What’d You Say?—Hearing Loss and Skydiving
By Laura Galdamez, M.D.
Above photo by Kristian Caulder.
Let’s talk about hearing loss—the real kind, not the type people believe their spouses have. (That’s called “selective hearing” and is a whole different topic.)
First things first: How are sound waves converted into signals our brain can recognize?
Well, the outer ear funnels sound waves into the ear canal, where the tympanic membrane (ear drum) vibrates. (For more information on the tympanic membrane, see “Under Pressure—Barotrauma in Skydiving,” June Parachutist.) These vibrations are amplified along the three tiny bones of the middle ear that are attached to the tympanic membrane and transmit the sound energy to the inner ear. The inner ear houses a snail-shaped, fluid-filled structure (cochlea) that is lined with more than 16,000 tiny hair cells. Sound vibration energy transmitted from the middle ear generates waves in the cochlear fluid, which bend the hair cells and cause them to depolarize, converting the vibration energy to electrical signals. These signals are transmitted via the auditory nerve to the brain, where they are integrated and recognized as sounds.
Temporary hearing loss can occur from short term exposure to loud noises (e.g., music concerts, target shooting). In these cases, the hair cells bend like blades of grass. Temporarily dysfunctional while bent, they will straighten out and recover after a rest period. Permanent hearing loss is the result of long-term or repetitive exposure to loud noises, causing the destruction of the hair cells. Surprisingly, 30-50% of hair cells can be damaged or destroyed before hearing loss reaches detectable levels. Prolonged exposure to a single-pitch-frequency drone (e.g., aircraft engine noise, lawn mowing), is typically more damaging than sounds that change in frequency, like loud music at a rock concert. This is because the single-pitch sound wave targets a small patch of hair cells, like a wave repetitively crashing on the same part of the beach will cause localized erosion.
Sound is measured in decibels (dBA), which is a logarithmic scale. A 10dBA rise in sound level is a 10-fold leap in loudness. This means that sound at 20dBA is 10 times louder than sound at 10dBA, and sound at 100dBA is one billion times louder than sound at 10dBA. Some common examples of noise levels are listed in Table 1. The Environmental Protection Agency and the World Health Organization recommend maintaining noise levels below 70dBA over a 24-hour period or below 75dBA over an eight-hour period to prevent hearing loss. The Occupational Safety and Health Administration set a permissible exposure limit at 90dBA for an eight-hour work day with a 5dBA exchange rate (i.e., when noise levels increase by 5dBA, the amount of exposure time for a person to receive the same dose is reduced by half). The National Institute for Occupational Safety and Health recommends exposure to noise levels at 85dBA for no more than eight hours daily to avoid hearing loss.
How Noisy are Airplanes?
The Federal Aviation Administration sets guidelines for noise produced by general aviation aircraft. Noise levels vary based on engine type, cabin soundproofing and physical location within the aircraft. Interior noise levels for most general aviation planes vary from 80dBA to 110dBA (Table 2). Data gathered from a Cessna 172 demonstrated an average maximum noise level (during run-up with windows open) of 101.3dBA, with a time-weighted average over the entire flight of 86.26dBA. The Caravan and King Air, considered quieter aircraft, were found to produce sound levels of 62-76dBA in the cabin with takeoff levels reaching the higher range. The Twin Otter aircraft measured a whopping 96-103dBA in its base model, but can be refurbished to decrease interior noise levels to 85dBA. To impress an earlier point, these noise levels can result in even more hearing damage, given they are a single-pitch frequency that targets a specific area of hair cells.
What About Freefall?
That is a harder question to answer; research has not focused on this specific exposure. As an indirect comparison, sound levels in motorcyclist’s helmets traveling at 100mph were measured at 112dBA (Table 2), and wind noise—not engine noise—was determined to be the dominant factor. This speed is still shy of the 120 mph that most belly flyers achieve and far shy of the 150-200 mph that vertical flyers hit. Tina Penman, AuD, and Michael Epstein, PhD, performed one of the few studies looking at noise levels in freefall using noise-dosimeter headphones with no helmet. The majority of dives were tandem with some solo dives mixed in. They measured the majority of skydives exceeding 115dBA with a duration ranging from one to 52 seconds above this point. These jumps were performed without wearing a helmet, and it is difficult to predict how much hearing protection a helmet might provide.
What About in the Tunnel?
Early open wind tunnels and modern recirculating indoor tunnels have noise levels that peak at 120-130dBA (Table 2). Newer models reduce noise inside the flight zone to 90dBA, and reduce noise in the spectator area to 65dBA. With the newest technology of active air cooling, noise in the spectator area is as low as 55dBA with 112 mph winds. This is comparable to an office space during work hours, and allows spectators to hold conversations while standing next to the tunnel without raising their voices.
What To Do
Given the tight-fitting helmets skydivers wear, options for hearing protection are somewhat limited to devices that fit fairly flush in the ear canal. There are two models of hearing protection: passive and active. Passive hearing protection is modeled by the famous orange foam earplug, which works by physically blocking the ear canal from incoming sound waves, especially higher frequencies. The denser the material and tighter the fit, the better the blocking. Noise reduction can be on the magnitude of 15-20dBA ... not a bad strategy!
Active noise canceling headphones use a similar strategy, but also actively block lower frequency sound waves. This is achieved by generating sound waves that are 180 degrees out of phase to the intruding waves. As demonstrated in Figure 1, sound is basically a sine wave with crests and troughs. The active noise canceling headphones generate a sound wave with the exact same amplitude and frequency, but 180 degrees out of sync, so the crests of the generated waves line up with the troughs of the incoming waves and vice versa. In this way, harmful sound waves are canceled out. This results in an additional 15-20dBA decrease in sound intensity on top of the 15-20dBA reduction from passive protection properties.
So what is the bottom line based on this data? If you wear hearing protection of class 3 or stronger (most disposable and reusable ear plugs are class 3), you are unlikely to experience any hearing damage if you skydive 10 times or fewer within an eight-hour period. If you skydive without hearing protection (or more than 10 times within 8 hours with protection), there is some risk for hearing damage.
So, there you have it.
If you want to get the maximum years out of your hearing, consider inserting some earplugs when you are headed up on a jump or diving into the tunnel. No, you may not be able to hear all the spicy jokes and comments on the ride up, but you’ll still have the goofy high-fives with your friends and hear their laughter back on the ground.
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What is Tinnitus?
Tinnitus is the perception of sound without one being present. Put another way, tinnitus is the “ringing in my ears” that many older skydivers report having. It can present as more than ringing, though, and can be a buzzing, roaring, clicking, hissing or humming noise that can be either continuous or pulsating. About 90% of people in the United States with tinnitus have a form of hearing loss, and there is a strong connection between these two processes. There are several theories on the mechanism behind tinnitus, but most revolve around the concept of decreased input to the auditory nerve and auditory centers of the brain causing increased spontaneous activity in either the hair cells or central auditory circuits. Basically, damage to the inner ear cells (i.e., hair cells and cochlea) causes frequency-specific decrease in output from that area of the inner ear. So the central auditory pathways compensate to counteract the lack of signals, and effectively turn up the gain in that area specifically, leading to a false perception of non-existent sounds.
About the Author
Laura Galdamez, M.D., C-50829, began skydiving in early 2020. She works as an emergency medicine physician in Houston, Texas; is a Fellow of the Academy of Wilderness Medicine; and worked on the medical team for the StratEx high-altitude balloon mission. She competes in 4-way formation skydiving on a team out of Houston called Non-Toxic.