This may sound funny, but it really isn't. Hearing loss is very common with
people who work with heavy equipment and machinery. This includes Birddog pilots. The Continental O-470 IS NOT a quiet engine. The most important fact of hearing loss is that it is directly proportional to the amount of noise and the duration of the noise insult. So (dah), we all need to limit this threat. Some of my friends out there in "Birddog Land" already have the problem. You know who you are!

There are a few terms that should be remembered or introduced. Intensity is the sound pressure level and is measured in decibels (dB). Remember, a sound wave is a wave of pressure in the air. This is similar to a wave on top of water. Frequency indicates the pitch or tone of the sound and is measured in cycles per second (Hz). Humans can hear in a large range but usually use between 500 and 6000 Hz. Impulse/Impact noise is very hazardous. This is the type of loud, instantaneous noise a firearm would produce. An impact noise would be harmful if it was 140 dB or higher.

Basic anatomy of the ear does not stop with the external ear (pinna for you cross word puzzle buffs). The pinna collects the sounds and the sound waves then travel down the acoustic meatus (ear canal). The ear canal serves as a resonator. The sound wave then impacts the tympanic membrane (ear drum) and sets it vibrating. The three bones of the middle ear (malleus, incus and stapes) work as transformers. The force of the movement is increased about 90 times. The force then goes to the middle ear, into a section named the cochlea. The transformed waves are now (behind the ear drum) transmitted in a fluid or liquid medium (remember the waves on the water). The cochlea is a spiral organ that is lined with small cilia (hairs). These change the waves into nerve impulses that go to the brain through the cranial nerve VIII. Some hairs will transmit a wide range of frequencies, while other will only transmit one frequency. This is a "down and dirty" Birddog type of
explanation of how we hear.

As previously stated, the damage or injury to the ear is directly proportional to the intensity (dB) and the duration. Now comes the catch! The cilia are susceptible to high frequencies. After years of abuse (high frequency machinery use without protection) they loose there ability to transmit those frequencies. They can simply break (like taking a paper clip and bending it until it fatigues and breaks) or the cilia can still be present, but not transmit. Thus, High Frequency Hearing Loss. The "double whammy" is that the frequency that is commonly lost is in the range of normal conversation. So, long exposure to a sound that is not considered loud or
harmful and still permanently injure your ears.

I'm not going to go through everything, but exposure for the times listed
below can cause damage.
 

This is a significant reduction. If you wore a headset in a noise environment of 108 dB, instead of staying in that environment for 7.5 minutes, you could stay in the environment for almost 120 minutes without damaging your ears. A small decrease in dB goes a long way.

Let's all remember that noise is directly related to the level of noise (dB) and the length of exposure. We are more susceptible to high frequency loss. This noise exposure in accumulative and we may have wonderful hearing this year, but due to years of exposure, our hear can be in jeopardy for the future. I don't want to have to tell you to "Read my lips!"

Fly Safe! REX 79

Written for Birddog owners by Dr. Steve Frushour, IBDA Aero-Medical Officer