BASICS OF ACOUSTICS: Acoustic vibrations
1 BASICS OF ACOUSTICS
1.1 Acoustic vibrations.
Elastic vibrations of the particles of the medium near the equilibrium position is called acoustic vibrations.
The oscillation mechanism is determined by two basic physical properties of materials: elasticity and inertia.
Under elasticity we mean the ability of the body to restore after exposure discontinuation to the original shape or volume.
Inertia – the ability to maintain their body speed in the absence of external forces.
The dependence of media particles displacement while continuous vibrations from the time is described by the periodic function of time.(Fig. 1.1)
The period T is the minimum time interval between identical positions of the particles. Period – this one oscillation period is measured in seconds (s) milliseconds (mls) microseconds (mks).
Fig. 1.1 – Dependence of particle displacement from time, continuous fluctuations.
Pulse fluctuations have practical significance for the ultrasonic testing purposes. (Figure 1.2.) In contrast to the continuous, pulsed vibrations, (Fig. 1.2) Pulse fluctuations are limited in time, have beginning and end. Impulse is a part of the signal which amplitude of oscillations exceeds a certain threshold. Changing the amplitude of oscillations is described by an envelope.
Fig. 1.2 – Dependence of the particles from the time shift (pulse oscillation)
1.2 Acoustic waves.
The process of acoustic oscillations (Fig. 1.3) in the tested object is called an acoustic wave. In the process of wave propagation occurs the transfer of energy of oscillation from one medium particle to another.
Fig. 1.3 – Distribution of acoustic vibrations
The mechanism of wave propagation is illustrated in Figure 1.4. Fluctuations in different points of the medium are synchronized. For example, at points A and B oscillation offset from each other by a half period. When particles at the point A are shifted downwards, at point B particles are shifted upwards and vice versa. The arrows in Figure 1.4 shows the displacement of particles of the medium in the process of propagation of the wave to the right.
Fig. 1.4 – Propagation of acoustic vibrations.