December 8, 1998 ANR 101: A Tutorial on Active Noise Reduction Headsets (Section 4 — Ergonomic and Interface 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

In this section, we'll be looking at the comfort and human-factors issues associated with ANR headsets. The ergonomics of the human head demand a wide range of adjustments to deliver comfort over hours of use.

Optimizing the physical headset for pilot use

Active pilots have been searching for ways to improve the physical aspects of the flying experience, but it's not easy to design a comfortable headset, as you'll see. While active cancellation certainly helps you feel more relaxed during your hours aloft, you're still constrained by the physics of pressure, weight, and temperature.

In many ways these comfort and fit issues apply to all headsets...not just active ones. But the human factors relating to user convenience are certainly more complicated for ANR headsets. ANR places additional constraints on the cavity and ear seal design. Furthermore, the electronics for cancellation require power, and most are supported by some form of portable power pack. The size and weight of such packs can be a pilot convenience factor, as can various other features available in the newer offerings.

A comfortable headset...the search for the Holy Grail!

Comfort is a relatively recent addition to the pilot's want-list. Early on, hearing protection and hands-free communication were the principle reasons for using a headset. The original and most popular headset design was developed for optimal passive attenuation. The focus was on military needs, and thus for noise environments often much harsher than those we encounter in General Aviation.

As needs and materials evolved, some changes were introduced to reduce weight and improve comfort. But unfortunately, these evolutionary changes have done little to add comfort in this "classic" headset design.

Why is it so hard to design a headset that fits well and wears well?

It turns out there's substantial variation in the sizes and shapes of human heads. The chart below is data gathered from military ergonomic standards for head sizes. It compares the dimensional differences for men and women covering the 5^th to 95^th percentile of people...basically almost the whole population!

The challenge for fit: the 5-95% dimensions for head width, height, and ear position:

Source: Human Engineering Design Criteria for Military Systems, Mil-Std 1472C

As you can see, the challenge to get both adjustability and a comfortable fit is a difficult one. The classic design relied on side pressure to compensate for the width variations. The metal overhead band has some adjustment for circumference but no accommodation was provided for ear protrusion. In analyzing how to make a more versatile headset design, the issues boil down to size and side pressure!

Adjustability

Obviously the headset needs to adjust big enough (or small enough for ladies) to properly fit at the start of your flight.

Look at some of the variability in things like head width (over 1 inch!) or the position of ears on the side of an "average" head! Overhead circumference means the headband needs to have ample "length" adjustment to extend the needed 2.5 inches. That's a lot. Ladies and children need it to compress to fit their smaller heads. Yet many pilots are men with size 7 3/4 hats!! While all this is certainly possible, you'd be surprised how many existing designs cannot cover that range of heads.

Comfort over time

But for most of you, your "comfort problem" isn't that your headset can't fit you when you first put it on, but rather that it starts to hurt after a short time. If you have flown much at all, you also know your comfort needs change some over the 1 or 2 or 4 hours you might be flying.

Without getting too medical, let's try to understand why. There is very little "subcutaneous fat" (flesh, padding, etc!) under the skin covering your skull. The head is also extremely vascular (there is lots of blood flowing around under this thin layer). The area around the ears is particularly dense in blood vessels. Head surfaces get tender or "fatigued" by constant pressure and typically develop sore spots from wearing a headset. These "hot spots" are actually places where the blood flow has been constricted (from pressure) and become sources of pain! The combination of little fat and high blood flow makes this region hypersensitive to pressure.

With that understood, then, a premium is placed on "uniform" distribution of the weight...both over the top of the head and the compression exerted by the earseals around your ears. Making a headset lighter in total weight is good, but actually the distribution of that weight is the more important variable to comfort. Heavier headsets with pads that distribute weight evenly over a broad area will feel much more comfortable than one that is lighter but has limited contour distribution over the top of your head. That's accomplished by proper radial design of the headband system and effective padding. In summary, the comfort of your headset will have a lot to do with the overall design and the "head interface"...the way it fits and conforms to your head shape and ears.

