KingFriday013
Topic Author
Posts: 879
Joined: Fri May 18, 2007 7:56 am

Background Noise On Aircraft

Thu Sep 02, 2010 8:43 pm

So, this question has been bugging me for years.

http://www.youtube.com/watch?v=BzHFQu6F1H4

Around 39 seconds into the video, the fans turn off and you just hear the hum of... what? What is it that makes that background ambient hum that's unique to the 757?

The 737 has a similar hum, perhaps a little more distinct. You can hear it in this video, although it is quite loud for some reason: http://www.youtube.com/watch?v=o1od3I9fsvM

I've heard that it's pneumatic systems that cause it, the "air packs" (which I'm still not sure what are exactly), the APU, so many different things. Does anybody know for sure what creates this noise, and how it does so?

Thanks!

-J.
Tho' I've belted you an' flayed you, By the livin' Gawd that made you, You're a better man than I am, Gunga Din!
 
Fly2HMO
Posts: 7207
Joined: Sat Jan 24, 2004 12:14 pm

RE: Background Noise On Aircraft

Thu Sep 02, 2010 8:59 pm

Quoting KingFriday013 (Thread starter):
The 737 has a similar hum, perhaps a little more distinct. You can hear it in this video, although it is quite loud for some reason: http://www.youtube.com/watch?v=o1od3I9fsvM

Pretty sure tt's the air conditioning packs, which use a turbine to compress and expand air, hence the whine. They can be very loud.

Quoting KingFriday013 (Thread starter):

I've heard that it's pneumatic systems that cause it, the "air packs" (which I'm still not sure what are exactly
http://en.wikipedia.org/wiki/Air_cycle_machine
 
KingFriday013
Topic Author
Posts: 879
Joined: Fri May 18, 2007 7:56 am

RE: Background Noise On Aircraft

Thu Sep 02, 2010 9:10 pm

Quoting Fly2HMO (Reply 1):
http://en.wikipedia.org/wiki/Air_cycle_machine

It says most aircraft usually have two or three... I guess that's why the hum is louder in some areas than in others. I bet this is it!

Thanks Fly2HMO!  

-J.
Tho' I've belted you an' flayed you, By the livin' Gawd that made you, You're a better man than I am, Gunga Din!
 
PGNCS
Posts: 2249
Joined: Sun Apr 15, 2007 5:07 am

RE: Background Noise On Aircraft

Thu Sep 02, 2010 9:25 pm

The drop of ambient noise you refer to are the packs being turned off for engine start. The remaining background noise is a combination of other things and varies depending on where you are located. Most aircraft still have gasper air on at this point, and some may or may not have recirculation fans on as well (the 757 will have one recirculation fan on at a minimum due to the electronic cooling configuration of the aircraft.) There are also other electronics and electronics cooling components, hydraulic pumps, pneumatic components, and other contributors to ambient noise. I can't generally pick out single components though some mechanics no doubt can. The bottom line: I don't think the background sounds can be attributed to a single component, rather an array of items working simultaneously mostly under your feet.

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