December 8, 1998 ANR 101: A Tutorial on Active Noise Reduction Headsets (Section 3 — Airplane Issues)

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by	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.

Articles in This Series Introduction Section 1: The Basics of ANR Section 2: Acoustic Issues Section 3: Airplane Issues Section 4: Ergonomic and Interface Issues Section 5: Optimizing Your Flying Experience

Looking at actual cockpit noise profiles, and how well aviation headsets cope with it

In Section 2 of this series, you learned how engineers measure effectiveness of active noise canceling systems in the laboratory. But since we don't fly in the lab, our emphasis will now shift to studying the actual noise spectrums in various aircraft to focus on what ambient noise we're trying to cancel, and how serious a threat this noise poses to our hearing. Clearly, we want our aviation headset to cancel noise most effectively in the areas of the spectrum where there is the most noise! We'll look at some actual noise profiles in piston-powered single-engine cockpits, and see how well passive and active headsets deal with it.

Noise profiles

Let's start by reviewing some actual detailed noise data for two different single-engine airplanes. Below is data measured during flights in a Cessna 210 and a Piper Commanche.

Actual noise profiles, Cessna 210 and Piper Commanche.

Having analyzed dozens of aircraft noise spectrums, we know that these profiles are typical of most makes and models of single-engine planes. The noises generated by the propeller and its resonant (harmonic) frequencies make up the most predictable part of the noise spectrum.

The Commanche had a two-bladed propeller that creates its peak noise levels at about 80 Hz, while the Cessna 210 has a three-bladed propeller with a peak noise level at around 120-130 Hz. Engine, exhaust, and wind noise add most of the rest of the sound in the noise profile. Obviously engine size, aerodynamics, and many other aircraft-specific design features contribute to the actual profile of any specific plane.

While these two planes look different in many ways, there are two general characteristics that are evident:

1. There is a lot of noise in the low frequencies...between 70 and 300 Hz.

2. Noise levels decline in the higher frequencies...particularly beyond 500 Hz.

Both these characteristics create a perfect fit for using active cancellation for optimal sound reduction. Remember (from Section 2) that active cancellation works well only in the lower frequencies...it doesn't provide a noticeable dB reduction at frequencies over 500 Hz. Recall also that active systems require some tradeoffs in passive attenuation to support the needed modifications inside the domes. As such, they are not quite as effective in blocking out the higher frequency noise.

But isn't high-frequency hearing protection more important?

That all depends on both the level of noise and the duration you are exposed. In Section 2, we covered the "A-weighted" and "C-weighted" NRR measurements. The conclusion was that high levels of low frequency sound could actually be more damaging than the higher frequency noise. As you can see from the airplane noise spectrum graph, there is usually 20-30dB more noise at 100 Hz than at 1,000 Hz in a piston-engine airplane.

Typically people are focused on hearing damage and "saving what they have left." That's a key reason they're interested in getting a new headset. Prolonged exposure to noise has a variety of effects on the body and brain that have daily and direct effects on your ability to fly a plane safely. (Section 5 of this series will cover those issues in more detail.) Meantime, it's sufficient to state that active noise reduction headsets create both a quieter and safer environment for your ears.

What we know about hearing loss

The data relating to hearing loss is actually the simplest to understand and has been well studied. Below is the data gathered from studies done by the EPA correlating levels of noise to the length of time the subject is exposed.

Projected hearing loss from continued noise exposure.

These figures were calculated assuming exposure to the given level of noise for eight hours a day, five days a week. Not surprisingly, more noise for longer times means greater risk of hearing loss. But the most interesting piece of data here is that there is no projected loss from exposure to 80 dB for eight hours a day, and even 85 dB results in just nominal hearing loss. The real damage begins to develop with prolonged exposure to levels above 90 dB.

We've seen that in propeller aircraft, noise of this intensity occurs only at low frequencies. Now you can see why it is so important to reduce the very low frequencies.

Why ANR works so well in aircraft

One of the reasons active cancellation is so effective in airplanes is just this: there is lots of low frequency noise. The graph below is a smoothed output of the takeoff noise spectrum of a Cessna 172RG Cutlass. It has a similar profile to the planes we looked at earlier this section.

Effects of typical passive cancellation on noise spectrum.
(Cessna Cutlass with adjustments for passive cancellation over the full spectrum.)

The lower line is there to represent the attenuation you can expect from a typical passive headset. Note how poor the attenuation is at 100 Hz and how much better it gets at the higher frequencies. While there is substantial quieting at 1,000 Hz, you're still exposed to levels well over 80 dB at the lower frequencies.

As we've seen, passive hearing protection is very effective where there is less noise, or where the noise is predominantly at higher frequencies. It's just not the ideal solution for an aircraft noise environment.

Compare that to the protection provided by excellent active cancellation in the low frequencies.

ANR's additional low-frequency hearing protection over passive headsets.

This is a close-up of that same Cutlass noise spectrum focusing just on the low frequency area. The red line shows the additional attenuation provided with active cancellation over a typical passive headset. That reduction is very noticeable. Understand that there will be a slight reduction in higher frequency attenuation with ANR headsets. This is generally a good tradeoff because the residual cockpit noise levels are already very low at higher frequencies...well below any levels which can cause damage.

So where are we?

You should now have a good understanding of the noise levels and spectrum we live in as pilots. The high-decibel, low-frequency components are not effectively removed by a traditional passive headset. In contrast, active cancellation is specifically designed to reduce this portion of the noise spectrum and get it well below the hearing damage thresholds. All of that creates a quieter, safer, more relaxing environment to enjoy during your time aloft.

In the next section, we'll change gears and turn from noise to comfort. I don't know a single pilot who wouldn't like to have a more comfortable headset. But until the advent of ANR headsets, the words "headset" and "comfort" were not normally uttered in the same breath!! Our next section will focus on the ergonomic issues that affect the level of comfort of a headset. We'll examine the wide range of variables that make getting a comfortable headset something akin to the search for the Holy Grail! We'll also cover user features like battery box sizes, controls, and accessories that effect ease of use.

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