Section 1: The Basics of ANR
A good place to start is with one of the most-asked questions that we
receive from pilots:
Don't all active headsets work the same way... or do the same
The answer is a simple "yes"...and a much more detailed "no." It's yes
because the principles of additive cancellation with out-of-phase sound is
several decades old and is central to how all aviation active noise systems
work. We'll cover that part first.
There are three elements of a active cancellation system that will be
present in every headset. The basic sensing logic of the microphone and
"anti-noise" speaker for each ear include:
- a microphone that "hears" the sound levels that have gotten into the
- electronics that will "process" that signal and relay it to the
- a speaker (also known as the "ANR driver") that "adds" sound into the
This "added" sound combines with the existing ambient noise to reduces
the overall noise level. It's the "summing" of those signals that represents
the active cancellation. How much cancellation can be achieved is a function
of many variables we'll cover later in this section.
What "cancellation" means
let's be clear about what we mean by noise cancellation. We often get
questions about whether ANR is just a "masking" sound that "hides" noise,
much as a dentist might use music or white noise to "mask" the pain of
drilling. Such a "masking signal" approach would inject additional noise
into an acoustic environment and cause the ears and/or brain to perceive a
different, more acceptable overall frequency response than what was
originally present. But obviously, this would not be "cancellation" and is
not what an active-canceling headset does.
Active cancellation involves the "addition" of sound waves...one wave
mixing with another and dampening its energy. The coming discussion will
focus on "analog" cancellation techniques, which is the dominant approach
used for headsets today. (We'll also touch on digital cancellation.) It's
important you understand the physics of what is being done, since this is
not a simple task. Particularly as you get to higher levels of cancellation,
the demands for precision in each aspect of the design are critical to
proper function, as you'll see.
While it might seem odd to add large amounts of "new" sound energy into
the dome cavity and ear canal, that's exactly how these active systems work.
A microphone picks up the frequency and amplitude profile at an instant in
time inside the dome cavity, the signal is processed and amplified by the
electronics, and an inverted "anti-noise" signal is produced by the active
driver to provide canceling sound waves.
Limits on how much cancellation is achievable
The profile of the canceling signal must fall within the contour of the
existing noise spectrum to get effective cancellation. At low levels of
cancellation, it is relatively easy to provide an inverted signal that fits
within the existing noise profile. But the more you try to cancel, the
harder it is to keep it functioning properly. If the canceling signal is
either over-amplified or distorted in some way, the new signal will
partially fall outside of the existing noise profile, and will thus be
audible as new noise! In addition, a feedback condition (oscillation or
"squealing") can arise unless the phase and gain of the control loop is
This is what limits how much active noise cancellation a headset can
deliver - what technical folks call the "gain" of the system. The gain must
be carefully set to make sure the systems has good "stability" - in other
words, that it works repeatably and doesn't oscillate!
Obtaining stable performance with large amounts of cancellation is no
small task, particularly when you add in the variables of ear shape,
different sized heads, existing cavity shape and designs, etc. All of these
are part of the reasons why the answer to our original question "Don't all
active headsets work the same way...or do the same thing?" is an educated,
What affects how well ANC works?
We've already seen that any active noise cancellation system involves
three basic components:
- a noise-sensing microphone
- some processing electronics
- an anti-noise speaker
Getting exceptional performance from an ANC design requires attention to
each element of the system we've described. Let's look at each of these
three elements in detail.
The "accuracy" of the noise-sensing microphone
To begin with, we need to pick up the most accurate sound replication
within the dome if we hope to provide effective cancellation. When we talk
about accuracy, we're really addressing two separate issues:
- Sensitivity and fidelity: how well the microphone
reproduces the sound it hears.
- Correlation: how closely the sound that the microphone
picks up corresponds to the sound your ear hears.
The first issue (sensitivity and fidelity) is important, but it's an easy
problem to solve because there are many very good mics available for a
manufacturer to use. It's the second issue (correlation) that's the
difficult part of getting good information into the cancellation system.
Simply put, an ideal system would cancel noise right in your ear canal, not
just at a mic positioned somewhere in the dome. While it's not practical to
stick a mic in your ear canal, a good headset design can go a long way to
ensure that what the mic picks up is very close to what your ear hears. To
get this right, it's important to know some things about the ear opening and
the acoustic cavity we're working in.
