New engine growl sound

[cagarini]

Sounds were never on my "checklist" for a flight simulator.

I kept seeing announcements of add-ons for MSFS and X-Plane with sound packs for different aircraft, and I agree that users with sophisticated sound systems might really profit from that, but for me it is so bellow in my list of items I consider important in any flight simulator that I never really paid much attention to the way PSX does it's sounds.

For the first time, with this last update, I focused on sounds. I am really enjoying the new "effects", specially with the GE engines, and comparing it to a few videos at youtube revealed that indeed they sound real close to what I can listen to on those youtubes.

I am simply using my LCD monitor speakers.

[Phil Bunch]

I have been puzzled by the very loud growling sound I usually hear on the ground if a 747 or certain other large airliners fly overhead, probably about 2-3000 feet above my position on the ground.  This is of course usually near an airport.

The thing that has puzzled me for years is "how can a turbine engine make such sounds?".   It sounds like very large metallic gears or other large parts are grinding against each other!  I suspect that jet turbine engineers could explain the sounds as due to acoustic resonances inside the engine.

Does anyone have a basic, simple explanation of how this growling engine sound is generated?

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I continue to be unsure how to adjust my home theater system to play the sounds created by PSX.  Its speakers range up to 18 inches in diameter (45 cm), and each speaker has its own amplifier (i.e., I have a powered subwoofer as an integral part of each of  my 7 home theater speakers).    At the moment, I don't think there is any unusually low frequency content that would require very large speakers.  With no special adjustments, PSX sounds "reasonably good" on my system without adjusting my audio receiver's parameters, at least to a first approximation.  I think this favorable outcome is probably due to the system having reasonably high fidelity for the audio frequencies that characterize the sound tracks of PSX.  In other words, my system doesn't introduce any major distortions for the PSX sound track. 

However, there is a big personal (subjective) preference component in this assessment - perhaps I just happen to be personally satisfied with the new PSX sounds as played on my system.  Another parameter is the fidelity of one's hearing - as an older person I doubt that my hearing is accurate or comparable to younger users.  This factor may not be too important in assessing the growling sound since (I think) it is mostly a low-frequency sound.

Another consideration - my home theater receiver comes with a calibrated microphone and software that notches out (removes) frequencies near the primary resonance frequencies of my listening room.  This probably helps my PSX experience if it has frequency content near a room resonance (approximately 50 Hz for my room).  Would activating a room resonance get confused with or distort the 747 engine growling sound pattern?

What is the approximate frequency of the growling sound?

Perhaps system-to-system variations are most stressful when the speaker size/quality and amplifier power vary a lot between systems.  As Hardy has posted, comparing very small PC speakers with say a 3-watt amplifier to 5-inch speakers with a 10-watt amplifier while playing PSX sounds may be hard to fully interpret.  Some laptop speakers are exceptionally small.

Would it help to obtain plots of the spectra of each of the PSX sound files of interest?  It might be informative to see what these frequency spectra are and to compare them with our various subjective impressions.  An interesting but difficult to implement experiment would be to compare the output audio spectra of each person's system while executing the same situ. 

Maybe there is PC or Mac software that could usefully modify the output audio spectrum of a user's PC/speaker system?

It would be very interesting to make career out of understanding and improving the sound files of PSX!   I think one would have to start with installing multiple calibrated, laboratory grade microphones around the cockpit and analyzing their output during various phases of flight.  Even if one could somehow do that, the measurements would (among other things) have to be converted into standardized human auditory response(s).
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(Just some musings while I'm mostly watching the US "World Series" of baseball.  Is baseball of interest anywhere outside of the USA, Japan, parts of the Caribbean, Mexico, and South America?)

[Hardy Heinlin]

I just split this discussion from the Updates thread.


|-|ardy

[Hardy Heinlin]

I have been puzzled by the very loud growling sound I usually hear on the ground if a 747 or certain other large airliners fly overhead, probably about 2-3000 feet above my position on the ground.  This is of course usually near an airport.

The thing that has puzzled me for years is "how can a turbine engine make such sounds?".   It sounds like very large metallic gears or other large parts are grinding against each other!  I suspect that jet turbine engineers could explain the sounds as due to acoustic resonances inside the engine.

Does anyone have a basic, simple explanation of how this growling engine sound is generated?

If you have two deep tones, one at, say, 100 Hz and another one at 101 Hz, for example, you will get interferences. At one point in time, the amplitudes of the 101 Hz tone are almost in line with those of the 100 Hz tone, and the sum of the amplitudes are twice as large. As the 101 Hz amplitudes oscillate faster than those of the 100 Hz tone, there will be a point in time where the positive amplitudes of the 101 Hz tone will be in line with the negative amplitudes of the 100 Hz tone, and the sum of the amplitudes will be nearly zero. This cycle continues and a tremolo effect occurs (not a vibrato, that would be a pitch modulation; the tremolo is a volume modulation). The tremolo is very slow when the two frequencies are almost equal. And it gets faster the more the two frequencies differ. It can get so fast that the tremolo itself reaches a frequency that lies within the human frequency spectrum. This way, another third deep tone overlays the existing tones, even though a third sound source doesn't exist. With these interference induced tremolo effects the sound gets more complex, just by slight mistunings. Engines and engine components are never 100% tuned. As for the often mentioned A320 rattle tone: I'm not sure if this is really A320 specific, but if it is, I guess the A320's engine control has the better "thrust equalizer", making the two engine tones so equal that the natural interference between the two engines is almost standing still, so that the high frequency rattle noise is louder than the bass tones themselves which are the cause of the rattle (the rattle being the rest of what remains after the low frequency elimination).

my home theater receiver comes with a calibrated microphone and software that notches out (removes) frequencies near the primary resonance frequencies of my listening room.  This probably helps my PSX experience if it has frequency content near a room resonance (approximately 50 Hz for my room).  Would activating a room resonance get confused with or distort the 747 engine growling sound pattern?

No. Feel free to use your automatic equalizer if you have one.

What is the approximate frequency of the growling sound?

It doesn't consist of a single sine tone. It contains multiple frequencies and they vary.


Regards,

|-|ardy

[emerydc8]

That's a good explanation, Hardy. By the way, I had a chance to test drive your new sound updates. The engine sound when setting climb thrust and when shutting them down on taxi in put a smile on my face. Very nice! And I'm not even a "sound" guy.

Jon