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 random air releases you’re hearing is from an automatic air dryer. The tank will automatically purge air to remove condensation from collecting in the bottom of the tank.
Every time you hit the brakes it uses some air pressure, say 5psi or more, and there could be minor random leaks. The air compressor has a kick in and kick out pressure regulator called an air governor. It’s usually set to kick off the compressor at 125-130psi. When it kicks off your air compressor (really it free wheels it but who cares), it activates the air dryer purge valve. It lets air pressure escape the dryer out the bottom for a few seconds, taking moisture with it, hopefully. You’re hearing the purge valve on the air dryer.
Large vehicles use air brakes which make that noise at different times for example:
When the brake pedal is pressed air pressure applies the brakes to slow the vehicle.
Air has to be replenished in the system so the compressor will kick on to fill the air tanks, the TSSS noise is the valve closing to stop taking in outside air, really just the compressor turning off.
The other time that sound is common is when parking. Large vehicles use “spring brakes” as a parking brake. Air has to be constantly supplied to release the brakes so when the parking brake is applied all the air that holds the spring brakes open leaves the canister that contains the spring brake.
Hope that’s not too much, or too complicated.
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.
Latest Answers