Sound appears due to the propagation of a wave from an oscillating body. Solid objects, in particular, metals and their alloys, air, water - all these are media. They may produce sound.
Many are surprised by the situation when the train is still out of sight and cannot be heard, and if you put your ear to the steel rails, the sound of the wheels will be distinct. Obviously, the reason is the different speed of sound in steel and air. This issue will be discussed in more detail in the article.
How a sound wave propagates in solids
Let's consider the physics of the process. Sound in steel, as well as in solids in general, does not propagate in the same way as in gases and liquids. This is explained by differences in the structure of substances. The atoms of a solid body are interconnected by invisible electrical forces. Together they form a crystal lattice. Links act like springs. If asome atom moves, then others move with it.
Sound in a solid is created by vibrations of particles and their propagation along the crystal lattice. Moreover, the movements of atoms are ordered, have the same frequency and direction. The process becomes possible due to elasticity, i.e., the ability of the body to resist pressure. This property and density determine how fast a sound wave propagates. In metals, this happens ten times faster than in air.
What determines the speed of sound propagation in steel
To answer this question, you need to know what else plays a role in this process. In addition to elasticity, the direction of the sound wave affects the speed of sound. It is longitudinal and transverse. The first diverges in the direction of the oscillatory motion, and the second - against it. In solids, unlike air, sound can travel in both directions. It is interesting that the speed of a longitudinal wave at the same oscillation frequency is always higher than that of a transverse one. The difference is a few seconds.
Steel grades differ in carbon content (it determines hardness), in the number of non-metallic inclusions, etc. Here is another interesting fact. It seems that if one type of this alloy is taken, then the speed of sound in steel will be constant, since it depends on elasticity. However, it is not. This property characterizes the resistance to deformation, which can be different: torsion, compression, bending. The type of impact also determines the speed of sound. Thus, a longitudinal wave diverges alongstainless steel at a speed of 5,800 m/s, compression wave - 5,000 m/s, shear and torsion wave - 3,100 m/s.
