14 minute read
COCKPIT AUDIO
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Technical
Heads up…
In Part III of how best to deal with audio issues in the cockpit, Ian Fraser looks at aviation headsets
The power of advertising is everywhere, beseeching us to spend our hard-earned money on wondrous gadgets – and it’s certainly true in the world of aviation headsets. A headset can cost us from £70 to about £300 for a Passive Noise Reduction (PNR) set, and up to £1,000 for an Active Noise Reduction (ANR) set.
In the early days of noise reduction headsets, claims of 20-40dB (see Note 1) noise reduction were common, but many of these performance values have now gone from the sales literature and device specifications.
Today, for the high-end sets, they are replaced by claims that they meet TSO C 139 (see Note 2) or are ‘30% better than conventional headsets’, whatever that means. And at the lower-cost end, they often claim to be just as good as the high-end devices.
Headsets now seem to be compared to some unobtainable or meaningless benchmark and we seem to have to rely on the suppliers, or a subjective test in a shop or on a trade stand, to work out if they are going to achieve what we want. So, do we get what we pay for? In this article I take a look at the differences between a collection of headsets and ask how you might determine if they are what you need or if they perform as implied.
In GA, our noisy cockpits generate two audio challenges – protecting our ears from discomfort and damage over time due to excessive noise, and being able to hear the radio, warnings and alarms while still being able to hear the engine clearly enough to identify
Above We put headsets under the microscope. However, is the one that is ‘best’ for you necessarily the most expensive?
potential problems. The Health and Safety Executive (HSE) say that the maximum noise our ears can tolerate (for a very short time) is 135dBA, while the average noise maximum without risking hearing damage is 80dBA. I have measured my RV-6’s cockpit noise to be about 90dBA at cruise power so, to protect my hearing alone, my headset needs at least 10dB of overall noise reduction. I will explain later how you can measure your cockpit noise and the effectiveness of your headset, which is actually quite easy.
For someone with normal hearing (by an audiologist’s definition) a good PNR headset will normally suffice to protect hearing, and no further noise reduction should be necessary to hear speech clearly. Cockpit audio alarms (such as stall warning) and the health of the engine, should still be audible despite the headset. Anyone with a hearing deficiency (and with the aging LAA membership, statistically that is most of us) may find that even ‘safe’ cockpit noise levels still frustrate hearing the spoken word clearly. Age-related hearing loss (see December 2019 article) is not just a reduction of the volume to the listener but a change to the dynamic range of the ear (the ability to hear loud noises and quieter noises simultaneously at several frequencies). It is generally manifested as an inability to hear high notes at the same time as louder low notes. This can result in words becoming muffled and having more difficulty in hearing female and children’s voices. The noise in an aircraft exacerbates this problem as it produces excessive low frequency noise, which drowns out the higher sounds. ANR headsets attempt to overcome this problem by specifically reducing lower frequency noise to optimise the chances of hearing the spoken word clearly. But be careful, too much headset noise reduction can also eliminate essential noises such as the engine note or a warning device.
What are the options?
Ignoring the microphone, there are three basic types of headset available. ■ On the ear, which is just what it says: A small loudspeaker with just a comfort pad, there is no serious attempt at sealing the ear from external noise. This is OK for a really quiet environment, which most GA aircraft are not, for our aircraft we have to address external noise. So serious GA headsets come in two categories. ■ The first are Passive Noise Reduction (PNR) types, in Right Old type ‘hard’ ear seals Picture 2
which the ear is surrounded by a cup with a large soft seal in an attempt to mechanically isolate unwanted noise from the outside (Picture 2). They are good at high frequencies, not so good at low, but are effective at overall noise energy reduction. Important to their effectiveness is the efficiency of the seal versus comfort.
Cheaper (and older) headset ear seals tend to be shallow with a stiff plastic film over foam and require a tighter grip or clamp pressure over the head to seal out the unwanted noise. More modern and costlier models use a variety of softer, deeper more pliable materials (thin leather and gel) to mould over the ears and glasses.
Their thicker but more pliable material requires a much lighter clamping grip to exclude noise, resulting in longer term comfort. ■ Active Noise Reduction (ANR) technology normally uses soft seals but also electronically cancels low frequencies typical of engine and propeller noise (Picture 3). They have an additional microphone on the outside of the ear case to detect the ambient sound then use electronics to generate an opposite signal, which should selectively cancel the unwanted noise.
