Science to Live By: Our Senses (Part Three): Hearing


© J. Dirk Nies, Ph.D.

The human ear
The human ear

The world is alive with reverberations and deep with silences. Our sense of hearing brings the world into our minds, touches our hearts, and speaks to our souls. And in our turn, we communicate to the world with the language of sound.  We sing, laugh, cry, speak out and we keep silent.

Our sense of hearing is integral to our lives. Our relationships are nourished by our active attentiveness to the meanings and nuances conveyed in sound. When we speak openly, we desire to be understood; when we listen well, we seek to comprehend.  And when we don’t, anger and frustration can arise. “That’s the most absurd argument I’ve ever heard,” he blurts out impatiently.  She quickly retorts, “Well dear, maybe that’s because you’re not truly listening to me!”  Our word ‘absurd’ comes from the Latin surdus, meaning deaf.

Like touch, hearing is a physical sense. Sounds arise when a vibrating material sets the air in motion.  Our capacity to hear is based upon our ability to detect and to discern these pulsing movements of air. The two most important characteristics of sound that we perceive are its pitch and its loudness.  Pitch, the position of a note on a musical scale, is determined by the frequency of the sound wave measured in Hertz (Hz = one cycle per second). Higher frequencies yield notes of higher pitch. Loudness is determined by the amplitude (the height) of the sound wave measured in decibels (dB).  Larger amplitudes produce louder sounds.

To hear, our ears transform airborne vibrations into electrochemical signals transmitted to our brains. This transformation is a multistep process that occurs as sound traverses from the outer ear through the middle ear to the inner ear. Undulating pressure variations of air (sound) in the ear canal sets the flexible, circular, tympanic membrane (eardrum) to vibrate. The eardrum transmits these vibrations to the middle ear. To help keep the eardrum flexible, the body uses the Eustachian tube to equalize the air pressure in the middle ear with ambient atmospheric pressure. The middle ear holds the smallest bones in the body, the auditory ossicles—the malleus, incus, and stapes. Often referred to as the hammer, the anvil, and the stirrup, the ossicles transmit sound to the inner ear, the cochlea, which is a spiral shaped, fluid filled tube. In the cochlea, sound undergoes yet another change as the vibrating cochlear fluid selectively stimulates, depending upon frequency, specialized hair cells. It is these hair cells that generate electrochemical impulses that are transmitted by the auditory nerve to the brain.  From start to finish, all of this happens in a split second!

As we age, the range of sounds audible to our ears tends to diminish. This most often occurs due to damage to the hair cells or to the fine nerve endings inside the cochlea, leading to reduced perception of sound intensity and quality. About one in six people have some degree of hearing impairment. Hearing aids that amplify sound to overcome the decrease in hearing sensitivity can improve communication for most people suffering hearing impairment.  In cases of more profound hearing loss, a cochlear implant, which replaces the functionality of the inner ear, can offer help.  Cochlear implants process sounds and transform them into electric impulses that stimulate the auditory nerve. And, of course, many people who are deaf compensate with heightened abilities in their other senses.

We can hear sound with frequencies that range in pitch from a very low 20 Hz to a very high 20,000 Hz and amplitudes that range from the barely audible 0 decibels to the exceedingly loud 100 decibels.  Most people find sounds louder than 100 dB uncomfortable and even painful.

Normal speech is conducted with auditory intensities between 50 and 60 dB, and with frequencies between 100 and 150 Hz. When we sing or play music, we often employ a higher pitch than our regular speaking voice. For example, the standard A note has a frequency of 440 Hz and a soprano’s vocal range extends up to “soprano C” at 1,046.5 Hz. Musical instruments can yield sounds beyond the range of our vocal chords. The first key on the piano’s keyboard is tuned to a frequency of 27.5 Hz, while the eighty-eight key is set at 4,186 Hz.

Sound waves with frequencies above what we can hear are referred to as ultrasonic, while those below are referred to as infrasonic. Many animals can generate and hear ultrasonic and infrasonic sounds.

Bats produce ultrasound with frequencies as high as 200 kHz (kHz = 1,000 Hz) by contracting their larynx (voice box) or by clicking their tongues. By rapidly interpreting the ultrasonic echoes that bounce off objects, bats “visualize” their environment, permitting them to navigate and forage for food even in total darkness. At 120-130 decibels, ultrasonic bat calls (which are louder than a smoke alarm blaring four inches from your ear) are among the most intense of all airborne animal sounds.  Bats on the wing are extremely loud, yet are silent to our ears.

At the other end of the frequency spectrum, large land animals such as lions and elephants use infrasound to communicate their location and territory. Similarly, the songs of whales–which can travel hundreds of miles underwater–contain notes sung in infrasound.

Fascinating and potentially calamitous phenomena can occur when an object preferentially absorbs energy at its natural frequency of vibration, called its resonant frequency. If bridges, buildings, trains or aircraft continuously take in energy at their resonant frequency, oscillations with ever-greater amplitude can result, causing violent swayings and even catastrophic failure.

Does this sort of stressful resonance happen in our bodies? The Department of Defense has reported the resonant frequency of the eye to be 18 Hz, while the resonant frequency for the entire human body is in the vicinity of 3-8 Hz. Experimental exposure to a low-level, inaudible 17 Hz tone caused more than 1 in 5 human respondents to report feeling “anxiety, uneasiness, extreme sorrow, nervous feelings of revulsion or fear, chills down the spine, and feelings of pressure on the chest.”  In a real life case in England, Vic Tandy, a university employee, became very anxious while working late at night in a lab. “I felt the hairs rise on the back of my neck.” He saw a grey blob out of the corner of his eye. “It seemed to be between me and the door, so the only thing I could do was turn and face it.” After observing, the following day, an object vibrating wildly in the middle of the lab, Mr. Tandy sought a natural explanation for these phenomena. He eventually discovered that a large building fan was vibrating at a frequency of 18.98 Hz. He postulated that the fan was the culprit of his emotional sensations and visual artifacts. Published reports such as these have led to the hypothesis that, in some cases, infrasound may be the source of eerie visual apparitions and uncanny sensations people have experienced in haunted houses!

Our ability to recognize and interpret sound is a marvelous gift. We possess the extraordinary capacity to sort through cacophony and tune in to the relevant; to hear the sound of a baby’s cry above the noise of the vacuum cleaner. Sound conveys not only information, but music, which is the “art of the Muses.” Oscar Hammerstein and Richard Rodgers captured well the inspiration we all can find in Nature’s melodies:

To laugh like a brook when it trips and falls

Over stones on its way

To sing through the night

Like a lark that is learning to pray

I go to the hills, when my heart is lonely.

I know I will hear what I’ve heard before.

My heart will be blessed with the sound of music,

And I’ll sing once more.