pilotboi
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What Makes Engine Sounds Unique?

Wed Mar 11, 2009 12:09 pm

I'm very well aware of the fact that different manufacturer's engines sound different. But I have two questions really...the first is simple, how can you distinguish each type by the sound? In other words, how can you tell them apart? The second is a little more tricky (for some) and really is what I'm interesting in finding out with this thread...what makes each one unique? In other words, why do they sound the way they do?
 
rutankrd
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 12:54 pm

Simply sonics and acoustic properties

Each engine design differs in materials used size/shape number of moving parts amount of gas forced through and therefore differing resonances.

The external baffles and cowlings of jets also effect the noise.

Props having no baffles produce that deep bass through vibration as they cut through the air and in the case of the helicopter blade these may be running close to the sound barrier at the tip resulting in mini sonic booms
 
CanadianNorth
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 12:55 pm

Where I come from it's pretty easy, since the airliners up there are so vastly different. However, here are a few I'm generally able to call:
-JT8D powered aircraft have what I would call a thundering sound, I believe it's mainly due to simply being a relatively low bypass engine.
-RR Darts to me are instantly recognizable by their screaming, especially at low power settings, and I've been told it's the gearbox that does the screaming.


CanadianNorth
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JQflightie
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 1:04 pm

well i would love to meet the creator of the IAE engines on our A320's!! they are sooooooooo noise on take off! as im up the front of the aircraft alot as R1 Cabin Crew, but then if i am PAX'ing sitting at the Over-Wing exits it is extreemly quite, and then if i am working down the back of the aircraft its another sound!
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fatmirjusufi
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 1:10 pm



Quoting JQFlightie (Reply 4):
well i would love to meet the creator of the IAE engines on our A320's!!

Haha! Just my thoughts...

Check it out
http://www.youtube.com/watch?v=P1SjtT6GUdI

Fatmir
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JQflightie
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 1:18 pm



Quoting FatmirJusufi (Reply 5):
Haha! Just my thoughts...

Check it out
http://www.youtube.com/watch?v=P1SjtT6GUdI

Fatmir

OMG lol that is the exact sound from the front of the cabin a loud wining sound....thanks for that.....
but to make it worse i was on a BA A320 last november and their A320's are extreemly old bird... and alot louder!
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comorin
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 1:51 pm

The GE90's are famous for their 'mooing' sound on engine start - quite a few videos on Youtube! I'll post one later when I get back from work.
 
khobar
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 2:24 pm

Interesting question.

We have a good mix of 737's and comparable Airbii fly over on approach, and the general difference between the Boeing's and Airbii is very apparent. 737 on approach has more of a roar while the Airbii generally have more of a whine/scream.

I haven't paid attention to the differences between Airbii with differing engine types however.
 
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seabosdca
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 2:32 pm

Love the CFM56-5/7 buzz saw sound on takeoff... immediately recognizable because the pitch and timbre are different from other high-bypass engines.

PW2000-powered 757s are cool too, because the pitch is lower and they sound relaxed even at full power.
 
comorin
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 3:06 pm

So what are the sources of these sounds?

My humble guesses:

1. Subsonic airfoil sounds from the fan (hairdryer, vacuum cleaner).
2. Compressor and geartooth whine proportional to rotation speed.
3. Supersonic roar of air exiting exhaust nozzles.

I am not sure if there are any cavitation effects, and I am not sure how turbulent flow generates noise.

Engineers, please correct !
 
CFMTurboFan
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 3:29 pm

The RB211 D4s BA used to run on the 747-200 had a very distinct growling sound they made during start up. This was caused by the fan blade clapper faces. As the engine accelerates, the fan blade centrifuge out into the dovetail slots of the fan hub, which reduces the amount of growling (but does not eliminate it).

The RB211 G/H engine BA has fitted on the 747-400 & 767 fleets did not make this noise as they have wide chord fan blades fitted. These engines made more of a whoosh noise on start up and engine running.

Generally any high bypass engine that has clapper faces midway up the fan blade will made some sort of growling during normal engine running. I watched a US Navy Viking two years ago at an air display, which made these distinct sounds. I was explaining this to some of my work colleagues who were wondering what the noise was.

Hope this answers some of the questions.

