Sound creates a body that moves, vibrates at a certain frequency. If the vibrations are too slow, then the air simply bypasses the object and sound does not occur. In the article we will tell you what prevents the propagation of sound, but first we will understand what a sound wave is. Consider the process of the appearance of sound in air, water, solids.
How a sound wave arises and diverges in air
The sound source moves and thereby changes the air pressure in closely spaced layers. With each deviation of the body, the air is alternately compressed and rarefied. Pressure changes are transmitted from layer to layer - this is how an elastic wave propagates. The distance at which sound can be perceived is determined by the wavelength, i.e., the distance between the nearest air compression points. The wavelength in turn depends on the oscillation frequency. Sounds of high frequency we call high, and low - low.
Acoustic wave in different environments
Sound propagation in an environment depends on its structure and characteristics. Liquids, air, solids - all of these substances are arranged differently, so they conduct sound differently.
Particles of water and solids are held together by a crystal lattice. Atoms are connected by electric forces, so water cannot completely flow, and solid objects retain their shape. As soon as sound pressure displaces one particle, others follow it. This property is called elasticity and means the ability of the medium, the body to withstand deformation. The more resilient the medium, the faster it conducts sound.
Compared to solids and liquids, air is the least elastic. This is due to its structure. Particles do not hold any bonds between themselves, so the air always tends to disperse. This is prevented by gravity and the constant collisions of atoms with each other.
At 0 ° C, the speed of sound propagation in air is 340 m / s, and water - 1480 m / s. In solids, especially metals, sound travels much faster (up to 5-6 thousand m / s).
What prevents the spread of sound
From the body, sound diverges in all directions equally, but only if there are no obstacles in its path. Not all obstacles interfere with sound propagation. Obviously, you won’t hide from the noise with a sheet of cardboard, like from light. The fact is that sound waves bypass obstacles if their size is less than the wavelength. The wavelength that we hear is 0.015-15 m. A wave can go around a tree, but a building or rocky mountains cannot. From such large objects it is reflected. Like light, a sound wave is reflected at an angle equal to the angle of incidence. At the moment of reflection, we hear an echo.
Sound transition from medium to medium
It is possible only if the densities of the two media are not too different. For example, the difference between air and water is too great. Sound, approaching the border, is reflected from the surface of the river. Only a small part of the wave energy is spent on the vibration of the upper layers of water. Under the water, near its surface, sounds are still audible, but at a meter depth no longer.
Soundproofing media
In buildings with thin walls, good audibility, because sound causes them to oscillate. The walls recreate the noise in the adjoining room. What prevents the propagation of sound, what isolates an acoustic wave? Cork chips, mineral wool, stucco with microparticles, foam rubber - all these materials have a common property: they have many compartments, pores. Sound falling into these voids is repeatedly reflected and absorbed.
What prevents the spread of sound in nature? An example of the absorption of an acoustic wave in natural conditions is fog. In clear weather, you can hear better and at a greater distance. Fog is heterogeneous air, it contains droplets of water. Part of the wave is absorbed by the "compartments" between water and air.
Absorption of sounds of different frequencies
There are sounds that are absorbed with difficulty, it all depends on their frequency. Low sounds (ship horn, ringing of a large bell) can be heard for tens of kilometers. Their frequency is 30-50 Hz, so they are poorly absorbed by the environment. High sounds do not travel that far because they are easily absorbed. For example, we do not perceive ultrasound with its frequency exceeding 20 thousand Hz.