Change in engine noise with altitude?

[TotalBeginner]

This may seem like a strange question, but it's one that has always fascinated me.

Why is it that the audible sound (heard on the flight deck) produced from an aircraft's jet engines decreases with altitude?

Is it purely because the noise is "drowned out" by the slipstream running around the aircraft as it accelerates, or is it because of the air density having an effect on the transmission of sound?

For example, changes in "thrust note" can be heard quite clearly during the approach, or if an initial level off is required during the SID. But you can hardly hear a change when the aircraft levels in the cruise, or when the engines decrease from CRZ power to IDLE at the TOD?

[Rainboe]

At altitude, there is hardly any engine noise at the front. You can barely hear the engines come down to idle power. The vast majority of the noise is the high-TAS airflow noise around the fuselage. Things like windscreen wipers, window frames and pitot probes sticking out into the airflow make an awful lot of noise.

I like taking a 737 up to 40,000'+. Even over 39,000', there suddenly seems to be a decrease in sound- it seems more peaceful.

[TotalBeginner]

Yes, I've noticed this up-front on the 737 as well.

Does the air density play any part at all? When are the engines more likely to be audible? Crusing along at 3000ft with a speed of 300KIAS, or crusing along at 40,000ft with a speed of 250KIAS?

[AtoBsafely]

I am just guessing a bit, but consider the following:

The actual thrust in cruise at altitude is much less than takeoff thrust, so the exhaust noise will be less at altitude. At descent point I find the reduction from cruise thrust to idle quite noticeable.

Exhaust noise is caused by the shear between the exhaust and the air that you are flying through. In cruise, you have a TAS of say 450K, so there is less shear (I think).

The mach number at cruise is much higher, so in the cockpit you are effectively "leaving more of the noise behind you". The noise generated by the engine has to travel a greater distance through the air to reach you in the cockpit.

Thrust, TAS and Mach are related to the density of the air at altitude, so you are correct.

[Rainboe]

But when you travel in the cabin, you can be sitting by the engine and still hear positively when the engines go from cruise thrust to idle. The only place you can hear this best is in the rear cabin of a 737/A320 where the blast noise is reduced, but very often even the cabin crew can't hear when you have started descent if you do it carefully (B737 'Descend now' gives you 1000'fpm until you hit the descent profile when it comes fully back to idle.

I don't know why there is a noticeable quitening above 38,000, but I find long cruise at 41,000' much less fatiguing because of it. I think it is not the engines. Maybe it's because IAS/TAS are slightly lower.

[TotalBeginner]

The mach number at cruise is much higher, so in the cockpit you are effectively "leaving more of the noise behind you". The noise generated by the engine has to travel a greater distance through the air to reach you in the cockpit.

This is very interesting and something that I haven't considered!

[cavortingcheetah]

So at the soft and silent 40,000ft, you are travelling much faster inside the aircraft than is the speed of sound outside?

[AtoBsafely]

Cheeta,

I am considering any noise generated by the engine that is travelling through the outside air to the the cockpit. That sound will travel through the air at the local speed of sound m, and at the velocity of m-TAS relative to you in the cockpit. The energy of the sound wave will dissipate at a constant rate with time, and since it is taking longer to reach you the volume will be reduced.

NB Naturally this does not apply to any noise reaching the cockpit through the airframe.