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I understand that (from other answers) planes taxi using engines only. But, when I'm a passenger, then why do I hear the "real" engine noise only when the plane is on the runway?

So, if planes taxi till runway using engines only, then why don't I hear that much noise until the aircraft reaches the runway?

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2  
Airplane can start moving at engine idle (not sure how many % power) and take off can be as high as 100% power – vasin1987 Feb 14 at 17:18
11  
Same reason you don't hear a Ferrari idle, but its loud when it rockets down the street... – Ron Beyer Feb 14 at 17:25
    
@RonBeyer: I have a suspicion that's very intentional, unlike the plane. Ferrari's aren't even that bad. Loud cars have turbo's which at high RPM's dump excess pressure into the exhaust, but Ferrari isn't a big fan of turbo's. – MSalters Feb 15 at 23:02
    
@MSalters Jet engines are compressors that dump excess pressure into the exhaust, and planes are intentionally quiet at idle, there are noise restrictions around many major airports. In fact many aircraft can't take off at full power because of noise restrictions, so there is a big push to make them quiet. If you see the engine nacelles of newer aircraft, the chevrons at the back are specifically to reduce noise... – Ron Beyer Feb 16 at 1:49
up vote 33 down vote accepted

When you have the thrust levers of, say, a 747 all the way back to idle, the airplane still has enough power to move. In fact, you will have to use the brakes to keep it from taxiing too fast. If you didn't use the brakes, it would finally accelerate to somewhere around 45 knots or so, far too fast for taxiing, but much too slow to take off. At the idle power setting, relatively little noise is produced by the engines.

To take off, if the 747 were at max gross, you would have to accelerate to around 180 knots. That would require a power setting approaching the maximum, depending on the length of the runway. The noise produced by the engines is relative to the amount of power they're producing. Hence, taxiing at idle produces little noise relatively speaking, on the runway taking off a whole lot of noise.

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As an engineer, I strongly suspect the amount of noise is NOT proportional to the amount of power. Noise is an inefficiency, and there's quite some engineering done to reduce it, but you'd focus on the most frequent power setting. Full power just isn't that frequent. – MSalters Feb 15 at 11:39
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@MSalters I would read "A is proportional to B" in a vague sense of "A increases with increasing value of B"... :) But you're right, it is an imprecision. – yo' Feb 15 at 14:19
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@MSalters You are certainly correct insofar as the engineering and mathematical sense of the word, and I have changed it to "relative". However, for a non-technical audience, which I believe would include the person asking the question, my thought is that using "proportional" as in "the punishment should be proportional to the crime" would be appropriate. – Terry Feb 15 at 20:16

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