CFM Turbo Fan
 
ktachiya
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 3:31 pm

Very interesting thread. Although not right the whole time, this is a technique that I've developed over the years to distinguish the three engine makers at least in Japanese heavies (and some RR powered engines). I can usually guess when the aircraft starts rolling and the pilot applies full thrust upon takeoff.

RR - Sounds very unique to me and actually checked it out on a few CX B744 takeoff videos but it makes an extremely low rumbling sound when applied to full power. It also continues this sound even during its climb.

PW - I see the most often when I am spotting at ITM because it is on the T7's that NH/JL has. It sounds like a sportscar when it goes right past me.

CF - This sounds the most normal and does not have any rumbling sounds associated with it based on what I have seen so far during the whole takeoff roll phase. It also tends to make a high pitched noise? from what I've noticed.

Of course this is just IMHO and many people have different perspectives. Also, this can only be said when you are viewing the takeoff aircraft from 90 degrees (so when it goes from your left to right or vice versa) in airports such as ITM or NGO. It can't really be used elsewhere but I would love to hear what others say.

Also while we are on this topic, can I also ask about reverse thrust? I don't notice any differences on reverse thrust but is there a way that somebody can tell the difference?
Flown on: DC-10-30, B747-200B, B747-300, B747-300SR, B747-400, B747-400D, B767-300, B777-200, B777-200ER, B777-300
 
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JBo
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 3:40 pm



Quoting SeaBosDca (Reply 9):
Love the CFM56-5/7 buzz saw sound on takeoff... immediately recognizable because the pitch and timbre are different from other high-bypass engines.

No doubt inherited from the TF-39 ... http://www.youtube.com/watch?v=uJvI9bdteko Big grin
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Aaron747
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 4:07 pm

I still have no idea what makes them sound so unique, but I've always been in love with the PW4000 vacuum cleaners on the 777, especially the tinny hum of the PW4090:

outside (my video)

http://www.youtube.com/watch?v=UuLMJXemd-Y

inside

http://www.youtube.com/watch?v=phogRUBSP5U

Listen to the latter around 1:05...absolute music to my ears. To be fair though, all 777 engine options sound incredible on takeoff.
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2H4
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 4:15 pm



Quoting Khobar (Reply 7):
the general difference between the Boeing's and Airbii is very apparent. 737 on approach has more of a roar while the Airbii generally have more of a whine/scream.

NWA A320s regularly pass over my house on approach to MSN. They usually pass overhead at around 3,000 or 4,000 AGL with the engines at (or near) flight idle, and those engines make a very distinctive sound as they pass by.

I think it's a combination of the engine itself and the doppler effect as the aircraft passes overhead. The frequency/pitch change that occurs is very strange. None of the other aircraft (ERJs, CRJs, DC-9s) sound anything like it.

2H4
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keta
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 5:05 pm



Quoting Comorin (Reply 9):
Supersonic roar of air exiting exhaust nozzles

In airliner's turbofans (you're perhaps referring to fighter aircraft?) exhaust velocity is subsonic.
Disclaimer: I'm speaking out of memory and thought, will have to check that  Smile
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BMI727
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RE: What Makes Engine Sounds Unique?

Wed Mar 11, 2009 10:26 pm

Generally, the roaring sound is due to the exhaust. This sound will be louder on lower bypass engines. (MD-80s and military aircraft for example) The buzzsaw sound is from the fan blades.
As was stated before, the nacelle of the engine has an effect on the sound so the same engine on two different types may sound different.
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tdscanuck
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 4:42 am

Quoting Pilotboi (Thread starter):
The second is a little more tricky (for some) and really is what I'm interesting in finding out with this thread...what makes each one unique?

A whole bunch of things. Aerodynamics (especially the fan blades) are a biggie. Nacelle design is a contributor (especially the acoustic treatments), and engine architecture (where the exhaust goes and how it's mixed with the fan stream, and how many spools).

There are some very strange engine architectures out there. I'm sure this one has some noise effects due to the very odd way of mixing the exhaust and the fan flows:
http://www.flightglobal.com/airspace...ays/garrett-atf3-cutaway-5603.aspx

Quoting Comorin (Reply 9):
1. Subsonic airfoil sounds from the fan (hairdryer, vacuum cleaner).

