PPRuNe Forums - Why cockpit noise reduces with altitude?

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Why cockpit noise reduces with altitude?

Hello guys,

I noticed on the A320 family that if you keep the same IAS during climb, it seems that the cockpit noise is much less at 35000 feet vs 10 000 feet. But I really don’t know why:}. Also I noticed, the wind noise starts to build more and more the higher you go but overall cockpit noise is definitely quieter at high altitude versus low altitude for a constant IAS. Could someone provides a logical explanation?
Bonus question: Sometimes during climb, you hear like a « bang » or cracking sound on the side but I have no clue also what it is...
Thank you in advance.

Wind noise probably increases at altitude because TAS is higher.
The higher wind noise and the higher diff pressure may mask other noises, resulting in an overall impression of less noise.
Cracks and bangs are probably panels ‘tin-canning’ with increasing diff.

Without thinking too much about it, I'd expect the actual perceived noise to decrease as cabin diff increases coz there are simply less air molecules around to transmit the sound waves.

As for cracking/creaking noises, it's very likely the trim fasteners moving a bit as the fuselage expands. The B747 Classic used to creak a bit at the rear of the FEO panel so the locating pins were lubed every few hundred hours to calm the cockpit crew.

If you imagine that instead of air molecules, you were flying through glass marbles - (yes I know, bonkers, but stay with me here!!) - there would be a hell of a noise in lower levels (full density marbles), with billions of them all hitting and bouncing off all of the windscreens and structure. But, higher up, owing to the much lower density, there would be far fewer marbles hitting the windows etc., (even though, at a constant IAS they would be hitting at the same speed), so it would be much quieter.

Does that help?

PS, the Airbus FBW avionics and cockpit instrumentation cooling also makes a hell of a noise, but its mode changes as the outside air temperature reduces, so that noise reduces with height as well.

Thank you guys!

Uplinker. Thanks! I like this kind of analogy.:ok:

The A320 banging and cracking is often the door seals. Your Purser will most likely agree.

Ah but uplinker, by that analogy you need the same number of molecules passing around the wing to create the lift wouldn't you? So the same number must be present at any given time? :):}

Haha. Good Point Compressor Stall. I undestand we have the same amount of molecules horizontally impacting the aircraft as it’s the same IAS but what about the vertical axis? Shouldn’t be less molecules per cm3 as air is less dense so overall less molecules impacting the windscreen? I drew a picture to try to explain how I picture things.
https://ibb.co/mWmwAJ
Sorry I’m terrible at drawing but I would think at sea level an aircraft will hit more molecules than at high altitude as they are more close together as seen on my drawing. I’m no expert tho.
Please correct me if I’m talking rubbish. Another thing came to my mind: At high altitude IAS is actually overreading due to compressibility by about let’s assume a random value of 10kt. So our 280kt IAS at FL360 is actually equivalent to 270kt IAS at sea level if we consider the overreading due compressibility. Surely 10kt makes a difference in term of noise?
Thanks.

Quote:

Originally Posted by compressor stall (Post 10175702)

.............but uplinker, by that analogy you need the same number of molecules passing around the wing to create the lift wouldn't you? So the same number must be present at any given time? :):}

I am no physicist, but you seem to be suggesting that the number of air molecules passing over the wing per unit time needs to be constant to provide constant lift? If that were true, IAS would always be the same as TAS no matter what altitude you flew at, but it isn’t as we all know.

The lift equation is 1/2 x air density x velocity(sq) x wing area x coeficient of lift.

At high altitude, the air density is much less, so to provide the same lift, the velocity of the molecules has to be much more, and that term is squared. So the reduced lift owing to many fewer air molecules passing over the wing is compensated for by the greater force imparted by each molecule resulting from the increased speed (squared) of those molecules.

If we think of the air molecules as ping pong balls* then the difference in noise caused by flying at low level through the maximum density of ping pong balls, (each one touching their neighbours), to flying through many fewer ping pong balls at altitude, (albeit faster), then the reduction in noise will be apparent?

*a bit kinder to the aircraft structure than marbles !!

.

I think differential pressure plays a role as the cabin becomes sealed and sort of muffles the sound a bit.
the temporary blocking of the ears can also play a role in than sensation.

My wild arsed guess is that as you get higher your speed, as a percentage of the speed of sound, gets much higher. You're cruising somewhere around M0.8 right? This would presumably have a significant affect on the quality of sound from noise sources that are behind you such as the engines.