A sound perceived as being too loud, disagreeable, or distracting is consid- ered noise. This is a subjective cue to each pilot because sound that is noise to one pilot might be an important cue to another. In any case, noise still
SCIENCE OF SOUND
Sounds from different sources have different frequencies. For exam- ple, the sound of jingling keys ranges from 700 to 15,000 hertz.
Human voices produce frequencies from about 85 to 1,100 hertz. The tones of a piano have frequencies from about 30 to 15,000 hertz.
Scientists often use a unit called the phon (pronounced FAHN) to mea- sure loudness. Measured in phons, the loudness of a tone is equal to the intensity level in decibels of a 1,000-hertz tone that seems equally loud. For example, a tone with an intensity of 80 dB and a frequency of 20 hertz seems as loud as a 20 dB tone with a frequency of 1,000 hertz. Thus, the 80 dB tone has a loudness level of 20 phons.
implies sounds that need to be filtered out or become a background level over which more important sounds, such as conversations, can be heard and understood. Not all noise is to be ignored, and it is critical to be aware of the source of the noise and what it means. Blocking all noise could lead to missed sounds of changing performance.
Types of noise
Two types of noise are steady-state and impulse. Steady-state is a type encountered around and in an operating aircraft, often during flight. Here is where duration of exposure becomes important because there is a direct correlation between intensity, duration, and amount of damage to the ear (Fig. 6-3). Sources include prop noise, which ranges from 90 to 113 dB, ven- tilation sounds in small aircraft and even in the new glass cockpits, and the turbulence of air around the fuselage.
Impulse noise is the short burst type that can be produced by a backfir- ing engine, switching to a communication channel already turned up to full volume, or other short-term (often lasting only a few milliseconds) but loud sounds. Often these sounds come as a surprise, and there is no way to antici- pate the need for added protection; however, within the middle ear, where the ossicles are located, there is a small muscle attached to one of the bones that functions like a “bungee” protection against sudden and extreme movement of the joints.
At the very least, noise is annoying and distracting in flight and can seriously interfere with concentration, communication, and performance.
Noise and fatigue
An often overlooked physiological effect of noise on the entire body is fatigue.
Noise, as a form of energy acting directly on the body as well as on the hear- ing mechanism, therefore, is more than just an annoyance. Pilots return- ing from a long flight in a noisy aircraft are often more fatigued than those who are wearing noise protection. Some have recognized that fatigue is more
Figure 6-3
Figure 6-3 Allowable noise exposure.
90 Hearing and vibration
Definition of noise 91 noticeable in the pilot who is mentally active than in someone who is resting in a noisy environment.
Other nonauditory effects of noise include interference with sleep, rest, and the proprioceptive system, which will be discussed in Chapter 8. Other sub- jective symptoms include nausea, disorientation, headaches, and general irritability.
Another result of noise exposure is tinnitus, or ringing or hissing in the ears.
Everyone has some degree of tinnitus, and this is very noticeable to an indi- vidual when one is in a superquiet room with no background noise. There are very few places without some noise: circulating air, vehicle traffic, wind, voices, and the like. Tinnitus is especially noticeable at night, when one wakes up and becomes aware of the ringing or hissing.
After exposure to noise, tinnitus is more noticeable but tends to diminish over time; however, with significant noise-induced hearing loss through pro- longed exposure, increased tinnitus is common. There is no cure for tinnitus.
Trying to ignore the ringing or hissing is the best solution.
Maintaining a heightened awareness of tinnitus seems to increase the sub- jective perception of this sound to the point that a pilot is overly distracted or annoyed. If tinnitus is severe and more than just distracting, an ear specialist should be seen.
There also appears to be an individual variation of tolerance to noise. Some are able to withstand periods of intense noise, a physical energy, without detrimental effects. Some recover more efficiently from noise than others,
“SAY AGAIN” OR “THE COCKPIT OF BABEL”
Enunciation and diction is taken for granted until you notice the fre- quent query of “Say Again” or having to repeat a comment. Consider the roles of sound and the human voice in communicating with others using spoken vowels and consonants. Poor communication results in babel, or noise and confusion. Confusion ends in missing key com- munications.
There are two kinds of sounds in communication. The open sounds with free unobstructed breath are called vowels (A, E, I, O, U). The closed sounds, called consonants, are made with the breath wholly or partly checked by the tongue, teeth, or lips. If consonants (B, K, M, T, etc.) aren’t used effectively, the vowels become indistinguishable—without diction, babel. Poorly spoken consonants result from fatigue, hypoxia, intoxication, and medications, among other contaminants.
GOTCHA!
even without protection; however, this is the exception, and only through adequate monitoring of effects to noise exposure by audiometric testing can one minimize unexpected damage or impairment.