Fans are usually locally supersonic at takeoff, actually.

Quoting Comorin (Reply 9):
2. Compressor and geartooth whine proportional to rotation speed.

I'm not sure you can hear the gearbox over the general engine noise...that sounds to me like a gearbox in trouble.

Quoting Comorin (Reply 9):
3. Supersonic roar of air exiting exhaust nozzles.

Only on supersonic aircraft:

Quoting Keta (Reply 15):
In airliner's turbofans (you're perhaps referring to fighter aircraft?) exhaust velocity is subsonic.
Disclaimer: I'm speaking out of memory and thought, will have to check that

You don't want supersonic exhaust on a subsonic aircraft. It lowers propulsion efficiency without any counterveiling benefit (and it's loud).

You will be very close to sonic at the exit of many turbofans, but the nacelle design generally wouldn't even allow you to get a supersonic exhaust flow.

Quoting Comorin (Reply 9):
I am not sure if there are any cavitation effects, and I am not sure how turbulent flow generates noise.

You can't actually cavitate a jet engine, because cavitation is a phase change brought about by low pressure. Since the working fluid is already a gas, it's got nowhere to go. You may be thinking of separation, which is qualitatively similar to ventilation on liquid-propellors, but somewhat different physically.

Turbulent flow generates noise because the turbulence is driven by steep local pressure gradients, which radiate pressure waves away. You hear the pressure waves as sound.

Tom.

[Edited 2009-03-11 21:43:45]
 
osiris30
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 5:34 am



Quoting Tdscanuck (Reply 17):
There are some very strange engine architectures out there. I'm sure this one has some noise effects due to the very odd way of mixing the exhaust and the fan flows:
http://www.flightglobal.com/airspace....aspx

I seem to recall we had someone on this forum who may have some first hand experience with that beasty.. Dougloid I believe. He may be able to confirm that for you  Smile
I don't care what you think of my opinion. It's my opinion, so have a nice day :)
 
Viscount724
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 6:34 am



Quoting CanadianNorth (Reply 2):
RR Darts to me are instantly recognizable by their screaming, especially at low power settings, and I've been told it's the gearbox that does the screaming.

Agree, the R-R Dart has one of the most recognizable engine sounds of any aircraft. Strangely, the high-pitched whine wasn't noticeable inside the aircraft, at least on Viscounts and F27s.

Good example of the Dart scream on an F27 on these old videos at AEP, the domestic/regional airport in Buenos Aires.
http://www.youtube.com/watch?v=tfbO50S-qmo
http://www.youtube.com/watch?v=rYPIYFJ8d6A&
http://www.youtube.com/watch?v=Vr63k4ZoT8M
 
ThirtyEcho
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 7:33 am

Sound? You want sound?

There is nothing on Earth like the Rolls Royce Merlin engine; there are many examples on YouTube. Just look up P-51 or Spitfire and enjoy.
 
2H4
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 2:59 pm



Quoting ThirtyEcho (Reply 20):
There is nothing on Earth like the Rolls Royce Merlin engine; there are many examples on YouTube. Just look up P-51 or Spitfire and enjoy.

Better yet, come to Oshkosh this July and bask in the glory that is multiple P-51s advancing throttles for takeoff simultaneously.....from about 100 feet away.  cloudnine 

2H4
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pilotboi
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 3:06 pm



Quoting 2H4 (Reply 21):
Better yet, come to Oshkosh this July and bask in the glory that is multiple P-51s advancing throttles for takeoff simultaneously.....from about 100 feet away.

Was there, myself, last year. I preferred the thunder of the Raptor doing a high speed pass. But to each their own.
 
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HAWK21M
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RE: What Makes Engine Sounds Unique?

Thu Mar 12, 2009 3:55 pm



Quoting Pilotboi (Thread starter):
the first is simple, how can you distinguish each type by the sound

Its all about practice.....Hearing them often one can tell the difference.Its like watching aircraft & knowing the type over time.
regds
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kl671
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RE: What Makes Engine Sounds Unique?

Sat Mar 14, 2009 10:28 pm



Quoting Pilotboi (Thread starter):
The second is a little more tricky (for some) and really is what I'm interesting in finding out with this thread...what makes each one unique? In other words, why do they sound the way they do?

