The Sounds of Silence: Part 2

Shooting suppressed weapons will protect hearing far better than using hearing protection devices, because the sound is attenuated at the source. Hearing damage by bone conduction may not be prevented with muff protectors, and certainly is not prevented by in-the-ear protection. Shooting with a suppressor makes it significantly easier to recognize ambient conditions or threats. (Photo by author)

It is easy to spot older, long-time shooters.  These are the guys who can’t understand a word you are saying while regaling you with tales of the Great Hunts in exotic locations, competitions won, or battles bravely fought.  Some will use hearing aids, which are second choice to natural hearing.  Others will be able to understand you if they can see your lips move and there is little ambient noise.  There are those who combine both categories.  What has happened to these guys?

Simply put, they have had significant hearing damage due to chronic exposure to loud noises.  The ear is a sensitive mechanism designed to translate rapidly changing air pressure variations we refer to as sound into nerve impulses that the brain can use for communication.  The hearing mechanism is more than just the floppy appendage on the side of the head (the pinna, which most animals can change the shape of to concentrate sound).  In addition, the external ear consists of the ear canal and the ear drum (tympanic membrane).  Considering only the hearing mechanism (not balance), the middle ear contains a series of three tiny bones connecting the tympanic membrane with a similar structure (the oval window), that forms the boundary of the inner ear.  These bones, the smallest in the body, form an ingenious mechanical sound amplifier.  With aging, their joints can develop arthritis, decreasing their ability to transmit sound.  The inner ear consists of the cochlea, which is a fluid-filled spiral structure having small sensory hairs.  These hair-like structures resonate at different frequencies, stimulating the underlying nerve endings, sending the sound bits to the brain.  These sensors are fairly fragile, and loud noises will over-stimulate them, leading to early failure.  The shorter ones (representing the higher frequencies) are the closest to the oval window and are more easily damaged.  There is some attenuation of the sound level in the liquid in the cochlea as the sound wave traverses to the far end, which explains why high frequency hearing is damaged sooner than the lower frequencies.   Once failed, the associated frequency can no longer be perceived.  We do get a little warning that the hearing mechanism is being damaged by “ringing” in the ears (tinnitus).  Early on, this is transient and indicates an insult to the hearing mechanism.

There are two basic mechanisms of hearing damage.  Sustained loud noises cause an inflammatory reaction in the tissues supporting these sensory hairs.  With time and continued inflammation, these sensory hairs eventually fall out.  The second mechanism is a shock from a really loud sound (over 140 dB), which mechanically shears some of the sensory hairs.  An analogy would be tooth loss from either gum disease (chronic inflammation) or acute trauma, such as a blow to the mouth.  Either way, there is permanent damage.

Drawing of the ear and hearing mechanism showing the components of the ear. The outer ear consists of the pinna, external auditory canal, and tympanic membrane, which is the boundary with the middle ear. The middle ear consists of the three tiny bones (maleolus, incus, and stapes) that form a mechanical amplifier and ends with the oval window, which is the boundary of the inner ear. The inner ear consists of the cochlea (hearing mechanism starting with the oval window) and the semi-circular canals relating to the balance mechanism. (Drawing courtesy of Brüel & Kjaer)

Other than totally avoiding loud sounds, there are several protective measures available.  The first is directly protecting the hearing mechanism with mechanical barriers of some sort.  These generally consist of either insulated earmuffs or insulating plugs that are inserted into the ear canal.  Of the two, muffs provide far superior protection.  The plugs in the ear canal do an acceptable job of reducing the sound to the tympanic membrane and the normal hearing path.  However, sounds can be (and are) transmitted directly to the cochlea through the bones of the base of the skull.  Bone conduction is the way we normally hear our own voices when we talk and is the reason why our voice sounds so strange to us from a tape recorder where we hear it through the normal pathway.

In-the-ear hearing protection, while better than nothing, is not as effective as muff-type protectors, which reduce sound levels by 28+ dB (including the bone conduction pathway).  Until about 30 years ago, few people used hearing protection when shooting, and even today hunters rarely use protection in the field.  Because of the logarithmic nature of sound measurements, using both muffs and plugs adds at best 3 dB to the reduction of the muffs alone.

The second protective measure is suppressing the sound at the source.  The past several years have seen a proliferation of new firearm suppressor manufacturers, some of whom offer highly affordable suppressors.  There are those who preach that actual reduction (or absolute sound level) measurements are not meaningful and preach that perceived loudness is all that counts.  Nothing could be further from the truth, and believing this fallacy can lead to significant hearing loss.  Perceived loudness depends on three factors: the frequency of the sound itself, the duration of the sound pulse, and the observer’s existing degree of hearing loss.  Significant hearing damage can be sustained from gunfire, regardless of how quiet something sounds. In addition, hearing damage is caused by more than just gunfire, and the dose is cumulative.  Other sources are industrial noise, wind noise (motorcycles, driving with the window open), entertainment, etc.

OSHA (Occupational Safety and Health Administration) requirements for industrial exposure (sound levels in the workplace) are that hearing protection must be used if the sustained sound level is above 85 dB, or at least the sound must be reduced to below that level at the ear.  This assumes an exposure of 85 dB day in and day out, 40 hours a week for the entire work year.  This standard also assumes that the worker is exposed to no sounds over this level.  In theory, if there are no sounds over this level, there will be no hearing damage.

