Sound
Sound is vibrations, that is. Periodic mechanical disturbance in elastic media: gaseous, liquid and solid. Such a disturbance, which is some physical change in the medium (for example, a change in density or pressure, displacement of particles), propagates in it in the form of a sound wave. The field of physics, which deals with the questions of the origin, propagation of reception and processing of sound waves, is called acoustics. Sound may be inaudible if its frequency is beyond the sensitivity of the human ear, or if it propagates in an environment such as a solid that cannot have direct contact with the ear, or its energy dissipates rapidly in the environment. So our usual process of perceiving sound is just one side of acoustics.
Acoustics is one of the oldest areas of knowledge. It arose several centuries before Christ. me. as a doctrine of sound, that is, of elastic waves perceived by the human ear (hence the origin of the name). The beginning of the formation of acoustics as physics. Science (17th century) is associated with the study of the system, musical tones, their sources (strings, tubes), with measurements of the speed of sound propagation. Until the beginning of the 20th century. Acoustics developed as a branch of mechanics. The creation of methods to decompose a complex oscillatory process into simple components (Fourier method) laid the foundation for sound analysis and synthesis of complex sound from simple components. All this classic. the stage of development of acoustics was summarized at the beginning. 20th century Rayleigh (J.W. Strutt).
A new stage in the development of acoustics began in the 1920s. 20th century in connection with the development of radio engineering and broadcasting, which generated the need to develop methods and means of converting electrical energy into acoustic energy , and vice versa.
Consider a long pipe filled with air. From the extreme left, a piston is inserted into it that fits snugly against the walls (Fig. 1). If the piston jerks to the right and stops, then the air in the immediate vicinity of it will momentarily compress (Fig. 1, a). Then the compressed air will expand, pushing the air adjacent to it to the right, and the compression region, which initially appeared near the piston, will move along the pipe at a constant speed (Fig. 1, b) . This compression wave is the sound wave in gas.
Figure 1. Occurrence of a sound wave in a pipe
A sound wave in a gas is characterized by excess pressure, excess density, particle displacement, and velocity. For sound waves, these deviations from equilibrium values are always small. Therefore, the excess pressure associated with the wave is much less than the static pressure of the gas. Otherwise, we are faced with another phenomenon: a shock wave. In a sound wave corresponding to ordinary speech, the excess pressure is only about one millionth of atmospheric pressure.
It is important that the substance is not carried away by the sound wave. The wave is just a temporary disturbance that passes through the air, after which the air returns to a state of equilibrium. The movement of waves, of course, is not characteristic only of sound: light and radio signals propagate in the form of waves, and everyone knows the waves on the surface of the water. All types of waves are mathematically described by the so-called wave equation.
The wave in the pipe is called a sound pulse. A very important type of wave is generated when the piston vibrates back and forth like a weight suspended from a spring. These vibrations are called simple or sinusoidal harmonics, and the excited wave in this case is called harmonic.Sound waves in gases and liquids can only be longitudinal, since these media have elasticity only with respect to compression (stress) strains. In solids, sound waves can be both longitudinal and transverse, since solids have elasticity with respect to compression (tension) and shear deformations.