- Is this
a "left" or a "right" ear cup? Although ears come in a lot of
shapes and sizes, it turns out statistically that the ear opening is not
symmetrically "centered" within the dome...it is forward and below center.
The mic placement must accommodate human physiology to deliver optimal
performance. Consequently, the best performance is attained when ear cups
are designed to fit specific ears (left and right). You'll notice in the
diagram that the domes are tilted and shaped to better fit the ear, and that
the mounting plates that hold the mic and active speaker are also angled to
better profile the ear shape in the dome. Attention to these ergonomic and
acoustic details substantially improve both comfort and performance. (It's
interesting to note now that more manufacturers are at least noting this
important feature in their documentation. Even headsets that can be reversed
should not be worn that way!)
- What is the position of the microphone relative to the ANR
driver and the ear opening? The location of the mic has everything
to do with the effects the active driver output has on it. If the mic is
directly above or next to the driver, you will get a stable, closed loop
cancellation (with good predictable cancellation) but have noticeably less
cancellation at the ear opening! Many of the "claimed" cancellation numbers
you read in ads and spec sheets are measured at the mic...not the ear
opening. Obviously, what we want is the highest cancellation at the ear.
Without getting too technical, then, mic position is a key contributor to
just how much cancellation you'll get in your ear. This is not just acoustic
theory and principles...you really can hear a difference when attention has
been paid to these details of design. When it's all said and done, you just
have to listen to various ANR headsets and compare them in your plane!
The sophistication of the processing electronics
signal from the microphone is sent through the electronics and is amplified
or "processed." There will be substantial differences in delivered
cancellation depending on the design of the acoustic cavity, the mic
placement issues already covered, and the gain levels the design will
support. The actual performance can be measured and plotted for comparison.
Such a profile of performance for several headests is shown in this
You'll note the graph charts frequency and depth (in dB) of the
cancellation. There are significant variations in active canceling
effectiveness for headsets on the market today. We'll cover this graph and
similar ones in great detail in Section
2 of this series....so you can begin to make educated judgements about
The sophistication of the electronics, amplifiers, and audio filters has
a great deal to do with overall headset performance. The differences can be
seen in more than just the effectiveness of cancellation. The way the audio
is processed will affect what you actually hear. Examples include:
- The communications audio...what you hear from your comm
radio. The less sophisticated ANR systems process the audio together with
the ambient noise. The result is that the low frequency components of the
radio and audio communications are cancelled along with the noise. This
causes frequency response to be degraded, sounding tinny and unnatural. More
sophisticated ANR systems process the signal and noise separately, allowing
for full reproduction of the original audio signal.
- The Signal-to-Noise ratio...what you're looking to ANR
to improve. All active products provide at least a modest S/N improvement
when the masking effects of loud, low frequency noise are reduced. More
sophisticated designs boost the signal levels to further enhance
intelligibility, producing clearer communications...particularly for those
who have suffered some hearing losses already.
Both of these are tangible examples of real differences among different
ANR headset designs. Again...hearing is understanding!
The effectiveness of the speaker
After sampling the ambient dome sound profile (hopefully well-correlated
to the ear opening!) and processing that signal for proper fidelity, the
system needs to produce the canceling "waves" of sound effectively and
Speaker "effectiveness" would be measured by how closely the wave profile
produced matches the input mic signal. The long, low-frequency "waves" are
most critical to canceling of the 50-300 Hz. noise so prevalent in piston
engine airplanes. More effectiveness will translate into better
low-frequency cancellation. With the highest decibel levels in most planes
between 80-120 Hz., you want a headset that provides maximum performance in
that part of the noise spectrum. (In Sections 2 and 3 of this series, we'll
learn more about specific headset cancellation performance and the actual
airplane noise spectrum.)
Many people are put off by the size and hassle of the external battery
modules associated with ANR headsets. Some of today's headsets come with
large power packs that hang on separate cables, while others have small
modules that are attached in-line to the communications cable. To achieve
30+ hours of operation, some systems require as many as ten AA batteries,
while others require just two! That's a big difference and worth
investigating before you finally purchase a headset.
Go to Section 2 >>