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Everyone is familiar with the whirring sound of jet engines when seeing an aircraft taking off from a nearby airport. It is distinctly very loud on the ground and one can hear it even when the airplane is miles away.

Although one can hear a 'white noise' like sound when inside an airplane, the engines don't sound very loud in spite of being just meters away from them. I understand that the cabin is well insulated from the outside, but I would expect to hear a similar whirring sound of the engines.

So what is the phenomenon that makes jet engines sound louder on earth compared to inside the aircraft cabin?

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    $\begingroup$ The jet engines point away from the cabin... $\endgroup$ – user207455 Jul 18 at 16:48
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    $\begingroup$ This article has some good insights. It sounds like the answer is good old fashioned engineering rather than some fancy physics principle. $\endgroup$ – GenlyAi Jul 18 at 23:30
  • $\begingroup$ Fwiw the phenomenon is known as "attenuation". $\endgroup$ – Richard Barber Jul 20 at 6:59
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Sound is a pressure & velocity wave in fluid medium, i.e. air. Air molecules wiggle back and forth and bump into other air molecules so they wiggle too so you have a whole chain of wiggling air molecules.

The jet engine moves air molecules A LOT, hence it's extremely loud. As the sound moves away from the jet engine the energy disperses over a larger and larger area and so the sound pressure level drops. The pressure drops by half every time you double the distance. That's 6 dB per doubling of distance or 20 dB per decade. If it's 120 dB at 10 meters, it's still 100 dB at 100m, 80 dB at 1km and 60 dB at 10km. That's why you can easily hear it on the ground.

There is no easy way for sound to get into the cabin, because the cabin is air tight and fully sealed. The air molecules outside can wiggle like crazy but the air molecules inside don't care.

It's still fairly loud in the cabin but that's due to mechanical sound transmission through the wings and the fuselage. The vibration of the jet engine wiggles the wings which will wiggle the fuselage which will wiggle the panels which will wiggle the air molecules inside the cabin, which will wiggle your ear drum. Planes are carefully designed to minimize the transmission but the amount of energy from the jet engine is enormous, so even if you eliminate 99.999% of the energy, it's still quite loud and getting to 99.9999% or 99.99999% is difficult and very expensive.

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  • $\begingroup$ Just out of extended curiosity: Is the effect of the sound of the engine vibrating the hull (i.e. via the airwaves) completely drowned out by the vibration of the engine (imparted to the hull through physical contact), or are these working in tandem to supply the sound you hear in the cabin? $\endgroup$ – Flater Jul 19 at 9:09
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    $\begingroup$ "The pressure drops by half every time you double the distance": why? Considering the area of a sphere is proportional to its squared radius, I'd expect that doubling the distance would cause the pressure to be reduced by a factor of 4, not 2. This is what happens with radio waves coming from an antenna. Shouldn't it be the same with sound waves? $\endgroup$ – Fabio Turati Jul 19 at 9:49
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    $\begingroup$ @FabioTurati the energy of a sound wave is proportional to the square of the pressure. When you average it over a surface, the pressure decreases as 1/r. $\endgroup$ – Davidmh Jul 19 at 14:27
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    $\begingroup$ "20 dB per decade" made me think that if you start with 120 dB then 10 years later it would be 100 dB. Not complaining, just sounds funny. $\endgroup$ – Blueriver Jul 19 at 18:38
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    $\begingroup$ @Blueriver In physics, a decade simply means "a ratio of 10". $\endgroup$ – Blackhole Jul 19 at 23:07
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First, the cabin is quieter because the fuselage walls are designed to limit the transmission of sound from the engines.

Second, on most commercial aircraft, the engines are suspended beneath the wings, which block the noise from the engines before it can strike the fuselage.

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There is a howling 500 - 600 mph gale blowing outside the aircraft, the like of which you have never experienced on the ground, and this carries away most of the sound before it can enter the fuselage. Some sound is also reflected by the fuselage. A few passenger aircraft like the Caravelle, VC10 and Tristar have engines at the rear, which also helps to reduce sound levels in the cabin.

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    $\begingroup$ Not when waiting on the runway at takeoff... unless you have different wind patterns. $\endgroup$ – user207455 Jul 18 at 21:05
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    $\begingroup$ There is still a howling gale blowing through the engine relative to the speed of the plane. Each engine on a B747 has about one ton of air per second going through it at maximum power, even before the plane starts moving for takeoff. The airflow speed at the tips of the front fan blades is transonic. $\endgroup$ – alephzero Jul 19 at 2:03
  • $\begingroup$ The planes you list are all out of service but many small regional jets (e.g., the Bombardier CRJ series, Embraer ERJ series and Boeing 717) are still in flying. $\endgroup$ – David Richerby Jul 19 at 18:41
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While sound waves can be impacted by the particles in the air to transmit sound, the sound waves can bounce off of the ground much like how you can have a louder or longer lasting sound in an enclosed room as it bounces off walls and such

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