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The Technical Staff of LightSPEED Technologies |
We hope you find this series informative. If you have additional questions on what we've covered or have points of interest to share relating to ANR, please e-mail us at the link above.
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NOTE: Dr. Jim Yates provided
substantial contributions to this section of ANR 101. Dr. Yates is
Professor and chair, S.P.A. of John A. Burns School of Medicine,
University of Hawaii, and principal of Audiology Associates Hawaii.
Previous
sections of this series have discussed active noise cancellation and
the technical issues associated specifically with general aviation
headsets. We've discussed how ANR works, why it's so difficult to get
optimum levels of cancellation, and how to make ANR more effective in the
airplane noise spectrum. We've also talked about the basic physiology of
the human head and the difficulties faced in designing a headset that's
really comfortable.
Naturally, as with any other aspect of flying, safety and occupant
protection should come first, and comfort should follow. Although the
history of powered flight dates back nearly 100 years, concerns about the
effects of aircraft noise on hearing are relatively new. We've all heard
the axiom "there are old pilots; there are bold pilots; but there are no
old, bold pilots." We could add "and precious few with normal hearing." In
fact, in the days of open cockpits and wire struts, pilots flew with ears
unprotected and gradually damaged the hearing they desperately needed to
be safe while flying! It is said that the whisper test, still used in the
flight physical, has its roots in the perceived necessity for pilots to
hear the "singing" of the support wires while in flight.
The effects of "noise"
While it is true that most cockpits today are closed and there have
been advances in sound treatment (i.e. mufflers, soundproofing insulation,
etc), increases in engine power and extended aircraft range have largely
negated any advances in average "quietness." Today's pilots and passengers
are subjected to noise that can have numerous adverse effects, including:
Permanent hearing loss
The human auditory system is a bio-electrical marvel. It's capable of
hearing sounds from 20 cycles per second to roughly 20,000! Under ideal
conditions, the human ear is said to be capable of hearing a sound roughly
equivalent to pressure exerted by a gnat's wing, yet able to tolerate
sounds literally millions of times greater. So responsive is the ear that
early researchers adopted a logarithmic scale for hearing measurement. In
this scaling system, a sound of 100 decibels (dB) is 100,000 times greater
than a sound of zero dB, and a 120 dB sound is a million times
greater than that same zero dB sound.
Noise-induced permanent threshold shift can occur from repeated
exposure to damaging noise levels. In Section 2, we presented a chart containing
A, B, and C weighted noise curves. The "A" weighted curve represents a
rough equivalent risk associated with exposure to noise at various
frequencies. 1-3Khz noise levels have more damage potential for human
hearing than do those of lower frequencies, though sufficient levels of
low-frequency sounds can produce permanent hearing loss.
One thing is certain: Beyond certain levels (about 85dB, A-weighed),
increased intensity and exposure time will produce increased hearing loss.
The hazard is most pronounced in the region nearer 4 kHz but spreads over
a frequency range as exposure time and dB level increases. In the typical
small aircraft, cabin noises tend to be concentrated in the lower
frequencies. Even so, noise levels in most aircraft are sufficient to
cause threshold shift losses if the exposure is sufficient in duration.
Flying without ear protection for long periods and with repeated exposures
places us at a risk of hearing loss.
Active noise canceling headsets provide relief in two distinct ways:
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They reduce the levels of low frequency noise
exposure. This issue was covered in detail in Section 3. Most
GA aircraft have high levels of low frequency noise and even the best
passive protectors are relatively poor at attenuating this part of the
noise spectrum sufficiently. The best active headsets are exceptional at
this specific range.
-
They permit the communications audio signals from radios and
intercom to be intelligible at reduced volume levels. Because
of the masking effects low frequency noise has on speech, we have to
turn the volume up to be able to (partially) hear normal voice
frequencies. We often will deliver more than 110db of communications
audio directly to our own ears (through the headset speakers) in an
effort to better hear and understand what's being said! With ANC
systems, you will turn down your audio signal because you can
understand ATC much better. (We'll discuss the audiological reasons for
that shortly.)
Stress and fatigue
Noise clearly has the capability of producing stress and fatigue.
However, measuring and quantifying these effects on the pilot has proved
difficult. Stress is colored by previous experiences, conditioning, and
the character of the noise itself. Higher frequency sounds are generally
more disturbing and higher pitched sounds are often associated with
danger.
One obvious aspect of stress levels is the perceived loudness of the
sound. Loud sounds are more stressful than ones of similar character but
quieter. Vibration and lower frequency sounds produce fatigue and
influence our ability to focus. The physiological effects of this constant
"pounding" on the body was studied in depth by the military as a source of
mission errors. The effect on the recreational pilot is less well
documented, but no less a concern in carrying out your flying "mission."
Long-term exposure to aircraft noise not only can produce fatigue but may
also lead to mental errors.
