Why is there a sound of jets when the bus closes the back door?

The sound of the jet when the back door of the bus opens and closes is actually the sound of gas being ejected from the cylinder. The opening and closing of the bus door is controlled by a pneumatic control system, and the actuator is a cylinder with a piston rod. When the compressed gas pushes the piston rod, the cylinder moves, and the gas charged in the previous process on the other side is expelled. The sound we hear is the sound of the cylinder exhaust.

So, why do buses use pneumatic devices instead of hydraulic transmissions, which are much less noisy than pneumatics, because pneumatic transmissions are cheaper and more suitable for the bumpy environment of buses. 

The compressed gas used for pneumatics does not need to be recovered, and the control of a bus door does not require too much precision and stability. As for the noise problem, in fact, the sound we heard has been processed by the muffler, and the sound is not too loud, so the bus doors all use a pneumatic transmission system.

If you pay attention, when the bus or truck is parked, there will be a sound of gas spouting. This is because the large vehicle has a lot of kinetic energy, and it takes a lot of force to make it stop, so compressed air is needed to assist the brakes, and the compressed air needs to be exhausted to release the brake pads. The reason is the same.