Section 3: Airplane Issues
Looking at actual cockpit noise profiles, and how well aviation headsets
cope with it
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.
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.
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:
- There is a lot of noise in the low frequencies...between 70 and 300 Hz.
- Noise levels decline in the higher frequencies...particularly beyond 500
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
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 nineBlack and
brain that have daily and direct effects on your ability to fly a plane
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
Projected hearing loss from continued noise
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
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
(Cessna Cutlass with adjustmentsfor
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
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
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
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
Go to Section 4 >>