Noise from a turbine engine is created from both the inlet and the exhaust.

The inlet noise from the compressor is at higher frequencies. When a rotating compressor blade passes a stationary object such as a vane, it creates a pressure fluctuation. This pressure fluctuation creates noise at a frequency which is a function of the rotational speed of the rotor multiplied by the number of blades. Turbines have many stages with different numbers of blades and vanes and so generate many different frequencies.

The exhaust noise is much lower in frequency and is caused by the combustion process.

Different manufacturers use different shaft rotational speeds, number of blades, number of vanes and cumbuster configurations. Thus each engine model produces a unique sound.

Hope this helps.
 
keta
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RE: What Makes Engine Sounds Unique?

Fri Mar 20, 2009 7:54 pm

In addition to what KL671 and others overall correctly stated, I will go deeper into how engines make noise.

Primarily, the noise is generated by the fan and the exhaust, and to a lesser extent the turbine, compressor, combustion chamber and all internal mechanisms. Typically, when an airplane approaches, there are two peaks of noise, the first one when it is approaching due to the fan, and the second one once passed due to the exhaust. The mechanism by which the sound is produced is different in each case.

In the exhaust, the noise, of low frequency, is generated by the strong fluctuation present in the mixing region between the exhaust and the outer flow. In this highly turbulent region, fluctuation speeds reach 0.14*V, where V is the exhaust speed, in the subsonic case. The acoustic power radiated by the exhaust is P = A*G*V^7, where A is a factor depending of the mixing conditions, G is the mass flow and V is the exhaust speed.

From there, there are three means of reducing exhaust noise. One would be to reduce the mass flow, but a decrease of 3 dB would require a 50% reduction in thrust, clearly not the solution. Another way is to modify the A factor, by modifying the geometry of the engine so as to get a less fluctuating mixing flow. We all have seen this method implemented in the undulating (can't describe better) nozzles found in some engines. I believe this is also the method proposed by Boeing for the B-787. This method achieves noise reductions of 5-7 dB with 1% less thrust per dB; the JT8D-209 engine produces 7 dB less noise at takeoff with a modified nozzle. The third method is to reduce exhaust speed. So as to not lose thrust, the way to do it is to increase the bypass-ratio: a 6:1 bypass-ratio turbofan reduces 20 dB the exhaust noise compared to its turbojet equivalent. Of course, this increases the noise generated by the fan, but the overall effect is positive.

The principles by which the fan and the compressor create noise are the same. This noise is characterized by a wide spectrum of frequencies, with some peaks corresponding to pure tones, and other tones and their harmonics. In the compressor, 75% of the noise is generated by just the first two stages, so the number of stages is not the most relevant factor. The wide spectrum of the background noise is caused by random fluctuating forces. These are generated both by the turbulence of the upstream flow and the turbulence in the boundary layer of each blade, which create fluctuating forces, vibrations and noise. The randomness of this processes make the frequencies cover a wide range.

After each stator stage, the velocity varies in direction and magnitude tangentially (in the perimeter, so as to understand), primarily in magnitude due to the effect of each blade. If the blades in the stator are separated a distance S, the following rotor, at speed U, will encounter a S-periodic velocity field in space, and the forces will be U/S-periodic in time. Now, since S is proportional to 1/n where n is the number of blades, and U=wR, where w is the rotational speed, the frequency of the noise will be proportional to wRn. These are the pure tones encountered in the spectrum.

For a stationary observer, the pressure field of each blade would generate noise also at frequency wRn. In reality, though, small inequalities in the geometry and angles of attack of the blades destroy the periodicity; however, the structure is repeated on each revolution, so there is a noise of frequency wR. This is what causes the existence of multiple tones and harmonics.

It has to be noted, however, that this noise is much attenuated in the nacelle prior to exiting to the outside. There are many ways of controlling the noise explained. Controlling the geometry of the compressor (spacing between stages, number of blades...) the noise can be highly reduced, both because of reducing the effects of the mechanisms discussed, and by choosing a correct frequency which takes best advantage of the attenuation of the atmosphere. The noise reduction in the nacelle comes from a correct design in geometry and several ways of treating the inner surface.

