What kind of a sound will a subsonic object make?

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I get that going at the speed of sound creates sonic a sonic boom. But what happens when the object is close to that speed, but not there yet? Say at 1200 kmph.

Is the sound louder with the size? If so, how loud?

For example, how different would a bullet, a baseball, and a bowling ball, going at the same speed sound?

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It doesn’t get significantly louder until a shockwave forms. Sound from an approaching object will not be audible until the object is much closer than would happen if it were stationary. Volume changes do not occur until air begins to compress when it can no longer leap out of the way of a transonic object.

Subsonic object make noise primarily from turbulent flow around and behind them. the object can shed vortices on alternate sides, forming a “vortex street”. (You can see this easily by putting a stick into flowing water.) The alternation produces low frequency sounds, while collapse of the vortices produces a broader spectrum. Turbulence where the boundary layer separates and recirculates is noisy.

In general you cannot compute this. Many practical studies have been done. For instance, NASA Ames had a program devoted to listening to helicopter rotors in flight, and testing new designs. Search on “Rotor Systems Research Aircraft RSRA” and “Quiet Shorthaul Research Aircraft QSRA”.

The sonic boom happens because the normal sound the object makes piles up on top of itself. If you had a perfectly silent plane that made literally no noise whatsoever, when it broke the sound barrier, no sonic boom would happen. Of course this is physically impossible. But it’s not like the sound of a sonic boom comes from being supersonic. The sound comes from the fact that the thing make noise all the time. When the noise can’t escape faster than new noise is made (very loose description for eli5, I’ll see if I can find [my comment ](https://www.reddit.com/r/explainlikeimfive/comments/yu39ee/eli5_sonic_booms/iw7p7x6?utm_medium=android_app&utm_source=share&context=3) explaining it in more depth to link if you’re interested), then it just piles up and piles up on top of itself creating the sonic boom.

Anyway, long story short, a subsonic object makes whatever noise it always makes. Planes typically have engines, bullets will make a bit of a whistling noise from moving the air out of its way, stuff like that.