December 8, 1998 ANR 101: A Tutorial on Active Noise Reduction Headsets
(Section 2 — Acoustic Issues) |
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Questions about ANR? LightSPEED Technologies answers all of them in this five-part series for AVweb.
December 8, 1998
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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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 Understanding and comparing cancellation specifications
Section 1 of
this series dealt with the variables and complexity involved in canceling
the repetitive and random aspects of airplane noise. It explained that although
the basic elements of all active noise reduction systems are pretty much the
same, the details of how they are designed and implemented has everything to do
with the results you'll experience.
The focus of this session is on the ways of measuring noise cancellation and
determining comparative performance. Many claims are made about the amount of
cancellation a given headset will provide. As we've said all along, the very
best way to choose an ANR headset is to actually try out various headset
models in the aircraft you usually fly. No chart, graph, or product review is as
effective in determining your best choice of headsets as actually flying with
them. Unfortunately, that is often not feasible, so you're left to make a
purchase decision based on the information you get from the manufacturers, the
product reviews you read in the aviation press, and word-of-mouth
recommendations from other pilots.
Our goal in this section, then, is to give you the knowledge you need to ask
better questions and make a more critical analysis of the active and passive
cancellation you can expect from a particular ANR headset model.
The cancellation profile
All active headsets have a profile of sound they are able to cancel. The
profile can be graphed to chart the amount of cancellation and where in the
noise frequency spectrum that cancellation occurs. The measurement process is
relatively simple but requires fairly sophisticated laboratory equipment and
techniques. The testing is done inside a closed, controlled acoustic chamber.
The headset is mounted on a fixture that has a microphone located inside an
artificial "ear" whose purpose is to sense or "hear" what you would hear. The
headset is then swept with a tone from 10 hz to 10,000 hz...with the "ear"
recording what was heard. The sweep is done twice, once with the ANR circuitry
turned off and another with it turned on. The difference between
the first and second sweeps indicates what was incrementally cancelled when the
active system was engaged. The results are then usually plotted in the form of a
graph, with frequency in Hertz (Hz) on the horizontal axis and cancellation in
negative deciBels (dB) on the vertical axis:
Of course, different headsets provide different results based on how well
they have managed the variables we learned about in Section 1 of this
series. Before we begin to do any comparisons, lets first talk about the
attributes of this cancellation curve that will be present in every active
headset. They include:
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Depth: how deep is the cancellation at its deepest point.
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Breadth: how wide is the cancellation frequency spectrum.
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Position: at what frequency the cancellation profile is
centered.
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Boosting: places in the cancellation spectrum where ANR actually
does more harm than good (i.e., noise is amplified rather than cancelled).
Each attribute tells us something about the effectiveness you can expect to
hear from the cancellation.
 Depth
The depth of the curve tells us the maximum amount of cancellation you would
generally expect. Normally this is the active cancellation number that a
manufacturer will advertise. Typically a range of 2-4 dB is given since
variations in mic sensitivity, calibration, and even specific ergonomics of the
pilot will have some modest effect on the final cancellation. It's important to
make sure you know how the cancellation measurements were made. Some
manufacturers publish noise reduction figures that were not measured at
the "ear" but at the headset's ANR sensing mic instead. Recall from Section 1 of this
series that the best ANR systems position the sensing mic to provide optimal
correlation to the ear canal position, but correlation is never perfect. For
example, headset "T" in the above graph claims 14-16 dB of cancellation but only
measured about 11dB at the ear. Since every 3 dB represents roughly a doubling
of sound pressure, that discrepancy is quite large and is probably explained by
that different measurement technique.
 Breadth
Here we're looking at the overall frequency band that is cancelled by the
electronics. Given the measurement deviation and sensitivity of our ears to
small changes in dB, cancellation greater than 5 dB would begin to be of value
in comparisons. When combined with "depth," these two measurements constitute
the "total" cancellation the electronics can deliver.
 Position
You'll notice in the graph that one headset cancels more low frequency than
the other.While every plane has its own unique noise signature, it turns out
there are some common things we can say about most single-engine piston planes:
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The loudest part of the noise spectrum is generated by the resonant
frequency of the propeller. For direct-drive engines red-lined at 2400-2700
RPM, that will occur at 80-90 Hz for two-bladed props and 120-130 Hz for
three-bladed ones. That is the specific point in the spectrum where the noise
is loudest.
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The overall noise envelope in the cabin of these airplanes is
loudest from about 40 Hz to 250 Hz, so this is the range where cancellation is
most important. Noise levels at even 500 Hz are typically down at least 10dB
from those at 100 Hz.
Section 3 of
this series deals in depth with airplane noise spectrums and how they
"overlay" with active and passive performance. For now, it's sufficient to
understand simply that the loudest noises are at the lower frequencies. So a
cancellation profile "position" that is centered between 85 and 130 Hz will
probably be the most effective in the typical piston-powered airplane.
Boosting
This is the last characteristic evident in all ANR cancellation profiles, and
perhaps the least understood by most prospective purchasers of ANR headsets.
Boosting is actually amplified, out-of-phase noise created by the ANR
electronics that is not cancelled by existing ambient noise. Simply put, it
represents the "overshoot" of the electronic "anti-noise" signal introduced into
the acoustic chamber. It is present to some extent in virtually all active
headsets, and here's why. (Refer to the cancellation profile graph above to
follow this).
The physics of sound waves and the acoustic cavity makes significant
cancellation much more difficult as frequencies increase. This is due to the
shorter wavelengths of sound at higher frequencies, in relation to the physical
distance between the sensing microphone and the anti-noise speaker. The graph
above shows this phenomenon on various headsets, all of which have declining
cancellation at frequencies above 300 Hz. The deeper the 100 Hz cancellation,
typically the steeper the profile drops off between 300 and 600 Hz. The steeper
the drop-off, the harder it is to avoid some "overshoot" of uncancelled noise.
