ANR 101: A Tutorial on Active Noise Reduction Headsets
(Section 5 — Optimizing Your Flying Experience)

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Questions about ANR? LightSPEED Technologies answers all of them in this five-part series for AVweb.

LightSPEED 20KNOTE: 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

  • Stress and fatigue

  • Masking of "wanted" sounds

  • Negative effects on speech perception and recall

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:

  • 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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