September 24, 2010
Facts and insight on how you can prevent hearing loss afield.
Turning the corner on what passed as a trail in the great swamps of South Carolina, I spotted a bobcat. The feline was walking away and the wind favored a life-sized mount. I discreetly racked one in, shouldered the muzzle-braked rifle and BOOM! Well, not "boom" to me, I didn't hear it go off as I was transfixed by the process. But two years later, my hunting partner Paul still hassles me about making him near deaf in his left ear as I neglected to call his attention to what was going on as he rounded the corner a half step behind.
It's odd isn't it, that so often we just don't hear the blast when we're on game. Far less odd is that this altered perception does nothing to protect our hearing. Predictably we firearm folks are at a higher risk of traumatic hearing loss because of the "boom" we all love so much. The hearing loss from repeated trauma is usually so gradual that it isn't noticed until somebody (usually a significant other) insists we get our ears checked. Unfortunately, this gradual process is not an inviolate rule. Sometimes traumatic hearing loss can occur with just a single significant incident.
What exactly is the "trauma" of a gunshot that may damage our hearing? Further, if a decibel is exactly equal to 0.1 bels, what's a bel anyway? The true working components of the ear are hidden away and buried in our skulls leaving their function as a near total mystery. What we want to do in this article is to provide technical information about how the ear is structured, how it works, and consequently, what can be done to protect this precious sense. We realize that most people are not biology majors. However most people are not physics majors either yet we hunters love to learn about bullets, terminal ballistics, recoil, foot pounds of energy, trajectories and the like. So class...sit up and pay attention.
The outside part of the ear is called the pinna. Its function is to round up waves of air pressure (sound) and focus them into our external ear canal as well as keep debris out of the canal. In many of our game animals, the pinna is mobile and aids in sound localization, but much less so in us. Stretched across the external ear canal about an inch or so from the outside is the eardrum. The eardrum is a thin membrane with a surface area of about 70mm that seals off the cavity of the middle ear from the external ear canal. Air pressure is equalized on both sides of the eardrum by the descent of the Eustachian tube that starts in the middle ear and goes into the posterior part of the nasal canal. A significant difference in pressure across the eardrum can be painful and distort hearing (popping ears at altitude or depth equalizes the pressure).
The eardrum is the place where the air pressure waves (sounds) are converted into mechanical vibrations. Our hearing, which really doesn't rank near the top in the animal kingdom, is still quite amazing. At its most sensitive range (3000 Hz) the ear drum's deflection when vibrating is remarkably subtle (0.000,000,01 centimeters). In order to transmit and amplify these subtle mechanical vibrations, three very small bones are linked from the eardrum to the organ which converts mechanical vibration into neurological impulses, the cochlea. Skipping the Latin, these bones, which are held together by firm ligaments, are known as the hammer, anvil, and stirrup. The hammer is connected to the eardrum and at the other side of this bony linkage is the stirrup's footplate. This footplate acts as a piston generating mechanical waves at its attachment to the cochlea, the oval window.
This complex set up allows for small air pressure changes (sound) distributed over the relatively large surface area of the eardrum to be concentrated at the oval window with an increase in vibratory intensity. Herein lies the problem with gunshots. Gunshots generate intense air pressure waves (sounds) and when concentrated, can result in far too much mechanical vibration for the cochlea to safely and repeatedly handle.
The cochlea is a very fascinating organ. It's curled like a snail shell in appearance. There are three chambers that transverse the length of the cochlea. The complexity of this system is extraordinary even when compared to our other senses. To simplify, all the action takes place in the middle chamber known as the scala media. Along the floor of the scala media is a complex of hair cells and support tissue collectively known as the organ of Corti.
The phonograph is perhaps the best analogy for what happens to convert vibration into electrical nerve impulses that we interpret as sound. A phonograph needle is mechanically moved through a magnet by the uneven surface of the vinyl record and that creates an electrical impulse, which creates sound when it's amplified and that in turn drives a membrane into motion (the speaker). The alterations in electrical frequency correspond to different sounds. Hence a range of sound can be recorded on a phonograph record that extends beyond what we can normally hear at both the high and low end of our sound spectrum.
