I do understand that it’s compressed air, but what exactly is happening when it makes that sound? After driving one around for a few days (moving truck), it seems to be somewhat randomly occurring.
In: Physics
The sound itself is caused by opening a valve where air behind it is at high pressure. When air escapes into low pressure, it forms a jet. When a jet of air mixes with non-moving air, at the boundary there is mixing and small vortices (little pockets of rotating air) form. The size of those vortices depends on the size of the jet and the speed of the air in the jet. There will be a mixture of larger and smaller vortices, both at any points and as the jet expands and mixes. The frequency of sound created by each vortex depends on its size, and the loudness on its intensity. The combination of vortices in the jet creates a sound that you hear as the Tsss, and the dropping off of intensity happens because as the trapped air escapes, the pressure behind the valve drops, so the jet velocity slows down.
This is the same mechanism that causes the roaring noise behind a jet engine or the rumble of a rocket, but because those jets are different sizes and speeds, the sound is different.
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