Hunters and people of military specialties are familiar with the sound of flying bullets and shells. This sound is sibilant-hissing, not distinguished by the purity of tone. In the short time of the bullet's flight, you will notice that the tone of this sound changes from high to low.
To understand the cause of the characteristic sound when a bullet is flying, look at the shape of the bullets you know. Hunting bullets for smoothbore guns are round or cylinder shaped (yakan, Mayer's bullet). For sports and military weapons, conical bullets or bullets with a rounded front in the direction of flight are used. Obviously, the aerodynamics of the bullet is not perfect and does not contribute to its good flow around.
Studying the behavior of bluff bodies in flows of liquids or gases, the scientist Theodor von Karman discovered that a vortex trail forms behind such bodies. This phenomenon is called the "Karman track". The density of air flows in vortices is different and changes cyclically, respectively, a vortex can be imagined as a generator of acoustic waves. And sound is an acoustic wave.
You are probably familiar with the omen according to which the fighter hears the whistle of only the bullet that flew past. This sign has a completely scientific basis. The bullet flies at subsonic speed, and the vortex path is located behind it along the flight path. Moreover, a person hears not the vortices of the "Karman path" themselves, but the waves that it forms in the surrounding airspace when it comes into contact with it. That is, a person hearing the sound of a passing bullet is not on the trajectory of the bullet, but next to this trajectory.
A simple experience will help you understand what a vortex street looks like. Put some water in the tub and add a small amount of foam of any kind of detergent to the surface. Launch a dummy bullet into the tub. It can be a children's boat with a sharp bow and blunt stern, or a flat foam model of any shape. Sweep the layout over the surface of the water. In the wake jet of the model, you will see vortices consisting of foam bubbles. This is the "Karman track".
Note that when you are close to the trajectory of the bullet, you are observing this trajectory from a certain angle. If the vortex path is at an angle close to a straight line to you, the distance to the source of the vortices is minimal, then the sound will follow the shortest path. But the bullet flew past, and now the distance to the source of the vortices increases. The bullet speed is high and comparable to the speed of sound. This means that the distance between the centers of the vortices will be perceived as increasing due to the retardation of sound waves. Subjectively, this can be heard as a decrease in the audio tone. In physics, this phenomenon is called the Doppler effect. It is one of the proofs of the wave nature of sound.
