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Sssshhh... it's a secret!

ErickC

ErickC

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Hey kids! Did you ever get frustrated when making frequency curves for your sounds? Well, good news, I've got a way of doing it that requires no real effort, and I was even nice enough to make a comment about it on the YouTubes. Here's what I posted:

Tip: recording good audio is only half the battle. Blending the frequency curves of the clips is somewhere where the default aircraft in previous versions of MSFS always fell short, to the point where I doubt the people putting together audio understood how sound.cfg worked. But there's an easy way of figuring it out. There's a direct relationship between pitch and RPM on all rotating engines, be they jet or piston. Since I'm presuming that y'all were smart enough to record RPM values while you were recording audio, it's a simple matter of basic division to find the correct frequency curve values. For example, let's say I have a piston engine that idles at 275 RPM and maxes out at 835 RPM, and my clip was recorded at 515 RPM. To figure out the maximum value, I would divide 835 into 515 and get 1.621. To get the idle value, I would divide 275 by 515 to get 0.634. The resulting frequency curve, in MSFS format, would look like this:

rparams= 0.329,0.634, 1.000,1.621

The 0.329 value comes from 275/835. If you do some basic math, you'll realize that you can do these calculations based on percent values as well, making it equally easy for jet engines - but remember that jet engines often run at values above 100 percent, so your frequency curves need to take this into account. Moreover, most jet engines have multiple shafts and you will need to tie the right sounds...
Click to expand...

Super secret #2: a big part of figuring out what RPM the clip was recording at is figuring out the pitch of some harmonic and comparing it to a clip of known RPM. Whaaaaa? Calm down, tiger. Find a cockpit video on YouTube where the gauges are easily visible. Open your audio editor (it needs to have a parametric EQ that is tied to some kind of graphical display of the waveform, I use Goldwave) and set it up to record from your sound card's output. Record a short clip of the engine running at a constant RPM during a portion where the gauges are legible and write down the RPM or percent value. Use your sound editor's parametric EQ to find one of the larger frequency spikes and write that frequency down, as well as the RPM value. How do you do this? Look at the graphical output and use that to get a rough idea of where the spike is. Now turn the gain all the way up and adjust the frequency value. When you have it close, you'll notice that a single tone starts getting louder, but it sounds like it's "vibrating" a bit, because it's not quite right. When you're right on top of the harmonic, suddenly all you will hear is a pure tone. On the JT8D-17, I have worked out that the N1 whine has a dominant overtone of 3975Hz at 102.5% RPM (the maximum for this engine).

With that knowledge, you can then find the same spike on your particular clip. An easy way might be to tweak the pitch of the reference clip until it sounds close, then find that original spike. Let's say I've done this and my reference clip's spike is now at 2570 Hz. Well, I look at the looped clip that I'm using for my sounds, and it doesn't have a spike there, but it has one at 2545 Hz. That's probably the harmonic we're looking for. Write this number down. I can now divide 2545 into 3975 to get 0.64. This means that my clip was recorded at 64% N1, and I can create my frequency curve accordingly.

Easy peasy.
 
Outstanding work. Do you, by the way, use Aircraft Sound Manager? Click and drag those frequencies and amplitudes after calculation. Aircraft Sound Manager, Aircraft Container Manager, and Aircraft Airfile Manager were all a fantastic suite of tools, and sad to say, the only one left that'll run on Windows 10 is AAM. But the others'll run on Windows XP or Windows 7, so if you have an old machine just for running those (I do), ASM is a lot easier than typing in numbers. If you can find it.

ED
 
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