Cracklin’ Jets

Why, when military jets zip by over head, is there a crackling to be heard in the roar of the engines? Know what I’m talking about? If not, here’s an F-18 Super Hornet video. It’s most noticeable around 2:55 minutes in. Space shuttles do it too.

Super Hornet

Before I get into the nitty-gritty of the research, led by Kent L. Gee of Brigham Young University, you’d probably like to know why this matters. Can you imagine working around these planes all the time, hearing crackle-crackle-roar at work every day? Or living in an area that the jet planes pass through on a regular basis?

If physicists understand the noise, they can find a way to reduce it, improving life for military personnel and communities whose airspaces are frequented by these jets.

Apart from that, who doesn’t want to know about the acoustics of jet engines?

Let’s get down to business. A sound, traveling through air, alternately compresses and expands the air molecules, and our ears react to these pressure changes. These physicists wanted to be able to see the sounds of the engine, so they made a graph.

Seeing Sound
They measured the pressure of the air in the exhaust plume of an F/A-18E Super Hornet. Pressure is plotted on the y-axis to see how the pressure varied over time. In the plot below, all those fluctuations happened in a mere four hundredths of a second!

So, what marks this graph as a crackler? Notice how the peaks go further away from zero than the valleys, with a maximum of 3500 as opposed to the minimum of -1500.

Also notice that the peaks are generally narrower than the valleys. In a more ordinary engine roar, the peaks and valleys would be random but equal. Not so when your engine crackles.

Another Way to see Sound
To more clearly demonstrate these observations, the researchers took this data and plotted it another way. Now, the y-axis is the probability that the pressure is a certain distance away from zero. The “Gaussian” curve is just a model of what the graph would look like if the pressures were truly random and centered around zero.

There are two things to notice about this graph.

1) The majority of the graph is on the negative side. This shows that the pressure has a tendency to be below atmospheric pressure.

2) Notice that the graph stretches further on the positive side than the negative side. This shows that the air compressions tend to deviate from normal atmospheric pressure more than the expansions.

Simulating the Crackle (er, trying)
Now, the researchers thought they knew what sort of pattern makes the crackle, so they tried to create a sound with the same pattern. These are the patterns for the sound (AB Jet) and the simulated sound.

Have a listen… Hear the crackle? No, there’s nothing wrong with your ears. Apparently, the crackle is dependent on more than just the tendency toward lower pressures and brief, strong compressions.

What went wrong? The graphs of the sound and the simulation look pretty similar. But wait, take a closer look at the peaks for the jet. The rising side is almost vertical while the falling side is more gradual.

The new thought is that the quick jump in pressure causes the crackle, not the overall distribution of peaks and valleys. Can’t be sure without another experiment!

A lay-language version of this paper found here or you can check out all the lay-language papers from 153rd Acoustical Society of American meeting

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