For many, even years after graduation, it remains unknown what the actual speed of sound in air is. Someone inattentively listened to the teacher, and someone simply did not fully understand the material presented. Well, maybe it's time to fill this knowledge gap. Today we will not just indicate “dry” numbers, but explain the mechanism itself, which determines the speed of sound in air.
As you know, air is a combination of various gases. Nitrogen accounts for a little more than 78%, oxygen takes up almost 21%, the rest is carbon dioxide and inert gases. Therefore, we will talk about the speed of sound propagation in a gaseous medium.
First, let's decide what a sound is. Surely many have heard the saying "sound waves" or "sound vibrations." Indeed, for example, the diffuser of a sound reproducing column oscillates with a certain frequency, which is classified by a person’s hearing aid as sound. One of the laws of physics states that pressure in gases and liquids propagates unchanged in all directions. It follows that under ideal conditions the speed of sound in gases is uniform. Of course, in reality its natural attenuation takes place. You need to remember this feature, because it explains why the speed can change. But we are a little distracted from the main topic. So, if sound is an oscillation, then what exactly is an oscillation?
Any gas is a collection of atoms of a certain configuration. Unlike solids, there is a relatively large distance between the atoms in them (compared, for example, with the crystal lattice of metals). We can give an analogy with peas distributed over a container with a jelly-like mass. The source of sound vibrations reports a momentum of motion to the nearest gas atoms. They, in turn, like balls on a pool table, “hit” the neighboring ones, and the process repeats. The speed of sound in air just determines the intensity of the root cause pulse. But this is only one component. The denser the atoms of matter, the higher the speed of sound propagation in it. For example, the speed of sound in air is almost 10 times less than in monolithic granite. This is very easy to understand: in order for an atom in a gas to "fly" to a neighboring one and transfer momentum energy to it, it needs to travel a certain distance.
Consequence: with increasing temperature, the speed of wave propagation increases. Despite thermal expansion, the intrinsic velocity of atoms is higher; they move randomly and collide more often. It is also true that compressed gas conducts sound much faster, but the liquefied state of aggregation is still the champion . In the calculations of the speed of sound in gases, the initial density, compressibility, temperature and coefficient (gas constant) are taken into account. Actually, all this follows from the foregoing.
Still, what is the speed of sound in air? Many have already guessed that it is impossible to give a definite answer. Here are just some basic data:
- at zero degrees Celsius at zero point (sea level) the speed of sound is about 331 m / s;
- reducing the temperature to - 20 degrees Celsius, you can "slow down" the sound waves to 319 m / s, since initially the atoms in space move more slowly;
- raising it to 500 degrees accelerates the propagation of sound by almost one and a half times - up to 550 m / s.
However, the data given are approximate, since in addition to temperature, the ability of gases to conduct sound is also affected by pressure, the configuration of space (a room with objects or an open area), intrinsic mobility, etc.
Currently, the property of the atmosphere to conduct sound is being actively investigated. For example, one of the projects allows determining the temperature of air layers by registering a reflected sound signal (echo).