Sorry if this was little long, but I hope it explains why each engine sounds distinct.  Smile
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access-air
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RE: What Makes Engine Sounds Unique?

Fri Mar 20, 2009 8:24 pm



Quoting CanadianNorth (Reply 2):
-JT8D powered aircraft have what I would call a thundering sound, I believe it's mainly due to simply being a relatively low bypass engine.
-RR Darts to me are instantly recognizable by their screaming, especially at low power settings, and I've been told it's the gearbox that does the screaming.

Well to expound upon these two engine types, the JT8D in the earlier version (lower power) that have a more, as I call it, Jetty sounds like on the DC9-10/20/ealier versions of the 30 series or on the older 737-100/200s and 727-100.. The later versions that power 727-200 Advanced and 737-200Advanced later versions of the DC9-30s, and on the DC9-40 and 50...That engine sound is more of a whiney sound...
The later JT8D- 200 series has an even more different sound than the earlier two....

As for the Rolls Royce Dart Turboprops. In listening to them on different planes, such as the Herald, HS-748, YS-11, F-27 or FH-227, the basic screaming sound is pretty much the same (unless it has the hushkit on it to deaden the scream) but there is a secondary whiney sound that the Dart makes that sounds different on each planes listed above....


Access-Air
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Aaron747
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RE: What Makes Engine Sounds Unique?

Sat Mar 21, 2009 12:10 am



Quoting Keta (Reply 25):
It has to be noted, however, that this noise is much attenuated in the nacelle prior to exiting to the outside. There are many ways of controlling the noise explained. Controlling the geometry of the compressor (spacing between stages, number of blades...) the noise can be highly reduced, both because of reducing the effects of the mechanisms discussed, and by choosing a correct frequency which takes best advantage of the attenuation of the atmosphere. The noise reduction in the nacelle comes from a correct design in geometry and several ways of treating the inner surface.

Excellent detail and technical analysis. Thanks!

So are we to assume then that the PW4090 "woo woo woos" are by design?  Wink
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tdscanuck
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RE: What Makes Engine Sounds Unique?

Sat Mar 21, 2009 4:18 am



Quoting Keta (Reply 25):
We all have seen this method implemented in the undulating (can't describe better) nozzles found in some engines. I believe this is also the method proposed by Boeing for the B-787.

The physics are similar, but where the "fluted" (undulating) nozzles are usually seen between the core and fan streams, the 787 appears to have a conventional annular nozzle on the core flow. The 787 does have serrations ("chevrons" in Boeing-speak) on the nacelle between the fan stream and the free stream.

Tom.
 
FlyASAGuy2005
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RE: What Makes Engine Sounds Unique?

Sat Mar 21, 2009 4:28 am

Surprisingly, no one has mentioned the engines on the CRJ!

They have a particular high pitched"scream. Very prevelent on take-off and when the a/c is a dead stop and the throttle is advance a ways to get it moving. Gotta love it!
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LASoctoberB6
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RE: What Makes Engine Sounds Unique?

Sat Mar 21, 2009 5:32 am



Quoting Pilotboi (Thread starter):
how can you distinguish each type by the sound?

I've memorized the common types out at McCarran. I just got used to it over the years..

Quoting 2H4 (Reply 14):
I think it's a combination of the engine itself and the doppler effect as the aircraft passes overhead.

I asked about that particular thing in a thread around a year and a half ago or so. I only ever hear that unique sound on A320s of both engines.
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keta
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RE: What Makes Engine Sounds Unique?

Sat Mar 21, 2009 9:43 am



Quoting Tdscanuck (Reply 28):
The physics are similar, but where the "fluted" (undulating) nozzles are usually seen between the core and fan streams, the 787 appears to have a conventional annular nozzle on the core flow. The 787 does have serrations ("chevrons" in Boeing-speak) on the nacelle between the fan stream and the free stream.

Yes, well, that's what I meant when I cited the B-787. The chevrons aim to reduce noise of the bypass exhaust by the same means as the fluted nozzles do for the core exhaust, that is, by modifying the structure of the mixing flow. I didn't want to say that the B-787 uses this for the core stream, sorry if I wasn't clear.  Smile
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