The Sound Field of a Supersonic Source

Animations

The following clip is an animation of Fig. 5.46(a) in the book. It shows a slow motion of the sound field of a monopole sound source moving uniformly at 600 m/s (the Mach number is approx. 1.7) and radiating a monochromatic signal of 500 Hz. You can clearly see the Mach cone, which causes the so-called sonic boom.

The sound field shown above is composed of two components:

1) A forward travelling component shown in the following clip, which is an animation of Fig. 3.46(c). This field component stems from what the sound source radiated in direction of its motion. The source surpasses its own sound field. The latter is therefore reversed in time. The signal is transposed upward in frequency for this particular source speed.

2) A backward travelling component shown in the following clip, which is an animation of Fig. 5.46(d). This field component stems from what the sound source radiated in direction opposite of its motion. The sound waves are stretched and the signal is therefore transposed down in frequency.

Audio Example

The following clip contains the signal that the supersonic source emits:

 

The following clip contains the signal as it would be recorded by a microphone. Note that I put the reversed and upwards-transposed signal component (the trombone introduction) into the left channel and the forward and downwards-transposed signal component (the first verse) into the right channel so that you can listen to them independenly.

 

There is not really much to hear in the previous clip as the amplitude drops really fast. The following clip contains the same signal like the previous one but without the amplitude envelope meaning that the signal is always equally loud so that you can better appreciate the different components.

 

And finally, the same signal but with custom-designed amplitude envelope that creates a more useful audio effect.

 

Other Resources

Other illustrative animations of supersonic sound sources can be found at Daniel A. Russell’s webpage.