Question about 737 NG sounds in air (flight deck)

[deuzx]

Hello everyone. I am currently editing an existing soundpack for PMDG 737 NG (addon program for Flight Simulator 9) and wondering about following:
When the engine spool up (in start phase, when starter is active) and pass approx. 10-12 % N1, a load bass sound is observed. Does this sound orign from the engine or from resonans in a/c body?
When on crz, and the engine throttle is moved to IDLE and N1 pass app 65 % a bass sound is again heard. Does this sound come every time when passing approx this % N1, or is this a little bit variating from a/c to a/c? I never heard this bass resonance sound when spooling up engine, (under T/O situation). It really seems to only appear when spooling down to IDLE on crz. Anybody who can explain a little bit about these sound I have heard when I have been observer / visitor on flight deck?
Then I would have been very thanksfull!
A little promo of my soundwork (currently still under progress) can be heard here: http://www.home.no/nv31/737NGpromo.mp3 (Warning! File removed!)
regards

[fireflybob]

Dont know the reasons why but I recall when I was on the B737-300 with Orion Aiways. We were (I believe) the launch customer and we had "Jean-Claude" on the flight deck from CFM who was with us to observe engine observation. We had noticed this "noise" when the thrust levers were retarded for descent. We asked him what caused it and (imagine this in a French accent) he said "Of course - that is rumble!".

[I-2021]

When the engine spool up (in start phase, when starter is active) and pass approx. 10-12 % N1, a load bass sound is observed. Does this sound orign from the engine or from resonans in a/c body?

Hello,

are you sure about these N1 values ? The loud sound you are talking about makes me think to when we put the start levers on idle detent and hence we open the fuel shutoff valves. At that stage the fuel is lead into the combustion chamber and is ignited. That is what actually comes into my mind.

[deuzx]

Hello! You may have very right. I am not totally sure about the N1 values I gave here, it was only estimate from me. Same bass sound does also appear when shutting down (moving fuel sw. from IDLE to OFF).

When going from app 70 %N1 to idle in air, it is nearly about the same bass noise, but not as load as in the start/stop situation. It is a mix off deep (but weak) bass and some vibrations.

[deuzx]

Thank you! Any 737 NG pilots who knows about the magic bass sound around 70% N1? I have a little promo on the internett as an mp3file. Everyone who wants to listen to my sound work, could visit and have a try. Would have been glad to get comments from persons with experience from flight deck in the NG (pilots, techicans, observers, other interested personell).

www.home.no/nv31/promo2.mp3

[vapilot2004]

I have always thought the noise to be engine produced and amplified by the fan case. Every series (300 and up) I have been on has this characteristic rumble.

[NSEU]

You would have to ask if the rumble is observable outside the aircraft... If it is, then it's not likely to be fuselage resonance.

Can't think of anything special happening at 70% unless it's the high pressure engine bleed valve closing (and the bleed system switching to intermediate or low pressure stages)?

It's like when you're listening for the APU to shut down. The APU sound is hidden in the background when it's running (from the cockpit), but on shutdown, it goes through a certain rpm and is audible for a few seconds. Could be fuselage resonance, but I'm not sure.

RB211's have a very strong base sound on startup and is heard outside. Perhaps there is a harmonic at 70%? (Haven't listened for it though).

Maybe there is an interreaction between the N1 and other stages?

I guess unless you had feedback from accoustics engineer with lots of experience with aircraft, you may never really find out. Maintenance engineers certainly don't dwell on these kinds of things :}

Regards.
NSEU

[deuzx]

Okay. I see! Anybody who has comments to the sound (promo)? It would have been nice to hear how other people think they are.

regards