The electronics and SW algorithms to do this are very complex and like most of such ‘signal processing’ are a compromise between getting rid of the unwanted noise, while not affecting the wanted sound. The extent to which they remove noise is very much proprietary to each manufacturer and is based on their understanding of the noise an average aircraft might make and what they think needs to be removed. Not all aircraft make the
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same noise and not all ANR sets do the same cancellation.
The temptation is to believe that you must throw money at the most expensive headset you can find, as it probably does the noise rejection job best, but increasingly in the market place we see claims that much lower cost devices are just as good. But at what? I set out to see if I could identify an accessible and affordable method to measure headset performance once and for all, so that they could be matched to individual needs. My tests are intended to compare, rather than determine, absolute calibrated performance.
Performance measurement – that we can do…
In the earlier days of noise reduction headsets, they were specified by measuring the total sound energy difference between the exterior noise and ‘in-earphone’ noise. That is easy to do with a noise source – a small microphone and a smartphone ‘noise meter app’ (normally a free download). I recorded the cockpit noise of about 10 mins of normal cruise in my RV-6 (at 2,350rpm) on another free app, and also measured the noise level (90dBA) and captured a 0-4 Khz spectrum (yet another free app) displaying a graph of frequency vs sound level. Above ANR technology normally uses soft seals but also electronically cancels low frequencies typical of engine and propeller noise
Below Ian testing the headsets The tests
I took this data home and put the noise recording onto my computer. Then, replaying it through its speakers, I checked that the spectrum looked the same as the one from the aircraft (to check the function of the microphone, which forms part of the earbuds from my phone) and measured the noise energy. It was 82dBA, so while lower than in the aircraft, it was still representative enough for my test. Then, for each headset, I measured external noise level, ‘in ear cup’ noise level for passive and ‘in ear cup’ noise level with ANR on. I also recorded the spectrums at each stage to get an idea of how each headset was achieving its results.
Subjective test
I then conducted a more subjective test, while still playing the engine noise. I found an aviation RT course online with a female voice and played the same part of the RT dialogue repeatedly on my smart phone with its headphone ‘out socket’ connected to the test headset (I made a 3.5mm to ¼in headset jack adapter but you can buy one online for less than £2). I recorded the lowest volume setting at which I could clearly hear the RT
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dialogue with ANR selected and noted the quality of the sound. During all tests, I also observed the effect of increased clamp pressure and the comfort.
I did try doing an audiologist (hearing aid) test while subjected to the noise, but that didn’t work too well.
Please remember that this is an overview of the market using uncalibrated equipment. My numbers may be quite different to those measured by others, but they are all derived using the same method and instruments and are repeatable, so are valid for comparison. You can see a summary of my results in the table below.
Benchmark
My benchmark ‘conventional’ 20-year-old DC H10-20 (without ANR) still achieved a pleasing 20dB+ of passive attenuation and an extra 5dB could be seen if I squeezed the earpieces to my head, but the seals are old. Its reduction was roughly in line with its claimed 24dB specification, validating my methodology. Its noise reduction spectrum in green and the noise environment in red, showed that at lower frequencies it did not reduce noise as much as the higher ones (Picture 5). As a result, its spoken word test was, although comprehensible, the most muffled of all and required the most volume increase to hear over the noise background.
All the ANR sets, rather surprisingly (to me), exhibited passive attenuation of only 6-7dB and squeezing didn’t make much difference. The majority of their overall noise reduction comes from their electronics, and their total noise reduction (for a quiet cockpit) seems proportional to cost. They all had soft and comfortable ear pads and, compared with the DC, were much more comfortable.
Their voice clarity was all much better than the DC. The only difference was the volume required to meet the threshold, and again, this went with price. Once the volume threshold was achieved, there was not much clarity difference between them.
Analysis of the active cancellation (the comparative noise graph of Lightspeed and Sony ANR ) shows that they apply quite different levels of reduction across the spectrum, and this may be the biggest challenge in understanding what they can do for us – they each may be optimised to be best at removing the noise from a different type of engine. A Lycoming 4 or 6, Rotax 2T or 4T, two- or three-bladed props, or even a turbine, all have quite different noise signatures. A manufacturer can tune his algorithm to optimise to a specific engine, or they can use one that automatically adapts to cancel
Top The noise reduction spectrum (green) and the noise environment in red of the DC 10-20 PNR headset.
