What kind of layers would you use across the frequency spectrum?
Do you have any sources where this is talked about?
Thanks in advanced!
I had a listen to jetpack videos online and it sounds like there's two types. On one you can hear a combination of turbine noise (pitched) and filtered noise (unpitched) while on the others there is only filtered noise.
TLDR: white noise through a coplex network of filters and EQs.
So I'll start by the filtered noise which is not only more simple but also common to both jetpack types. We obviously need to start with a white noise source and use different filters to get to the end result. But first we need to analyse what modulates this noise and how. Turning up the throttle makes it brighter, louder and somewhat more distorted. Although the later might be an artifact from the recordings. So we need a control that shifts the cutoff frequency of a band pass filter upwards, makes the noise slightly louder and adds a bit of distortion.
In addition, when the jetpack is close to the ground, and even louder version of that filtered noise is produced when the jets hit uneven ground. These are faster modulations and have a larger effect on amplitude and hence distortion. Their modulation is largely independent from the throttle modulation although a slight amount of throttle mod->ground effect mod might be needed. That's because the pressure of the jet does affect the sound it produces when deflected but not as much as the shape of the deflecting surface like small bumps and crevices that seem to be producing obvious resonances.
And then you have the doppler effect/reverb. Jetpacks are used outdoors so obviously we don't need a hall reverb. It needs to have sparse reflections and a largeish pre-delay since the reflecting objects are few and far apart. More dry signal when the jetpack is close, more reverb when it's far. As for the doppler effect, you could try using a pitch shifter following the reverb dry/wet modulation if not a dedicated doppler effect plugin.
Now, for the jet sound take what I say with a pinch of salt because I'm not very sure, but you could start with the initial BPF noise through a series of EQs or even a filter bank (or both). You could achieve the pitching by shifting a bell eq on the the right (high) frequencies. You would need high 'Q' to make that 'whistling' sound. We already have talked about a 'throttle' modulation source so this is a good modulator to re-use. You also need some other bell curves at set points to emulate the resonances due to the turbine shape.
Finally, the modulators for throttle and distance for ground are just linear user controls but to add some randomness I would add a small amount red noise. I wouldn't do the same for the 'distance' modulator though.
I suspect you could have a kinda realistic result doing only half of the above but I don't have the means to try it where I am.
First start with some imaging of the type of jetpack you want to design a sound for - this will help focus the mind on what sort of sounds you need.
The jetpack will likely have the following top-layers:
Engine/s will be the most significant layer and you will probably want to start with the sound of a small jet turbine, modified with pitch change to cover for the fact that it is smaller.
Also consider that the sound will be different depending on whether the jetpack is actually flying, or whether it is close to the ground. Also consider doppler effects as the unit flies past camera.
The approach you take will likely be different depending on whether it is for linear or interactive media.