The slow squeak of a door hinge at 1 a.m. or the screech of a subway train taking a sharp turn may send a shiver down your spine, but why do these noises happen? Why does metal squeak?
It turns out, it’s all about the periodic shift between metal pieces sticking and slipping against one another. Metal’s stiffness and density also make that squeak extra loud, experts told Live Science.
The mechanism behind metal squeaking is no different from the squeal of skidding tires or the squeak of a floorboard, Robert Hyers, a professor of mechanical and materials engineering at Worcester Polytechnic Institute in Massachusetts, told Live Science in an email.
“The squeaks are really periodic thumps,” Hyers said. “If the thumps are close enough together, you perceive the high-frequency thumps as squeaks.” In other words, when these thumps happen in quick succession, they emit a high-pitched squeaking sound.
This sticking occurs when lubricants on the metal, like oil or water, are “squeezed” out of the way during a moment of high contact stress, Yip-Wah Chung, a professor of materials science and engineering at Northwestern University, told Live Science in an email.
This slipping and sticking also occur when you drag your fingers down a glass window. While friction is key here, Hyers said it accounts only for the sticking part.
“It’s the periodic alternation between sticking and slipping that makes the squeak,” he said.
Slipping and sticking cause metal materials to vibrate and emit noise Chengzhi Shi, an associate professor of mechanical engineering at the University of Michigan, told Live Science in an email. You could think of this like plucking a guitar string. As the metal sticks and slips, it vibrates. The frequency and strength of the vibration also depend on the shape and material of the metal.
“It is the vibration modes of the metal excited by the friction that radiate the squeaking sound we hear,” Shi said.
Related: Why does striking flint against steel start a fire?
While other materials also resonate in this way, metals are particularly noisy, Naresh Thadhani, a professor of materials science and engineering at Georgia Tech, told Live Science in an email.
“Metals are generally denser and have high stiffness, which provides them with certain acoustic properties that enhance sonic effects and sonority,” Thadhani said. In other words, they create faster moving sound waves with higher amplitudes.
As for why the squeaking of a subway train is so much louder than the squeak of a door hinge, Thadhani said that greater pressure, speed and size cause louder screeches.
So can you do anything about it? “There are two ways to remove squeaking,” Chung said. “One is to reduce the load on the contact. The other is to improve the lubrication.”
For a squeaky door hinge, this might involve applying a lubricant, like WD-40, to reduce friction. However, sometimes you need the squeak: “The same additional friction that lets the trains stop and accelerate better also causes the squeak,” Hyers said.
Removing this friction by lubricating a subway train’s tracks might take away the train’s horrible screech, but it could also set up the train for dangerous accidents. So be grateful the next time you hear the loud screech of a subway car.