?

Hearing loss occurs when the hairs within the cochlea (called stereocilia)  become damaged. While certain animals are capable of regenerating damaged stereocilia, humans are not.  In humans, once stereocilia are damaged they can not be restored.

Stereocilia damage can occur following exposure to noises that are either too loud or last too long. This is typically referred to as noise-induced hearing loss. Sound is measured in the unit decibels.  Sounds less than 75 decibels won’t cause hearing loss, but sounds above 85 decibels will. The louder the sound (the higher the decibels), the less time it takes to develop hearing loss.  The following table lists the decibel level of some common sounds.

Age-related hearing loss is thought to arise from a combination of years of long-term noise exposure and the process of growing older.  A degree of hearing loss is inevitable as we age, as a lifetime of daily exposure to noise takes a toll on our ears. Additionally, certain health conditions which are more common in older people (including high blood pressure and diabetes), damage to the eardrum, or reduced friction in the ear bones can also damage the auditory system and lead to hearing loss. Scientists have also demonstrated that genetic mutations to the DNA within the mitochondria of the cells within our ears can result in age-related hearing loss*.

Typically, hearing loss begins with an inability to perceive higher pitched sounds. Over time, hearing loss may extend to sounds at lower frequencies.  If the hearing loss is severe enough to impact daily life, hearing aids are available.

This short video explains how hearing aids work to restore hearing.

How Well Do You Hear?

Disclaimer: these hearing tests should not take the place of a hearing test performed at a doctor’s office. Additionally, an inability to hear certain frequencies may not be due to actual hearing loss. Instead, it may relate to the speakers you are using to listen to the sounds. If you are concerned about your hearing, please see your doctor or an audiologist.

Now that you understand about how our ears are designed to hear and how hearing loss occurs, it’s time for you to test your hearing. Simply watch this video and listen to the sounds. Note when you stop hearing the beeps and you will learn how “old” your ears are. I have watched this video in a classroom of middle school students and they continued to hear sound long after I no longer could.  In fact, they heard sounds so easily that they thought I was only pretending not to hear them.

Next, if you want to really pin down your range of hearing, watch this video and note when you start hearing sound and when you can no longer hear.  If you have pets, they might not like the high frequency sounds at the end of the video.

So what were your results?  Have you already experienced hearing loss? Unfortunately for me, it appears that I may already have experienced hearing loss at both the upper and lower frequencies.  

While there’s nothing you can do to repair the damage to your ears that may already have occured, you can do your part to prevent further hearing loss.  Avoid loud noises if at all possible and wear earplugs if you must be exposed to high decibel sounds.

Take care of your hearing!

Further Information

*If you’d like to learn more about mitochondrial DNA, consider enrolling in my course Genetics and Heredity. In it, students will discover how the information contained within DNA is expressed as observable traits.  They will learn about gene alleles and understand the difference between genotype and phenotype as they investigate what traits they and their family members express.  Students will learn about meiosis and how it leads to genetic diversity in offspring. Students will learn about Mendelian inheritance and how to use Punnett Squares to predict the outcome of monohybrid and dihybrid crosses.  Additionally, students will learn about patterns of non-Mendelian inheritance including sex-linked traits, the inheritance of mitochondrial DNA, incomplete dominance, codominance, and polygenic traits. Throughout the course, students will be exposed to many real-life examples of genetics in action as they learn about the genetics of calico cats, human blood types, how hemophilia was spread throughout the royal houses of Europe, and much more.  

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What is sound? How do we hear? What causes hearing loss? How "old" are your ears? Take these hearing tests to find out. #homeschool #science

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