ANR for the whole cockpit

[Check Airman]

Hello,

From my basic understanding of ANR headsets, the mic detects the "background" noise and the electronics do the magic to remove it. Given that the cockpit is an unhealthy place for our ears to be (70-80 dB), and that many airline pilots have some hearing loss in one ear more than the other, why build ANR into the cockpit sound system? There are lots of mics and lots of speakers all over the place, why not cancel the background noise so the cockpit can be quieter? My current headset gives about 20dB passive and 15dB active, and I must say, the difference is a world apart.

I must say that for all the physics I've done, I've never studied acoustics past the Doppler effect, so my idea may be totally daft, but I'd like to know why.

[Aspen20]

Some years back Beechcraft experimented with the idea on a King Air to reduce cabin noise. Don't know how far that went

[Gooneyone]

Being very cynical, the answer is simply cost. It is cheaper to replace a pilot (probably nearing retirement anyway) who loses hearing due to cockpit noise than it is to install and maintain ANR in the cockpit.
If we wish to protect our hearing, the bean counters will say buy your own ANR headsets.

[Check Airman]

I can't see it being that expensive. It shouldn't add much (if anything) in terms of weight, and should be a part of a basic requirement for a safe and comfortable work environment.

I there any other idea why this isn't a part of the basic equipment in the cockpit?

Glad that Beechcraft started something, I'd really like to know what came of it. I really couldn't care less about cabin noise though. Pax are only in the cabin a few hours a year, plus it's quieter back there anyway.

[PaulW]

Saab 2000 and Dash 8-Q400 have ANR equipment in the cabin for passenger comfort. In the case of the Saab it works very well. No louder than a jet. The Saab is one of the aviation worlds best keep secrets in my opinion. There arent many turbo props that cruise easily at 370knots TAS at FL310 and above, full glass cockpit and reliable.

[GlueBall]

Sennheiser noise cancelling headset or foam earplugs will magically eliminate or substantially reduce the background buzz in the 747 cockpit.

[PaulW]

Seinnheiser HMEC-25s work well in turbo-props too. (ANR headsets used by BA and other jet operators)

[Check Airman]

Rainboe obviously has more knowledge in the subject than I, but PaulW notes that the Dash8-Q400's have ANR in the cabin. This seems to contradict Rainboe's argument. Anybody know Bombardier manages whole-cabin ANR in light of what Rainboe said?

[javelin]

And spookily, there is a set of Sennheisser HMEC25's on ebay at present !

ATR had ANR in the cabin - I remember positioning on one where the Captain said he was turning the system on in the cruise - very effective indeed.

[Stuck_in_an_ATR]

I know ATR has been experimenting with ANR in the cabin some years ago - but it didn't work for the reasons specified by Rainboe and they dropped it. I was told that if you cancelled the noise at one place, you'd get a hell of it in another. Me company even operatest the airframe that has been a testbed fo this - but with all the equipment removed

[Mercenary Pilot]

From what I understand of ANR, the device emits a noise in the opposite phase of the original sound which then makes it inaudible to the human ear. If this is the case then ANR is only a comfort thing and does nothing to actually protect your hearing (due to the volume being there but at an inaudible frequency).

[Check Airman]

I'm not a sound expert though I did do Physics up to advanced A level have some Open University studies.

It seems that we're just about equally qualified on the subject then:) Just kidding. You obviously remember/ know a LOT more than I do. I now appreciate why this tech isn't in the cockpit. Thanks a bunch Rainboe and everyone else!

[604guy]

The Beech Kingair 350 comes standard with ANC for the entire cabin. Made quite a noticable difference when you turned it on/off when I flew one back in the mid 90's.

There was a high end automobile advertising ANC for the passenger compartment a couple of years ago. Too lazy to look it up right now but I am thinking Lexus.

[arem]

<<thinking Lexus.>>

Could have been on the LS series but my RX is very quiet anyway - its the road noise ie old style tarmac thats the problem

[AtoBsafely]

In the Q400 cabin Bombardier used a combination of passive and active vibration suppression. ie they tried to reduce the cause of the noise. This technique doesn't translate well to a jet airframe.

One of the major causes of wind noise in the cockpit are the wipers. I wish they were retractable! If designers worked at it they could probably reduce cockpit noise quite a lot, but that would cost.

[balsa model]

I'm also not an expert but this is somewhat along my interests so maybe I can add a little.
Rainbow's pond visualisation and explanations are great. Maybe except that the thing can work over wider frequency range.

Essentially the bigger the room (or an accustic cavity enclosed between the headphone plate and the ear canal), the lower is its 1st resonant frequency.
For example a room with a length of 3.3m (10ft) would have its 1st resonance near 45 Hz (or 2700 rpm).

Freq_of_resonance = Speed_of_sound / (2 * Length)

For the purposes of active noise reduction, this is the frequency below which the simple ANR concept of creating opposite phase but equal amplitude waves will work evenly across the entire room.
Below the resonant frequency, all tones are essentially at the same phase across the room. The cancelling signal also fills the room with uniform phase and the two combine to produce quiet.

Approaching the resonance and above it, the sound is found at different phases throughout the room.
Since the cancelling wave is produced at a physically different spot than the noise, you will end up with the picture of multiple wave rings in the pond, with some spots where they combine and some spots where they cancel.

To make ANR work in an aircraft cabin (or a car), a few things can be done.
If the cabin is small relative to the expected noise (propellers are #1 culprit in twins), you could get away with a single mike and a single cancelling speaker. You have to make sure that the mike pickup is filtered out above the cabin resonant frequency. This will prevent the system from even trying to work where it can't.
If the cabin is larger, you will need "local area" mikes. With pilots, you have naturally co-located mikes to take advantage of. In one aftermarket car installation, I think that they hid the mike in the headrest (it used car's stereo for its speaker and worked well for the driver only, I was told).
Then, you have to use "local area" directional speakers, perhaps pointed across the cabin (for shorter dimension), plus some accustic padding to make sure that the cancelling sound doesn't reflect and spread to other "local areas". Overhead (P/A style) speakers would be my first try.
More sophisticated directional speakers could use speaker arrays.