Middle and below The spectrum and environment of the Lightspeed (right) and the Sony (below) Picture 6
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Summary of results
Headset Noise PNR ANR Total Comfort Audibility Clarity 3 KHz
level reduction vol Warning
DC10-20 82 dBA -20 0 -20 Tight 6 Muffled OK
Sennheiser HMEC 250 83 dBA -7 -6 -13 OK Not tested Not tested Not tested
SEHT 40-60 83 dBA -7 -13 -20 OK 5 Clear OK
Lightspeed Zulu 2 82 dBA -6 -18 -24 OK 4 Clear OK
Sony WH 1000 XM2 82 dBA -6 -24 -30 OK 2 Clear Marginal
Bose A20 82 dBA -6 -26 -32 OK 2 Clear Marginal
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any constant noise it detects. The latter are much more complex, and thus potentially the most expensive. Perhaps manufacturers could be more open on that in their literature.
So, you don’t need to spend a fortune to be able to hear clearly and protect your ears, but for the quiet flight experience, the more you spend the better.
Mind you…
My RV is equipped with a stall warner that produces a high-pitched, piercing 86dBA howl at 3Khz. While I know from experience that I can hear it through some of the sets, I was surprised at the level of reduction achieved by the Sony and Bose whose ANR reduced it to only just audible (to my ears) even without the engine running. I am not sure I would hear it during flight while wearing one of those sets.
Similarly, while the engine remains reasonably audible despite the noise cancellation, with the Sony and Bose it is reduced to a very comfortable low level and an engine problem might not be particularly attention grabbing – see picture 10 showing the noise meter graph of the Bose in-earpiece engine noise level making the transition from passive to active. The meter app reports the result as the equivalent of a quiet library.
If you were playing music, it would completely swamp any engine noise. A friend with a quieter aircraft than mine recounts a tale in which, during a long cross country, he was listening to music through his nice top-of-therange ANR headset and simply did not hear the engine falter. He did not pick it up until the aircraft started to descend uncommanded, but all ended well and an important lesson was learned. (Perhaps this is one for the human factor ‘distraction’ experts to think about).
Conclusion
As with any product, if you want to get it right you need to define clearly what you need your headset to do for you. If it is to protect your otherwise healthy ears then a good PNR set is the most cost-effective solution, and it will not eliminate essential noises.
If you have aging ears, then almost certainly ANR sets will make an improvement but, unless your ears are really bad, you don’t need to spend a fortune. And if you crave a quiet life, then a top-of-the-range set may be for you, but be careful, you may cancel out more than you bargain for – more than adequate may not be best or the safest. ■ Next month Nick Long and I look at the installation issues surrounding a modern cockpit audio system.
Note 1. What is a decibel (dB)? It is a (logarithmic) comparative measurement and is commonly used in audio or acoustics. It is used to describe how much sound energy is being produced, compared with a known reference. It can be found as both a positive number, in which it is describing how much louder the subject sound is than a reference, or negative, how much quieter. Because of the logarithmic scale, twice the sound energy is 3dB, 10 times is 10dB, and 100 times 20dB, whereas half the energy is -3dB etc. But sound energy and how loud something sounds are not linearly related. In the real world, a doubling of sound energy (3dB) is not discernable to the human ear – 10dB sounds about twice as loud and 20dB four times as loud. For example, in the case of noise cancelling, the engine noise in the Sony headset measured at 30dB will sound about half as loud as that in the DC (20dB). Don’t get too hung up on small differences in dBs because you won’t hear them. For example, you wouldn’t hear the noise difference between the Sony and Bose. However, because the dB is simply a ratio, it does lend itself to making quite accurate technical comparisons as long as we use the same tools and scales for all comparative tests.
Top right and middle The stall warner howl shown on the out the ear recording (picture 8) and (picture 9) in earpiece recording using the Bose ANR. Probably inaudible with the engine running.
Below right This graph shows the difference on the Bose headset between the ANR being on, and then it being switched off. Picture 9
Picture 10
Note 2 TSO (Technical Standard Order) C 139 is a FAA or EASA (ETSO) specification for audio systems in aircraft. From a customer’s point of view, it is useless as it doesn’t tell us what a ‘compliant’ gadget has to do. Instead it just refers to another (US) technical specification (RCTA DO 214). RCTA documents are normally electrical and mechanical parameters and may or may not specify what a headset must achieve acoustically. To find that out, you have to pay $150 or so to see the document (a PDF download). In my bitter experience with RCTA DO specs, what you may find is a reference to yet another specification ($150 more) and so on, or that what you want to know isn’t specified at all.
Customers should not have to do that to find out what the device they purchased does. Industry, be compliant with the TSO by all means but please specify what your devices do. Don’t hide behind obscure specifications. ■
