The racing season is here. Some people like the speed, some
the danger and the crashes, and some people actually like
the noise. But be careful, noise can cause pain and
When your vocal cords vibrate as air passes through the gap between them or as you pluck a guitar string, the movements cause air molecules to vibrate and move with increased energy. The atoms in the air are pushed in out away from the source, where they run into the molecules next to them. This increase in the energy and number of molecules translates as an increase in air pressure. The energy is passed on to the adjacent air molecules and they vibrate and move out to affect to next layer of atoms. Meanwhile the first set lose their additional energy and vibrate less. When the string vibration comes back to the same position just milliseconds later, the process is repeated. This sets up a series of pressure increases and decreases that when plotted on a graph of air pressure versus time looks like a sine wave.
When a sound wave reaches your middle ear, the acoustic pressure wave in air is converted to a mechanical wave by the tympanic membrane (ear drum) through the three smallest bones in the human body, the incus, the malleus and the stapes. The malleus is attached to the ear drum and pivots as the drum vibrates. The vibration is transferred and amplified through the three bones by a series of hinges, like the joints of a marionette. The stapes is connected to a second membrane called the oval window in the inner ear. Behind the oval window is the cochlea, a fluid-filled spiral formation with three linear cavities that communicate with one another. The vibration of the oval window creates a complex fluid wave relative to the pitches and amplitudes of the original acoustic wave. One of the cochlear cavities has a floor called the basilar membrane. This membrane runs the entire length of the cochlea and contains hair cells that stick up into the fluid.
Short exposure to very loud noise or long exposure to constant noise can cause damage to the hair cells, and once those stereocilia are lost, they don’t grow back. This can account for some forms of hearing loss. As humans age, the basilar membrane hair cells also die off due to natural causes or to high blood pressure or perhaps some antibiotics or other drugs that are toxic to hair cells, leading to age-related hearing loss (presbycusis). For reasons that have not been made clear yet, the basal region of the cochlea (where high notes are detected) is more susceptible to damage and age-related loss than is the apical region of the cochlea (where low pitches are sensed).
The stereocilia of the cochlear hair cells are damaged by loud or
constant noise. Note the difference between the normal and the
damage basilar membrane. The inset shows the normal
stereocilia under higher magnification.
For many years, the idea was that the tympanic membrane contained stretch fibers that were innervated by pain nerves and that loud noises would overstretch the ear drum and cause pain. This may be so, since anyone who has stuck a Q-tip in their ear a little too far is aware that it can be quite painful to contact the ear drum. However, other scientists have been studying the hair cells of the cochlea. For many years neuroanatomists have been aware of some unmyelinated neurons (nerve cells that don’t have the insulation around them that speeds up the nerve impulse along the long axon). Neurons that detect pain are typically unmyelinated, but the scientists had no evidence that this sub population of neurons from the outer hair cells of the cochlea transmitted pain – until late in 2015.
In hyperacusis, even the softest sounds can cause horrible pain.
The pain lingers for much longer than the actual sound, and can
be accompanied by painful tinnitus (ringing) even when there
is no sound.
Unfortunately, the other extreme is also possible. Every once in a while, a person will begin to have pain with everyday noises, and then with soft sounds, and perhaps even with almost inaudible noises. This is called hyperacusis and affects less than 200,000 people in the US. Scientists believe that, in rare cases, the damage to the cochlear hair cells that causes the pain fibers to fire, but they never turn off. Any noise after that will cause significant pain – enough that many sufferers must retreat from the world completely; the sound a person walking across the floor in their stocking feet is enough to bring agony. Many commit suicide rather than live with the pain. Luckily, in some cases, the pain subsides over time, with a gradual increase in the intensity of sound that causes the pain fibers to fire. Hopefully, the identification of the pain mechanism in the cochlea can lead to treatments for this condition, but the far better plan – wear your ear plugs at the race track.
Mark E. Lasbury, MS, MSEd, PhD