Those all-important ear seals

The ear seal design is probably the most important aspect of overall flying comfort. Size, shape, and material choice all affect the comfort of the headset, as well as its noise attenuation in both passive and active modes. It should be noted there are several "special" aspects to consider in designing a comfortable earcup cavity. Ideally, the ear would be floating free in the cavity...not touching any aspect of the seal or speaker system. That has a direct bearing on the size, shape, and construction of the optimal ear seal:

• Cavity opening orientation: As noted in Section 1 of this series, proper orientation and shape will also help isolate the ear from the seal. Many pilots actually tilt their 'oval' shaped domes backward to better align them with their ear.

• Cavity depth: Notice the >1/2" variation in "ear protrusion" on the above chart. This must be accommodated either in the depth of the ear seal or in the dome cavity opening.

• Cavity volume and ventilation: Greater depth also helps in keeping the ear cooler...more air space that heats up slower. The material used to surround the seal would also ideally be breathable...minimizing humidity (and sweat buildup) around the ear. Within reason, larger is better when designing for ear comfort.

Obviously we'd like the ear seal to create a cavity space that is both comfortable and quiet. It turns out there are tradeoffs between optimal comfort and getting the maximum passive attenuation. Let's take a look at different materials used in ear seals to learn more about the strengths and weaknesses of each.

¹ the ability to cross-sectionally block out noise.
² how easily the material will flow to cover an uneven surface.
³ the amount of pressure needed to seal against a vertical uneven surface.
⁴ the weight of a "normal" aviation earseal for various ANR headset.
⁵ compared to a silicone seal, how large is the cavity volume created by the ear seal.

You'll notice that the best materials for blocking out sound are liquid and silicone. Liquid certainly conforms the best to uneven surfaces (like the side of your head!) but requires reasonable pressure because of the vertical orientation...the liquid will stay at the "bottom" of the seal unless the side pressure is adequate to "squeeze" it up around your ears. In a headset application, the silicone is actually better since it will stay in place. Foam/liquid and Thermal-conforming foams don't have as much cross-sectional density to provide the best attenuation passively.

From a comfort standpoint, silicone gel seals may not be the best choice. It is a dense material...does not easily conform to the many variations around the back of your ears. It only conforms under relatively high pressure (compared to the other choices). It provides adequate relative volume but is quite heavy...twice the weight of the other ear seals! Liquid seals provide very little cavity volume and require medium side pressure for a good seal.

Temperature-sensitive foam materials are clear winners for conformability, minimum side pressure, and ear cavity volume. It's particularly effective when wearing glasses. The conformability helps minimize the localized pressure of the stems pushing on your temples! All of those characteristics will translate into a more comfortable headset.

As we have pointed out earlier, the best and only sure way to decide about comfort in a headset is to fly it.

What does this have to do with ANR performance?

Comfort and fit, by themselves, have little to do with active cancellation but ear seal conformability does. Remember back to Section 1...the discussion about needing a stable acoustic cavity for maximum cancellation. Even more than in a passive headset, an ANR headset needs to provide an "acoustically tight" fit. Tight doesn't have to be vise-like, but a good seal to the head is required to get proper cancellation If you doubt this, lift off the ear seal of an active headset while flying. You'll hear the system become unstable and start to oscillate.

Particularly when the amounts of active cancellation are high, a stable acoustic cavity with no "leaks" is important. That seal comes from either a dense material under high pressure or something of lower density with lower pressure.

What about lightweight ANR headsets... Do they work?

For many pilots, the "superaural" headsets might be more comfortable. These are the type that sit "on" the outer surface of the ear. They are light in weight and provide good ventilation for the ear. Their obvious disadvantages include limited noise isolation, limited passive protection, and a lack of basic headset features like volume controls.

