In the case of door hinges, squeaking can be addressed by adding lubricant, but this method is not applicable to subways.
The squeaking sound of door hinges or subways when navigating sharp turns can send shivers down the spine of those who hear it. So why do metals produce these sounds? The reason relates to the periodic transition between sticking and sliding of metal pieces. The hardness and density of these materials also contribute to the loudness of the squeaks.
Subways can produce annoying squeaks. (Photo: Doug Schneider).
The sticking phenomenon occurs when lubricants on metals, such as oil or water, are forced out under high contact pressure, according to Yip-Wah Chung, a professor of materials science and engineering at Northwestern University. Sliding and sticking also occur when you drag your finger down a glass window. Friction is a key factor, but it only explains part of the sticking process. “It is the alternating change between sticking and sliding that generates the squeaking sound”, he explains.
Sliding and sticking cause metal materials to vibrate and produce sound, explains Chengzhi Shi, an associate professor of mechanical engineering at the University of Michigan. This can be compared to plucking a guitar string. As sticking and sliding occur, the metal vibrates. The frequency and intensity of the vibrations also depend on the shape and material of the metal. “It is the vibrating state of the metal, stimulated by friction, that produces the squeaking sound we hear”, Shi states.
Other materials can resonate in similar ways, but metals tend to produce particularly loud sounds, according to Naresh Thadhani, a professor of materials science and engineering at Georgia Institute of Technology. “Metals are often denser and harder, giving them certain properties that enhance sound effects and resonance”, Thadhani explains. In other words, they generate sound waves that travel faster with greater amplitude. Thadhani also notes that the squeaking of subways is much louder than that of door hinges because the pressure, speed, and larger size contribute to greater squeaks.
“There are two ways to eliminate squeaking. The first is to reduce the load at the contact point. The other is to increase lubrication“, Chung states. With door hinges, one can apply lubricants like WD-40 to reduce friction. However, in the case of subways, the squeaking is actually necessary.
“The increased friction helps the train to stop and accelerate better, which causes the squeaking sound”, Hyers explains. Removing friction by lubricating the tracks may reduce the unpleasant noise, but it could also put the train in danger.