Anyhow, it's not easy but it can be done. If (big if) you have the development budget, time, and excess payload capacity to get rid off.
I think that the biggest challenge would be in avoiding heavy accustic padding and dividers. Oh yes, and the boom-box.

Javelin: Did anyone say anything why the system was only used during the cruise? (props kept out of sync in climb, power conservation, general mistrust?)

bm

PS: Disclaimer: I have never built such a thing. It's all theories and for all I know the world IS flat.

[Seat1APlease]

But it's surprsing how little the wipers are actually needed in anger, perhaps they could be reduced in size or faired into the surround better than they are.

[Self Loading Freight]

It's my understanding that you can do ANR over large areas, but it's not simple nor as effective as ANR in headphones, and is very frequency dependent. One system I've seen described has a number of sensors throughout the area and adaptively modifies the waveforms generated to create zones of cancellation, but I've no idea whether that's in production (or even proven to work!).
As an aside, work done on trying to apply ANR to aircraft ended up in flat speaker technology... from http://www.edn.com/index.asp?layout=article&articleid=CA438300:
"DML (distributed-mode-loudspeaker) technology began in the early 1990s as a research project of the United Kingdom's DERA (Defence Evaluation Research Agency), now QinetiQ. The research aimed to reduce the level of internal noise in military aircraft. As part of this research, DERA's scientists tried lining cockpits with high-stiffness sandwich materials but discovered that, rather than lowering the noise level, they increased it. As this effect might have significance for loudspeaker design, they filed a speculative patent. DERA itself had neither the resources nor the experience to develop this application, so it advertised for an audio-industry partner to license it."
The company that took it up, NXT, is doing rather a lot with it. (http://www.nxtsound.com/)

R

[ChristiaanJ]

balsa model,
Thanks a lot.
I was going to calculate the figures, but you already did.
At least it now makes perfect sense to me.

[BBK]

My knowledge of ANR may be a little out of date but here goes. ANR within a headset works very well and can cope with a range of frequencies. The RAE (as was) extensively tested ANR systems in a wide variety of aircraft eg Harrier, Chinook, Hercules etc. It was very good at reducing noise levels at the ear and used an analogue system. It certainly cancelled across a range of frequencies, maybe 100Hz to about 1kHZ, but I can't remember the specifics. I now use a Sennheiser ANR headset and subjectively it works very well especially at reducing boundary layer airflow noise at low level. The rules have changed now but until recently when ATC said "no speed restriction" the aircraft would wind up to about 340 kts and at 6000 feet it got pretty noisy!

I know that during the 80s and 90s there were companies researching a cabin system using an array of mics and speakers. As someone mentioned there was a design incorporating the speaker/mic combination into the headrest. Certainly, SAAB and Lotus cars were involved but I don't know how they fared.

I think it's probably correct to say that wide area ANR systems are not easy to build but not impossible to do so. What very likely does make a big difference is what type of noise you are trying to cancel and whether you are interested in cancelling noise at very specific locations eg pilot/pax head location as opposed to over a wider area. Some pretty hefty digital signal processing is required but computing power today should not be a problem. Another factor is whether the noise is broad band eg airflow generated or discrete such as prop/rotor noise (or even as in the case as helicopters from the gear box/transmission). It won't come as a surprise that a narrowband discrete source that can be tracked in some way is much easier to cancel. Also, the prop noise by having a longer wavelength is easier to cancel than a short wavelength (high frequency).

[Rainboe]

Cabin noise, especially turboprop noise, comes from a very wide band and source. You have the prop sounds, engine sounds, and extensive vibration sounds through the fuselage which comes from the walls, wing carry through. Airflow sounds as well. From all directions. And it comes from the other side. ANR can only really work well where the microphone is located very very close to the speaker, and the sound is mainly coming from one direction. You are essentially trying to double the sound energy in the volume involved to try and cancel out the initial sound energy. I can't see how even in a car cabin it can possibly work, and can make the problem worse in certain areas. There is another problem. If you do manage to neutralise the sound energy, how can your microphone pick up the original source? Over a larger volume, the mike picks up the sound, it's processed and the speaker pumps out 'deadening' sound. So the mike then picks up low noise- reduces the demand for deadening sound, so initial noise comes back- mike picks up loud noise again......in short, throbbing noises? Why don't you have a 'feedback' loop like when people are speaking into a mike at fetes when they get drowned out by a whistle?

It's been hard enough to get airlines to pay for ANR headsets (I bought my own). For the limited efficiency and great cost of installed systems, I can see why nobody really bothered buying such area systems. I can't see it getting much beyond ANR headsets.

[Jetstream Rider]

Mercenary Pilot - the noise is reduced in the same way that waves in a pond are cancelled out. If its quieter with ANR, its really quieter and does indeed save your hearing. The £220 I spent on ANR headsets in my previous aircraft (turboprop) were an absolute wonder.

Rainboe - If you can, wear them on both ears. Some people argue that they want to "hear their aircraft" for things like engine failures and tyre bursts etc. I have found that I can hear a lot more from the aircraft (albeit quieter) with ANR. I do accept that the sound environment is different though. I have no idea what you fly and if its practical to use both ears on, but there are some guys at work who insist on 1 ear off for no real reason. A number of times in the three aircraft I have flown I have heard noises that have meant something with both ears on, where the Captain has totally missed it even when it was pointed out.