At the oval window of the cochlea, the footplate of the stirrup sets up mechanical vibrations that pass through to the scala media and hence the hair cells. These hair cells act in a fashion analogous to the phonograph needle as they are the ultimate transducers of mechanical action into electricity (nerve impulses). These nerve impulses are sent directly to the brain where we experience them as sound. Vibrations of a high pitch are perceived very close to the oval window and vibrations of low pitch are perceived at the far end of the cochlea.
Getting back to gunshots and away from biology class, loudness is not related to pitch. The loudness of a tone appears to depend on the number and intensity of the hair cells set into motion by the mechanical stimulation of the eardrum and its bony amplification system. In a simple model, the more intense the air pressure wave (sound) the more intense the vibrations received at the oval window and the more violent the vibration of the organ of Corti. In essence that is how our hearing breaks down, either gradually or even with a single extra violent shaking.
Normal aging (our constant companion) results in a deterioration of this complex function. High-pitched sounds are lost first. This is known as Presbyacusis. The extent of this "normal loss" is of course highly variable and depends on an individuals, genetics, history of ear infection, trauma and as we are increasingly finding out, nutrition.
More pathologic hearing loss can be divided into two categories. Conduction hearing loss is due to malfunction in the eardrum and its bony partners. For shooters the much more common "nerve deafness" begins to play out. This occurs most commonly from damage to the organ of Corti and its closely linked nerve pathways, usually from mechanical vibratory trauma (the much desired boom). Clearly then a target shooter or avid bird shooter is at higher risk then say, the whitetail hunter simply on the basis of the number of shots fired.
One last science bit before we get into prevention and treatment options, namely understanding how we measure the intensity of air pressure waves (sound). We have all heard of the decibel (db), which is the most common measure of air pressure wave intensity. Here is how it is determined. This is important to know in order to map out a prevention plan. So take out a clean piece of lined paper on your desk and note:
Our ears have an astonishing capacity to detect subtle sounds. We are able to hear a single leaf drop onto the autumn forest floor or the ticking of a wristwatch as we sit motionless in the stand. The range of the intensity of pressure waves that we detect as sound is enormous. One researcher estimated this range in sensitivity from the weakest to the strongest sound that man can hear to be in the order of 100 billion to one! These sorts of numbers are best described using a log scale. This is where bels and decibels (db) come into play.
A bel is defined as the common logarithm of the ratio between two intensities of pressure waves. I can't say I really know what that means, but use the line at any cocktail party to sound intelligent. We use a tenth of a bel, or decibel to reduce this range of sound intensity from 0 to 160 instead of 0 to a 100 billion. Below is a common range sounds and their intensity in decibels:
No sound...0 db
Quiet office noise...40 db
Typical car...70 db
City bus...90 db
Subway train...100 db
Twin engine plane...110 db
Pain threshold...130 db
I left out gunshots because the sound intensity is so variable. A .22 pistol is louder to the shooter than the same round out of a rifle. Muzzlebreaks and the like disburse the intense sound waves and where you stand relative to the muzzle (shooter or spotter) makes a lot of difference. Suffice to say that any sound over 80db is bad for your ears and every gunshot is above 80db!
Prevention of hearing loss is therefore quite simple. Eliminate noises above 80db from reaching your eardrum. Custom fit earplugs are ideal and can be obtained from most audiologists and at many gun shows or hunting conventions. Generic plugs and earmuffs also work well. In many hunting scenarios, typical plugs or muffs may not be practical, but at the practice range, the dove field, and the clay games, they are a mandatory safety item.
There is an extensive category of hearing assisted ear protection devices. These electronically augment sounds below a certain db so we can hear our game's movement better. Importantly, they also electronically dampen out sounds above a certain db and reduce them to safe levels. Hence with these devices, your hearing is both improved and protected. Virtually all of the products on the market from earplugs to ear muffs work well. You have to just choose what is most comfortable for you and your budget but also consider what your hearing is worth to you.
There is an increasing large body of scientific medical evidence that suggests that Presbyacusis and even traumatic hearing loss can be restored to varying degrees with nutrition. A variety of vitamins, herbs and minerals in the correct proportion has been shown to be effective. Those individual ingredients can be purchased separately if one knows the correct studied doses or a newly, correctly formulated combination product such as Sportsmen's Edge is available.
As always, prevention is worth the proverbial pound of cure. Take care of your hearing. Machismo regarding hearing protection is not a good idea.