What is Noise?
Sound becomes noise when it is unwanted, bothersome or painful. The
basic unit of sound is the decibel (dB), the smallest unit of sound an
individual can hear. Sound is measured on a dB scale. The scale ranges
from zero, the average least perceptible level of sound, to about 130,
the average pain level. The dB scale is logarithmic; 90 dB of sound is
ten times louder than 80 dB and 100 times louder than 70 dB. When there
are multiple sound sources or multiple frequencies, as is the case in
an airplane cockpit, the decibels are weighted according to various
criteria to approximate the auditory sensitivity of the human ear. This
weighted measurement is called the A-weighted sound level (dBA). Many
researchers consider the sound pressure of 70dBA to approximate a quiet
ambient condition.
The FAA uses preferred-frequency speech interference levels (PSIL) to
measure noise in airplane cockpits. This is a measure of the noise
level at which the ability of crew members to understand each other is
impaired, and is roughly correlative to dBA. The PSIL is used as a
predictor for a more complex measure of speech intelligibility, the
articulation index (AI). AI ranges from 0.0 to 1.0, with a higher value
indicating better communication. The FAA considers an AI of between
0.25 and 0.3 acceptable for communication between crew members who are
approximately three feet apart. At an AI of 0.3, the upper end of the
acceptable range, test subjects understand 41% of unpredictable
syllables, 72% of rhyming words, and 92% of sentences. Understanding
increases as the content becomes more predictable. In Advisory Circular
20-133 (1989) the FAA states that "[I]f you hear part of a sentence,
you have a reasonably good chance to guess correctly what the rest of
it is." Pilots should demand a higher standard than 92%. The FAA notes
that visual communication between pilots can enhance the AI, but
recognizes that pilots would have difficulty orienting themselves for
direct face-to-face communication in the cockpit.
The FAA fails to consider the lack of visual communication between controllers and pilots when it considers an AI of 0.3 acceptable. The FAA also fails to note that while most routine communications may be predictable, it is important to be able to instantly understand non-routine communications. A remark from the tower controller that "your left engine is on fire" must be instantly understood, even though it is not expected. At an AI of 0.3, a pilot could correctly understand 92% of the sentence "CowAir 832, climb and maintain flight level three three zero" and fail to follow the instructions because 8% of the message is misunderstood.
The United States Air Force has a higher standard for noise protection than FAA, EPA, or OSHA. USAF auditory risk criteria are in line with European standards. The following auditory risk criteria is according to the USAF's Hazardous Noise Program (Air Force Occupational Safety and Health (AFOSH) Standard 48-19)
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Note for each 3dB rise in sound level, the allowable exposure time
is halved.