Taking Pictures of the Sound of a Rocket

Grant W. Hart – grant_hart@byu.edu
Brigham Young University
Provo, UT 84602
United States

Kent Gee (@KentLGee on X)
Eric Hintz
Giovanna Nuccitelli
Trevor Mahlmann (@TrevorMahlmann on X)

Popular version of 1pNSa8 – A photographic analysis of Mach wave radiation from a rocket plume
Presented at the 186th ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0026810

The rumble of a large rocket launching is one of the loudest non-explosive sounds that mankind has ever made. Where does that sound come from?  Surprisingly, it doesn’t come from the rocket itself, or even the exhaust nozzle, but rather from the plume of exhaust that shoots out of the back. The plume is supersonic when it comes out of the rocket, and it emits sound as it slows down in the atmosphere.

This process was visualized in some recent pictures taken by Trevor Mahlmann of a Falcon 9 launch from Cape Canaveral.  The launch was just after dawn, and Mahlmann took a series of striking pictures as the rocket passed in front of the sun. Two of those pictures are shown below. If you look at the edge of the sun in the later picture you can see distortions caused by the intense sound waves coming from the rocket.

Recognizing the possibility of gaining more information from these pictures, researchers at Brigham Young University got permission from Mr. Mahlmann to further analyze them.  The third picture below shows a portion of the difference between the first two pictures. The colors have been modified to show the sound waves more clearly.  The waves clearly are coming from a region far down the plume of the rocket, rather than the nozzle of the rocket. The source was typically about 10-25 times the diameter of the rocket down the plume.

The sound is also directional – it doesn’t go out evenly in all directions, but rather goes out most strongly at about 20-30 degrees below the horizontal. Most rockets sound loudest to people watching the launch when they are 20-30 degrees above the ground. This is all consistent with the models of the sound being produced by the processes that slow down the exhaust from supersonic speeds.  A good introduction to rocket noise is found in a recent article in Physics Today.

The researchers first had to line up the images so that the sun was in the same place in each frame. They were then able to subtract the later image from the first one to get the difference and leave just the distortions caused by the waves in the second image.  To find the source of the waves, it was necessary to draw a line backward from the wave’s image and find where it met the rocket’s path across the Sun. Since it took time for the wave to get from the source to where it was observed, they had to find where the rocket was at the time the sound wave was given off. They did this by finding how far the sound had traveled and used the speed of sound to find the time it took to get there. With that information the researchers could find the position of the source and the direction of the wave.

Figure 1. A Falcon 9 rocket about to pass in front of the Sun. Image courtesy of Trevor Mahlmann. Used by permission. Higher resolution versions available from the photographer.

Figure 2. A Falcon 9 rocket passing in front of the Sun. Note the distortions of the edge of the Sun caused by the sound waves produced by the rocket. Image courtesy of Trevor Mahlmann. Used by permission. Higher resolution versions available from the photographer.

Figure 3. A portion of the difference between the two previous figures, showing the enhanced sound waves. The bottom of the rocket is at the top of the image. Image adapted from Hart et al.’s original paper.