The family of equal loudness contours show the difference in sensitivity of the ear to sound levels at differing frequencies. The graphs show the actual sound level necessary to have the same apparent loudness as a 1 kHz tone at a given level. As an example, a 50 hZ tone will need to be approximately 15 dB louder than a 70 dB tone at 1 kHz to appear to have the same “loudness.” (Graph redrawn from Brüel & Kjaer)

Besides at work, there are a lot of other sources of potential sound damage to hearing.  Rock concerts come to mind, and there are a number of young fans of rock concerts sustaining significant hearing damage at concerts where the sound levels in the front row (where one can really “feel” the music) may be in excess of 130 dB.  The simple portable CD or cassette player with headphones can (and often is) cranked up to the point where the sound level delivered to the ear is dangerous.  As an aside, if you can hear the music when your child is wearing the headphones, it is too loud.  Other sources in the day-to-day environment include wind noise while riding a motorcycle, driving a convertible, or driving with a window open.

Other than the portable cassette/CD player, the dose level may not be sustained long enough to damage hearing if there are no other significant sources of dangerous noise in the workplace or environment.  The problem is that the dose to cause hearing damage is cumulative.  The maximum OSHA permitted level is for 85 dB for 2,080 hours in the year (the number of hours assuming a 40 hour work week).  The problem comes in when one considers the nature of sound.  Each 3 dB increase in sound level is an actual doubling of the absolute sound pressure level.  In other words, if 85 dB is the maximum acceptable dose for sound in the 2,080 hour year, if the exposure is raised to only 88 dB (a 3 dB increase), the subject will receive the same equivalent dose of sound in half the time (or 1,040 hours).  With each 3 dB increase in sound level, the acceptable time is cut in half.  For the 130 dB rock concert, hearing damage will start at about 17 hours, assuming no other damaging sound levels.  And this assumes the rock concert is only 130 dB.  Many rock musicians are prematurely deaf by age 30.

Firearm noises are another source of noise pollution, and they are unique due to their extremely short duration.  As such they may not sound terribly loud, and this perceived sound varies with the caliber and circumstances of firing.  An air rifle is thought to be essentially silent, but it may well generate a sound pressure level of 110-130 dB, depending on design, type, and caliber.  However, because the sound is of such short duration, it is almost impossible to come up to a dose equivalent to the OSHA standard of 85 dB over a work year of 2,080 hours.

As the firearm sound gets louder, however, the number of permissible exposures drops to the point where a 160 dB 9mm handgun will give enough exposure to reach the OSHA yearly equivalent in only 7 rounds.  This becomes a real concern, especially to the hunter (either shotgun or rifle) who doesn’t wish to wear hearing protection in the field.

The simplest method of measuring the sound reduction of muff-type protectors is to simply clamp a set around the microphone after making readings from a non-suppressed pistol. In this case, we measured a common set of Peltor electronic hearing protectors using an FN-57 pistol (163 dB) as the sound source. The reduction measured with the protectors rubber-banded around the microphone was over 32 dB (electronics off). We obtained the same reading (within 1/10 dB) with the electronics on and the volume set at maximum. With electronic hearing protectors, the batteries should be changed at regular intervals rather than waiting for battery exhaustion. (Photo by author)

Both OSHA and MIL-STD-1474D state that hearing protection MUST be used if the sound pressure levels of firearm noises are 140 dB or greater.  This is specified in Requirement 4 of MIL-STD-1474D.  While it is true that a sound level of, for example, 145 dB is not nearly as harmful as one of 160 dB, exposure will still cause damage.  Once the damage has occurred, it is forever and hearing will not come back.

MIL-STD-1474D has specific requirements for metering equipment, which eliminates the Radio Shack types of meters (and, interestingly, many of the current Type-2 field-portable units).  This was discussed in detail in the Spring issue of SADJ.

The standard location for sound measurement is 1 meter to the left of the muzzle at 90 degrees to the bore axis and 1.6 meters above grass.  This is the location that most of us use in the United States.  A second location, which is used to simulate the shooter’s ear, is 1 meter from the muzzle at an angle back from the bore axis of 15 degrees.  Experience has shown that this second location is generally 2-4 dB less than the one at 90 degrees.  Measuring actually at the shooter’s ear location is somewhat inconsistent because of shadowing by portions of the shooter’s head.  The bottom line is that if a sound suppressor does not meet the US standards specified by OSHA and MIL-STD-1474D, then it can cause hearing damage.

Although some suppressors may exhibit frequency shifting to where a significant portion of the sound level is at the upper limits (or perhaps even beyond) the range of human hearing, if the level is above 140 dB, hearing damage still can occur.  It is necessary to remember that each individual has a different hearing loss, and what may be beyond the hearing of an elderly industrial worker may well be within the range of hearing of a young person.

While sound measurements have been used as advertising hype, the important thing is that they indicate that the suppressor reduces the sound below the US standard of 140 dB.  The degree of reduction is not really as important as the absolute sound pressure level.

In the interest of not damaging the hearing of customers, all manufacturers should have their suppressors measured by someone knowledgeable in the field of gunfire sound measurement and who has in his possession equipment that meets the MIL-STD-1474D requirements for these measurements.

Firearm related hearing damage is a serious concern today and will continue to be a source of liability.  The largest percentage of claims in the Veterans Administration hospital is related to hearing damage and averages over $4,000 per year per veteran.  Hearing damage claims are starting to show up in law enforcement agencies, and noise pollution from law enforcement training is the source of numerous complaints in the general population.

In my opinion, it is a disservice with today’s knowledge to promote a suppressor that, while far better than nothing, still has a sound level that exceeds US standards for safety without hearing protection.  In the field of medicine, this would be considered contributory negligence, and in today’s litigious climate, it may well result in legal action against a manufacturer.