In an earlier post I gave an outline of my project, and mentioned that I am currently working on an aircraft emission model for auralization. Since then I developed a fully automated method to extract features from aircraft recordings. The method is roughly as follows:
- Backpropagate from source to receiver in time-domain, undoing the Doppler shift, atmospheric attenuation and spreading. The ground effect is ignored for now. We now have a signal that roughly corresponds to what is emitted from the airplane.
- Determine fundamental frequency. An aircraft spectrum consists mostly of noise and tones, which are mostly harmonics. Knowing the fundamental frequency, allows you to determine the power of not only the fundamental, but also of each harmonic.
- And that is the final step. Determine power of the tones, and consider the rest of the spectrum as noise.
The plan is now to determine these features from a large amount of events and develop a basic emission model. But before then, its important to already ask the following question. Are these features sufficient to create a realistic emission signal? Or, even better, are these features, taking into account the developed propagation model, providing a realistic signal at the receiver? Does, what you hear, really sound like an aircraft? That’s in the end what we’re after, right.
Synthesis of emission
To test whether the obtained features are sufficient to reproduce a realistic auralization, I considered a couple of events, and extracted the features for each. I synthesized an emission signal by summing all these time-varying components (about 200 tones and 30 1/3-octave bands), linearly interpolating the samples (features were obtained every second) and smoothing the interpolated components.
Click here to listen to a synthesis of the emission. It’s an Airbus A320 taking of from Zurich Airport.
The first part of the fragment you can hear the tonal components quite clearly but in the latter part noise takes over. This is due to the directivity of the components; tones generated by the fan blades radiate mostly forward.
For comparison, here is the backpropagated signal. You’ll notice it sounds very different. This is because the (unwanted) ground effect is still in it. And because of it, it sounds much more already like an actual aircraft fly-over!
At the receiver
So, let’s listen now at a receiver at a height of 4 meters above the ground. When the aircraft is closest by, the distance is about 180 meters. Here are two fragments: fragment A and fragment B. One is a full auralization, the other is a recording. Can you hear which is which?
(hint: look at the URL in the top part of your browser if you want to know for sure)
There are two quite noticeable differences between the recording and the auralization. This specific auralization does not include the effects of turbulence, and the level of the blade passing frequency is lower than it should be.