Why does a bus make a jetting sound when the rear door is closed? The answer to the long-standing question is finally here

The jet sound when the rear door of a 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. After the driver presses the pneumatic switch in the driver's seat, the compressed gas enters the cylinder through the air valve, pushing the piston rod to open or close the door. When the compressed gas pushes the piston rod, the cylinder moves, and the gas filled in the previous process on the other side is discharged. The sound we hear is the sound of the cylinder exhausting.

So, why do buses use pneumatic devices instead of hydraulic transmission devices which produce much less noise than pneumatic ones? Because pneumatic transmission is cheaper and more adaptable to the bumpy environment of buses.

The compressed gas used in pneumatics does not need to be recycled, and controlling a bus door does not require high precision and stability. As for the noise problem, in fact, the sound we hear has been processed by the muffler, and the sound is not loud, so bus doors all use pneumatic transmission systems.

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If you observe carefully, you will find that when buses and trucks are parking, there will be a sound of gas ejection. This is because large vehicles have great kinetic energy, and a lot of force is needed to stop them, so compressed air is needed to assist in braking. To loosen the brake pads, the compressed air needs to be discharged. The principle is the same.

References:

[1] Xu Binghui. Basic knowledge of pneumatic technology [J]. Hydraulics and Pneumatics. 1994, (05): 39-41

Source: Institute of Physics, Chinese Academy of Sciences

Author: fiufiu

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