Is boosting a bad thing?
While not disastrous, we'd always like to have less. Like so many things,
tradeoffs have to be made between the amount of boosting and the overall system
cancellation. Unfortunately, greater overall cancellation generally means higher
levels of boosting. This explains why the best canceling headsets often have the
largest boosting effects. All things considered, the additional cancellation is
generally worth it for at least two reasons:
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Typically the boosted dB levels are relatively low (3-6 dB) while the
additional cancellation at low frequencies are much larger. The actual amount
of boosted noise (shaded area under the curve) is quite small compared to the
total noise cancelled.
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The boosting generally occurs at higher frequencies around 1 kHz where
headsets typically have significant passive attenuation, and where most
airplanes have relatively low noise to begin with. The net effect is still
significant noise reduction at those higher frequencies...just a little less!
 The one
exception is low-frequency boosting like that shown for
Headset "C" in the graph. While only 3-5 dB, the headset might provide less than
5 dB of protection passive at 40 Hz, so the additional noise introduced by the
ANR circuitry could be more noticeable at those frequencies.
Congratulations! You now know more about interpreting
cancellation profiles of ANR headsets than 99% of the pilot population (not to
mention most so-called avionics experts). Of course, you can only use this data
to compare the performance of different ANR headsets accurately if the
data for the various headsets was derived using similar testing methods. Testing
variables such as mic placement, sensitivity, and fixture design will
substantially change the measured results. Again, the absolute best test
data comes from your own ears...comparing different headsets in different planes
and with different people!
"Total" Cancellation: an "apples and oranges" comparison
We can't leave the subject of cancellation measurement and comparison without
dealing with a very misleading notion: "Total" cancellation. What we're talking
about here is the practice of adding some measurements or claims of passive
cancellation to the ANR cancellation profiles we discussed above. While often
done in advertisements, this practice is inaccurate and quite misleading for
several reasons. To understand why, you must first understand the "NRR"
measurements that are normally used to quantify passive hearing protection
systems, and how this relates to aircraft noise.
Passive attenuation (NRR) measurements
There is still considerable debate about how well this "standard" fits the
aircraft environment. The NRR measurements were developed for industrial safety
hearing applications where the noise spectrum is higher in frequency, and
intermittent or impact based (e.g., the noise in a machine shop). The way the
noise was measured affects the relative importance of the frequencies in the
weighting of the noise effect on your hearing. The graph below shows the
"equivalent effect" curves for different scales of noise.
Hearing protectors (including headsets) were measured and rated based on "A"
weighted noise curve. As you can see, the ratings are heavily weighted to the 1
to 4 kHz noise spectrum for protection based on hearing protection needs of our
ears. Because of the way God designed our ears, we can tolerate about 20 dB more
100 Hz noise without hearing damage than we can we can 1,000 Hz noise. (That's
why the "A" scale curve crosses the 100 Hz line at about -20 dB.) "A" weighted
NRR measurements "discount" the 100 Hz noise by 20 dB in the calculations of
hearing protection.
The problem for pilots is that we don't fly machine shops, and our cockpits
have lots of 100 Hz noise...often 25-30 dB more there than at 1,000 Hz!!
While it's true that we are more susceptible to hearing damage from higher
frequencies, the dB levels we're exposed to at low frequencies often contribute
significantly to the hearing loss that pilots who fly without hearing protection
often suffer. Those excessive levels of low-frequency noise also have a
noticeable effect on speech intelligibility and human physiology...which we'll
cover more in Section 3 and Section 4 of this
series. Consequently, "A" weighted NRR measurements are really not the
best standard for judging the protection your headset will provide in your
single engine airplane.
Real NRR data for active headsets
Even if NRR could be used as a reference (by using a weighting more
appropriate to the cockpit noise spectrum), there is a problem with using such
figures for active headsets. We're not aware of a single manufacturer who
publishes actual NRR measurements for their active headset models.
When passive headsets are "converted" by adding active noise reduction
circuitry, sales literature often "implies" that the active cancellation is in
addition to the headset's original passive NRR. However, the fundamental laws of
acoustics dictate that passive attenuation will go down when active
circuitry is inserted and cavity space is consumed in the active version. Recent
product reviews of ANR insert kits make this point quite clear.
The NRR and active cancellation "numbers" don't add up!
Passive attenuation NRR is an accumulated measure of cancellation taken at
eight frequencies from 125 Hz to 8,000 Hz. Those numbers typically range from
about -10 dB to well over -30 dB...a very wide range! The NRR (which is a single
figure) is intended to provide a summary measure of cancellation over a very
broad spectrum of noise (weighted for hearing protection as discussed above). By
contrast, the active cancellation dB numbers that are normally quoted refer to
"peak" attenuation at one frequency where active noise cancellation is at
a maximum. When you add the broad spectrum-weighted NRR to the single-frequency
active cancellation figure, you get…nonsense! The figures simply cannot be added
with any validity.
So how can you judge which ANR headset is most effective?
If you've read this far, you already know the answer. Compare the active
cancellation profiles of headsets, provided they were derived using comparable
measurement techniques. Look for the depth, breadth, and position of the ANR
spectrum. Look also for areas where boosting occurs, particularly at the
low-frequency end of the spectrum, where it's likely to be noticeable.
Then once you've developed your "short list" of headset candidates, try to
actually fly with them so you can truly evaluate the performance, comfort and
convenience features of each one. After all, the ultimate laboratory test
fixture is your own head in the cockpit of your own airplane!
LightSPEED
Technologies, Inc.
15812 SW Upper Boones Ferry Road
Lake Oswego,
Oregon 97035 USA
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