Because of the sensitivity of the ear surfaces, sometimes these actually are less comfortable than a typical over-the-ear headset. For some cabin-class planes, they may be a nice solution. If you have a really quiet plane, give them a try and compare them to a circumaural (muff-type) for the tradeoffs between comfort and peaceful flying.

Powered headsets...what a hassle!

The one issue that is most noticeable about active systems is their need for an external power. With passive headsets the sidetone audio (what you hear) and the voltage to drive your microphone preamp (for what you say) are supplied by amplifiers in your avionics. The result is you have no need for external power for that headset to work. In active headsets, additional power is required to support the active electronics in the headset itself. In Section 1, we discussed the efficiency of various speakers and electronics and how they would effect cancellation performance and battery life. All that ties into this discussion of power and portability.

The majority of ANR headset users require portable power. For most pilots, a portable ANR system conjures up images of large, bulky battery packs with a rat's nest of extra cables. That was the norm up until just two years ago. As a result in improved efficiency, the "typical" ANR headset battery box went from 6 AA's and a separate cable to just 2 AA's placed inline with your normal headset cord. All that while battery life stayed in the 30 to 40 hour range...months of flying for many of us! Most pilots favor portable ANR headsets that use AA batteries rather than 9-volt batteries, since most pilots already carry a stash of AA's for their handheld GPS, mini Maglight flashlight, etc.

Then there's the problem of leaving the headset on at the conclusion of a flight, something that doesn't do much for battery life. Certainly the best solution for this dilemma is some "Auto-Off" feature built into the headset. It senses when you have it off your head and automatically shuts off. It will become more popular in the coming years.

Rechargeables

People have asked about using rechargeable batteries with ANR headsets. You can use rechargeable AA batteries in most battery-powered units, though the life expectancy drops about in half (compared to alkalines), and making sure that they are always recharged before flight can be a hassle. Some ANR headets have built-in rechargable batteries, offering the benefit of no extra "box." The built-in battery can be convenient as long as you remember to take the headset out of the plane after each flight and charge it fully for the next use. As with all rechargeables, battery life is substantially less than with alkalines, but more than adequate for normal flying...if you remember to charge them before flight.

While battery life is important, the truth is that more batteries are lost to forgetfulness (forgetting to turn the headset electronics off) than to sheer hours aloft. That's really the "Achilles heel" of a rechargeable headset...if you forgot to turn it off yesterday, you are out of luck with active cancellation today! The rechargeables can't be charged in-flight while you're using the headset, and you can't change the batteries before or during flight if the built-in ones run low.

Aircraft power

ANR headsets powered from aircraft power can solve these problems and completely eliminate the issues of batteries, battery boxes, etc. Most aircraft-powered ANR headsets have integrated panel jacks that allow for a clean, single connection for all audio and power connections. Others provide a separate power connection (via the cigarette lighter or a separate power jack), but this means you have to plug in three separate connectors (mic, phones, power).

The convenience of the panel-wired single connection is great, but installation is something that should be done by an avionics shop, so plan on additional $150-200 installation expense for each headset position. Unless you fly an awful lot, this could be equivalent to a lifetime supply of AA batteries! That's one reason that for many pilots, a portable model makes more sense.

Most of the latest generation of active headsets come with a full set of standard features for easy use. These would include dual volume controls, stereo/mono capability, some form of battery gauge, and a padded bag for carrying and protecting your investment. Different suppliers provide these features and functions in slightly different ways, so make sure you pay attention to them when you test-fly different headset models.

So there!

Now you know more than you probably thought possible concerning what headset design factors contribute to a comfortable flight. In the final section of this series, we'll tie together all that we've learned about hearing protection, improved communications, and comfort...when we look at the physiological issues that affect the pilot. We'll explore how low-frequency noise affects fatigue — both mental and physical — and how that can affect pilot reaction times and decision-making capabilties. A fellow pilot who is an audiologist will be contributing to this discussion, drawing from his professional knowledge and experience to help address these issues.

LightSPEED Technologies, Inc.
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Lake Oswego, Oregon 97035 USA