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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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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 thing?
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 dome
cavity
-
electronics that will "process" that signal and relay it to the anti-noise
speaker
-
a speaker (also known as the "ANR driver") that "adds" sound into the dome
cavity
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
First, 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 carefully managed.
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, qualified no.
What affects how well ANC works?
We've already seen that any active noise cancellation system involves three
basic components:
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:
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
The 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 graph.
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
comparative performance.
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 efficiently.
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.)
Speaker "efficiency" is a measured by the energy required to move the
diaphragm forward and back to create the "waves" of sound...the more energy
required to generate the waves, the lower the efficiency. Because most ANR
headsets sold today are portable models powered by batteries, speaker efficiency
is an important consideration. Higher efficiency means lower power consumption,
longer battery life and smaller battery boxes.
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.
Analog vs digital cancellation...what's the difference?
Before closing this section, it's worth making mention of a new type
of cancellation technique that has recently appeared on the market: digital
cancellation. While the basics elements of sensing, processing, and reproducing
sound are maintained, the way the signal is "processed" is different.
Analog systems re-insert the full spectrum of noise that is "sensed" with an
inverted phase from the original signal. In contrast, digital systems sample the
sound, analyze it, "predict" the noise levels, and then insert one or several
audio tones that will cancel specific frequencies. While not as effective at
canceling broadband noise, this is a technique that should be very effective
cancelling specific frequencies...like propeller resonance frequencies.
Once again, the best and only way to really know how well various
systems work is to fly with several different ones and compare.
LightSPEED
Technologies, Inc.
15812 SW Upper Boones Ferry Road
Lake Oswego,
Oregon 97035 USA
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