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?
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
KingFriday013 From United States of America, joined May 2007, 1277 posts, RR: 10 Reply 2, posted (2 years 8 months 3 weeks 5 days ago) and read 3253 times:
PGNCS From United States of America, joined Apr 2007, 2516 posts, RR: 45 Reply 3, posted (2 years 8 months 3 weeks 5 days ago) and read 3241 times:
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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