pure tone in a soundproof booth using a headset. For screening purposes, a good headset in a quiet room will do the job.
All FAA exams require testing of hearing. A common method is the whispered-voice test, now called a voice test. This is a poor substitute for the audiometer because it cannot quantify or identify the frequency at which a hearing loss might be present. Only with an audiometer can a reliable result be achieved.
The test results from an audiometer are recorded as an audiogram, which describes a hearing level curve for each ear. Normal hearing should be a straight line in the 5 dB range. Variations from normal are acceptable, but any frequency with a loss of greater than 20 dB is considered potentially significant and needs to be further evaluated to rule out a progressive loss of hearing.
Over time, by comparing audiograms, one can see shifts that are occurring. If high frequencies are being affected, then better hearing protection is neces- sary. A baseline audiogram is very important so as to compare subsequent curves of changes. These changes are called threshold shifts, meaning there is a shift from the baseline of years past and a strong indication of a noise- induced hearing loss.
NOISE AND HEARING LOSS
Hearing loss is not inevitable. Several preventive measures can be taken.
The first is to recognize that noise is present everywhere and is potentially detrimental, both in performance and in the health of the ear and hearing system. The next is to protect the ears from noise by wearing appropriate hearing defenders, or ear protection. The third is to be part of an ongoing hearing conservation program with annual monitoring of hearing abilities and retraining in noise avoidance.
Sources of noise
Airplanes and airport ramps are some of the noisiest sound sources to work around. Pilots are exposed to sounds varying from continuous low-level noise on a cross-country flight to short periods of engine runups on the ramp. In addition, pilots encounter noise associated with many activities not related to aviation, such as hobbies, mowing lawns, downtown traffic, and the like.
Increased background noise is inevitable in aviation and can erode effective communication unless the level is raised beyond the background level. Fur- thermore, if you have to raise your voice when a foot away from the source or shout three feet away, then you are in an environment that can do harm over time.
The Occupational Safety and Health Administration (OSHA) has determined certain levels of noise for many different events and activities (Fig. 6-4).
Note that each is “time weighed,” which means that damage is done with lower levels for longer periods of time. For example, eight hours of exposure to noise levels of 90 dB is as harmful as two hours at 100 dB. Then note
the sources. It is surprising how much energy there is from some sources.
Rock concerts near 120 dB levels and walking around with headsets at full volume virtually focus that energy right into the ear, much like a mega- phone. Studies are now proving that such exposure does cause permanent hearing loss.
Airplanes are not quiet. Many small reciprocating airplanes are very noisy.
Prop noise ranges from 90 to 115 dB. Even in the passenger cabin of airlin- ers, certain noise levels are potentially harmful over periods of time. As a general rule, whenever you must raise your voice to be heard by someone next to you, you are in a noisy area. Furthermore, for every 4–5 dB increase in sound above 85 dB, the safe time limit for exposure is cut in half.
Noise-induced hearing loss
Equally important as the interference with communication, noise also causes permanent high-frequency hearing loss. It is an insidious change, taking years to develop significant loss. Hearing aids do not help noise-induced hearing loss. In other words, noise alone can reduce hearing over the years and there is nothing that can be done to reverse the process. You won’t know there is a change occurring over time unless you check for it. As with so Figure 6-4
Figure 6-4 OSHA standards for noise exposure.
94 Hearing and vibration
Noise and hearing loss 95 many other issues in medicine and as frequently stated throughout this text, the best and only treatment is recognition, respect, and prevention.
The loss can be rapid in uncommon occurrences of extremely intense noise.
Sudden impulse noise is physically damaging to the eardrum as well as the ossicles. This usually occurs with noise in excess of 140 dB, such as gun- shots, explosions, and jet noise.
Many pilots have experienced the temporary loss of hearing after a long trip in a noisy airplane. There are stories about people who start their cars for the ride home and then try to restart the engine, which is already running. This sort of loss will disappear in a few days but is also an indication that there was a significant noise exposure that eventually could lead to a permanent loss.
Threshold shift
As unprotected exposure to noise persists, there is an increasing risk of hear- ing being impaired, especially in the higher frequencies. This threshold shift is not noticed by the pilot because there is no change in the pilot’s perception of sound, plus it occurs over several years. There is no pain to warn the pilot.
Short of undergoing testing, there is no way for someone with a threshold shift to compare his hearing to normal or what is expected from previous perceptions of sounds.
In other words, sounds are different, but the difference goes unnoticed over time. With even partial loss of high frequencies, such sounds as brass and string instruments sound bland and without clarity. Voices are still heard but are not sharp or clear. As the loss increases, sound becomes muffled.
Because this change occurs over several years, the pilot does not recognize it until extensive loss is noticed. As the loss becomes even more severe, speech recognition becomes difficult if not impossible, especially when background noise is also present, which is at a dB level equal to the hearing loss.
There is also a subtle reduction in the interpretation of certain words. Over a period of time, often months and years, some consonant sounds of words are diminished and only some of the vowel sounds are distinguishable because the higher frequencies are not heard. A word, therefore, that depends on a full range of frequencies will not be understood. It might sound like mum- bling. Communication then becomes more difficult.
Threshold shifts can be temporary, as when you are subjected to short peri- ods of sound infrequently; however, with continuous (over months) exposure to even moderate noise, this threshold shift becomes more and more perma- nent. Only monitoring and comparing results of audiograms will alert you to any shifts (Fig. 6-5).
The audiometer will detect even subtle changes before more permanent dam- age is done. Even without such documentation of loss, the pilot should expect loss if he/she continues to subject himself/herself to noise without protec- tion. The primary concern is a permanent threshold shift. A whisper voice test doesn’t tell you of a hearing change.