A320: rrrrrrrrr-sound (tech.)

[Malte]

Hi,

I guess everyone knows the typical "rrrrrrr"-sound of certain engine types when operating with maximum power (for example the Airbus series (A319, A320, A310, A300, A321 but also some 757 and the new 737-800).

Does anyone know by which technical factors this sound is caused?

I was just wondering cause I've noticed it so often (from the in- and outside of airplanes). The strange thing is that you don't hear it when you are "behind" the engines (when you sit in the last row, for example).

thanks

Malte, EDDH

[Jim lovell]

The A320 has a distinct sound of it's own. At lower power settings e.g approach they make a distinct whining sound which gives the a/c type away straight away, and on take-off they are very noisy from the outside. The A330 makes a similar type of sound from the outside. Anyone know exactly why?

[airbuswhiz]

The reason that you hear the '-r-r-r' noise is due to the high bypass ratio fan (N1) being at the front of the engine. This fan is turned by the power section(N2) of the engine and in turn "sucks" more air into the inlet the faster it turns. This is known as a "tractor fan", or air pulling unit, thus the N1 fan blades making the noise you hear. Most all high bypass fan engines make this noise to some extent.

The other reason that you can not hear the noise from the rear is the reason why these engines are Stage 2 certified. The N1 section of the fan moves high velocity air around the engine nacelle and also around the exhaust. This quietens the exhaust and thus makes the noise footprint smaller and almost indetectable, hence a quieter engine.

Hope this explains some of the phenomenon for ya...cheers

Airbuswhiz

[ft]

The reason that you hear the '-r-r-r' noise is due to the high bypass ratio fan (N1) being at the front of
the engine. This fan is turned by the power section(N2) of the engine and in turn "sucks" more air into the
inlet the faster it turns. This is known as a "tractor fan", or air pulling unit, thus the N1 fan blades making
the noise you hear. Most all high bypass fan engines make this noise to some extent.

Yes, all jet engines and especially high BPR ones have some fan noise (ever stand in front of a jet that doesn't have stator vanes in front of the first fan/compressor stage? :)) but I don't think that is what the question was about.

The A320s has this distinctive sound that is hard to describe. During my first week or two on the ramp I sometimes looked up to see what kind of turboprop it was that let out so much noise from it's jet cores - and was surprised to see a 320 on the takeoff roll.

As to the cause of that sound, well, I have no idea. Perhaps a beat frequency caused by slightly different N1 speeds on #1 and #2 and some kind of soundwaves generated at the fan blade tips? Does anyone know?

Cheers,
/ft

[Avman]

I've only heard it on those operated by Irish crews! :)

[RATBOY]

ft has it ... a peculiar "rrrrrrrr" sound. I think the engine in question is the CFM-56 family. I have heard it early on approach and it is not noticable (or as noticable) on takeoff. Never figured out how to listen for it in cruise from the outside.

Believe this is a common engine in A319, A320, A330s as well as latest mark 737s.

It sounds almost like a gear box or something or maybe one of the mechs has a way to fasten baseball cards to the nacelle so they bang on a fan when engine is in operation.

[buck-rogers]

Beat frequency. Multi engined large prop aircraft used to throb (sorry). Ever heard a lancaster in the air years ago? Other younger aircraft with large props can do it but it's a deeper noise than the turbofans. The smaller fan blades on the front fan of large diameter engines would explain the higher frequency.

It is particularly noticable from the ground when bigger turbofans are climbing from 2000 feet upwards, as the engines are under high load but the aircraft is moving slow and low enough for the sound to be heard from the ground. It dissappears as the aircraft passes overhead - for reasons explained above by my learned friend Airbuswhiz.

[This message has been edited by buck-rogers (edited 25 October 2000).]

[Mark 1]

Here's my 2p worth as an ex-RR acoustics engineer.
At take-off the fan tips are supersonic - typically Mach 1.4 - and generate shock waves. Normally fan related noise is heard at the fan blade passing frequency (Number of blades times RPM/60) and harmonics of this.
With shock related noise however the non-linear propagation of shock waves compound with small blade to blade differences and the spacing of the pulses becomes irregular.
The sound that this makes is usually referred to as "Buzz-saw" noise.
This has noise at all harmonics of the shaft frequency (rpm/60) peaking around 1KHz.
The low frequencies are also not easily attenuated by the acoustic lining of the nacelle.
The peak radiation of this noise is about 60 degrees to the intake axis but often appears to be higher because the aircraft has moved forward in the time it takes to propagate.
This is rarely a problem on approach unless you use more than about 65-70% N1.

Oh - and the whining noise at low powers is probably blade interaction noise from the LP turbine or compressor.



[This message has been edited by Mark 1 (edited 26 October 2000).]

[ft]

"Buzz-saw noise"! That's it. The rest fits as well. Thanks a lot Mark, nicely explained.

Now you're on my mental list of people to bug when I start wondering about something. Be afraid, be VERY afraid! :)

Cheers,
/ft