A creaking door in the middle of the night or the high-pitched squeal of a train taking a tight turn often awakens an instinctive sense of anxiety. But where do these sharp metallic noises come from?
These distinct metallic sounds result from a process of sliding and sticking between contacting parts. The rigidity and density of metals amplify these noises, making them especially high-pitched.
The mechanism behind these screeches is similar to that of squealing tires or a creaking floor, according to
Robert Hyers, a professor at the Worcester Polytechnic Institute. The screech is actually a series of quick, repetitive impacts, perceived as a high-pitched sound when they occur in rapid succession.
This sticking happens when lubricants, like oil or water, are "squeezed out" under the high contact pressure, explains
Yip-Wah Chung from Northwestern University. The sliding, on the other hand, is directly related to the friction between the metal surfaces.
When these metal surfaces slide and stick, they vibrate, creating sound. These vibrations, similar to the plucking of a guitar string, depend on the shape and composition of the metal, says
Chengzhi Shi from the University of Michigan.
Metals, due to their density and rigidity, are particularly suited to generating high-amplitude sound waves, notes
Naresh Thadhani from Georgia Tech. This is why metallic sounds are often louder than those made by other materials.
The screech of a train is more intense than that of a door because of the pressure, speed, and size of the elements involved. To reduce these noises, one could decrease the load on the contacts or improve lubrication,
Yip-Wah Chung suggests. However, in some cases, such as with trains, this screeching is necessary for safety, despite being unpleasant.