Obviously, active noise reduction systems reduce overall noise levels
overall and that "quieter" environment makes flying less stressful. The
effect of reductions of low frequency noise on a pilot's mental condition
hasn't been well studied, but the anecdotal reports we hear are
consistent: pilots regularly comment on how much better they feel both
during and after flights when they wear ANR headsets.
Masking of "wanted" sounds
While human speech ranges in frequency from slightly below 100 Hz to as
much as 10,000 Hz, the energy of speech is heavily concentrated in
the lower frequencies. 90 to 95 percent of the energy of speech is in
frequencies below 500 Hz. Typically this energy is concentrated in
the vowel sounds. The consonants carry most of the meaning of speech in
the 1 Khz to 6 Khz range but these are very weak (low energy) sounds.
Consequently, the weak consonants are easily "masked" by noise, making it
difficult to understand what's been said. It doesn't take much masking
before intelligibility is impaired, as the following data from one study
of adults with "normal" hearing shows:
| Loss of
Consonants |
Intelligibility for Adults |
| 4% or less |
Very good |
| 5%-8% |
Good |
| 9%-11% |
Fair |
| 12%-14% |
Minimal |
| 15% or more |
Unintelligible |
The implications for pilots are straightforward: even a 10% loss in
consonant sounds will substantially limit comprehension! Obviously the
danger of missing or mis-hearing verbal instructions is a very real
concern!
Given what we've already learned about the low-frequency dB levels
encountered when flying, excessive levels can easily overwhelm the
important consonant sounds. The danger of "masking noise" in an aircraft
is that we may miss important signals either from controllers or other
warning signals that we really need to hear accurately. Masking noises
contribute to stress levels as we "strain" to hear, process, and
understand the multitude of signals and data we manage as we're flying. In
a real sense, masking of "wanted" sounds may be the biggest danger in an
aircraft.
Active systems reduce the masking effects of the low frequency spectrum
your ear hears. The most effective systems will substantially enhance your
ability to hear and understand the audio communications. Virtually every
pilot who tries an ANR headset for the first time notices this benefit.
Negative effects on speech perception and recall
Elevated levels of noise substantially impair our ability to recognize
words. When noise levels exceed 85 dB, they begin to have adverse effects
on the speech comprehension. The key to greater word recognition is in
getting the communication audio "signal" significantly above the
ambient "noise." That's a hard task for pilots since we're exposed to
cabin noises with a significant low frequency component. When audiological
professionals want to create speech masking sounds, that's precisely the
type of noise they use. The typical cockpit is an excellent speech masker!
Studies have shown we need at least a 9 dB difference in the audio
signal above the "ambient" noise levels in our headset to achieve 80% word
recognition or better. That difference in dB level is typically expressed
as Signal-to-Noise Ratio (abbreviated S/N or SNR). A S/N ratio of 12-15 dB
allows a 90% recognition rate. We know how loud the noise spectrum is in
most planes...particularly at the lower frequencies. The combination of
masking effects and overall ambient sound levels makes hearing and
understanding communication very difficult. Most of us know what that
"sounds" like...from ATC calls we miss altogether to ones that prompt us
to make repeated "say again" requests. The higher background levels of
noise when flying also changes our hearing "threshold," making previously
understandable words unintelligible.
Beyond just the S/N ratio issues involved in protecting word
recognition, our ability to "process" speech and sounds is adversely
effected by higher background noises. For example:
-
Accuracy of comprehension declines as does response speed.
-
Time sharing of attention among several potential signals becomes
more difficult.
-
Recall (memory) is impaired, as is our ability to handled delayed
information effectively.
In other words, noise affects our understanding, attention sharing,
response time, and short-term recall of what was said. None of
these are good things for a pilot, so anything that can be done to quiet
things down is good!
Some final thoughts
The intent of this section is not to frighten, but to serve as a
reminder of all the sound-related issues we deal with every time we fly!
Being a pilot is a complex "job" with many responsibilities. You probably
have been using a basic passive headset and those help you stay on top of
your many tasks. ANR will make doing those tasks both easier and less
stressful. An ANR headset provides additional noise suppression in an area
of the noise spectrum that will make communication clearer and increase
attentiveness.
ANR will make you a safer pilot. It'll protect you from long-term
hearing damage. You'll finish the flight more relaxed and enjoy flying
more. A modest investment can enhance the health, comfort and safety of
your flying experience to levels unheard of just a few years ago. So keep
your altimeter up and your dB down!
| This concludes the five-part "ANR 101" series.
We hope you've enjoyed reading it as much as we've enjoyed putting
it together, and we'd welcome your email comments. Just use
the link in our byline to contact us!
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LightSPEED
Technologies, Inc.
15812 SW Upper Boones Ferry Road
Lake Oswego,
Oregon 97035 USA
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