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A.S.
11-05-2008, 10:36 AM
For about twenty years I've been trying noise cancelling headphones of various types because I just can not sleep on an aircraft due to the low/middle frequency noise inside. I believe that there are at least four sources of noise that I can detect:

1. The high velocity 'roar' of the jet engine's exhaust (which must be an air stream of at least 600mph otherwise the plane wouldn't move forward at 600mph!)

2. Structural noise and vibration from the engines carried along the engine pylons and into the metal body of the plane. Note: the engines cannot be rubber-decoupled from the pylons or they may fall off .... they have to be securely bolted-on

3. Noise from the air conditioning system

4. Skin noise. Every rivet on the outside of the plane (there are many thousands) has a head which is ground down at a lap-joint but is not perfectly smooth. Each one of these joints cause a mini-vortex, a tiny whirlpool which is a noise source. Add all these noise sources together and you have a random-noise like turbulence sound. If you look at a really well designed, smooth wing like that on an A340/380 you'll see how smoothly the air flows over the surface (at very high speed). Contrast that with an old-style 'gappy' wing which generates noise at every panel intersection. If you've been in a situation where the engines are reduced to idling turned off/failed you'll hear only this sound. In my experience, the Airbus A340 is noticeably quieter than the 747 inside. Can you believe that some people think that the new A380's interior is too quiet! Read here (http://blog.seattlepi.nwsource.com/aerospace/archives/121137.asp) or here. (http://boeingblogs.com/randy/archives/2007/08/sound_of_silence.html)

Not willing to pay top money for the market leading German or USA models but still chasing the dream of a good sleep, after some internet research I set off to by new Sony cans. In fact, I didn't. I studied the specs. on the cartons of all available types as I waited at London Heathrow's new (wonderful) Terminal 5. I found another model that I hadn't uncovered in my on-line research - the Panasonic RP-HC500. Very similar claimed specification for NR to the brand leaders at 30% less money and better construction. So I bought them*. They are excellent - highly recommended. Very comfortable. Almost what we would class as hi-fi quality, and the noise reduction is very good indeed, in part due to the good seal around the ear ('supra-aural') and in part due to the electronics. They are very light, the cable unplugs at one 'phone, but they don't have a volume control. The battery life is at least 15 hours, and probably much more. I also used them overnight in bed at the airport hotel to eliminate aircraft noise and the aircon fan, not connected to anything but with the noise-reduction on. Perhaps a little sweaty around the ears after a few hours but at least I could sleep.

Attached is the spec. sheet the Panasonics came with. I've highlighted the number you need to look out for if you are considering buying another type - the more dBs noise reduction the better .... 22dB is very good - most of the cheap ones are around 12dB which I've found to be nearly useless. 200Hz or lower is an ideal frequency for this to be specified at. A higher frequency (say, 600Hz) would imply far less worthwhile noise reduction.

* Word of warning! Buy a battery as astonishingly, the box does not contain a battery.

OTHER RELATED LINKS (that interest me)

Noise reduction by shaping jet exhaust outlet (http://www.boeing.com/news/releases/2001/photorelease/q4/pr_011120a.html)

Incidentally, when next flying look out for the logo painted on the side of the engines. If it's Rolls Royce (about 40% of jet engines are) these are fundamentally different in design from the other big turbofans. More complex but a shorter, stronger, more fuel efficient engine. And they just keep on working. They're the launch engine for the new Boeing 787 and Airbus 380 double-decker. And they're made here in the UK as are all Airbus wings.

P.S. I read the following "...people inside the industry know that the 787 Dreamliner suffers appallingly low resonant frequencies along the upper fuselage. These problems were not solvable within the timescale and mean stress, creep and noise problems until the problem is resolved later. Most likely this will involve retrospective and expensive re-working of the structural members until a full re-design can be funded/implemented." I do not think that noise cancelling headphones can eliminate very low frequency sound but if true, it proves once again that 'sound' is a difficult science to predict at the design stage.

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eelekim
11-05-2008, 05:13 PM
Thanks for the tip! Is it good for non-flight listening? I've found some info here (http://ctlg.panasonic.jp/product/info.do?pg=04&hb=RP-HC500). It does mention a battery included... I guess the unit you bought is in an export version that no battery is included.

mike

A.S.
11-05-2008, 06:44 PM
Yes, the sound quality is good and connected to the aircraft in-flight system or a personal audio player (or laptop) the sound is clear and lifted well above the background noise - which is suppressed by the anti-noise technology. When 'on' the is some gain in the 'phones so you do need to have a means of controlling the volume level. They are OK for all general purpose listening at home- good bass, warm mid and a sweet top. Just a tiny hiss from the internal electronics.

The single AAA battery is definitely not included in the UK version. How much did that save the manufacturer? Five or ten cents? If I'd not checked when in the store, I'd have been most irritated to discover that absence on board the plane.

A.S.
12-05-2008, 10:26 AM
As mentioned, the smoothness and shape of the exterior of the aircraft has a big influence on the noise inside the cabin.

Here is a superbly designed "green" video presentation from my eldest son's company, EADS, who make the Airbus planes. Notice how clean the wing design is - very smooth air flow = low cabin noise. British made wings and British made Rolls Royce engines; in this example, one has been converted to run on bio-fuel. Watch here (http://www.airbus.com/store/photolibrary/MISCELLANEOUS/video/att00011404/media_object_file_highres_Airbus_GTL.swf).