BASICS OF ACOUSTICS: Types of waves & Laws of propagation of acoustic waves – acoustic field

1.7 Types of waves
Several types of waves can be distributed In an infinite medium: longitudinal, transverse and surface. Each of these waves types differ from each other by direction of the fluctuations in the wave, and propagation speed.

Longitudinal waves – are acoustic waves, in which the particles of the medium oscillate along the direction of the wave propagation. Under the influence of the wave material undergoes compression and tensile deformation. The waves can propagate in all bodies except the vacuum and have the highest rate of propagation in steels Cl = 5950 m / s.

Fig. 1.9 – Diagram of propagation of longitudinal waves
Transverse waves – are acoustic waves, in which the particles of the medium vibrate perpendicular to the direction of wave propagation. Under the influence of waves the material undergoes shear deformation. This type of waves can propagate only in solids. In liquids and gases not covered by transverse waves. The rate of shear waves (Ct) in one material is always less than (Cl) the velocity of longitudinal waves, to become the following relation:

C_t ≈ 0,55·C_l.

Fig. 1.10 -schemes shear wave propagation
Surface waves (Rayleigh waves) – are the acoustic waves that propagate along the boundary between two media, in layer with a typical thickness of 1.5 to 2 wavelength (1,5-2) ∙ λ. Fluctuations medium particles of the wave combine the longitudinal and transverse vibrations, the particles move along locked elliptical trajectories. The speed (C) of such waves the lowest, in steels roughly the works the relation:

С_п = 0,96∙С_t.

Fig. 1.11 – Diagram of the propagation of surface waves
In general, for any given material the following ratio is true :

C_l > C_t > C_п

2 Laws of propagation of acoustic waves
2.1 Acoustic Field
In ultrasonic testing, in most practical problems the law of the rectilinear propagation of sound waves is used. To describe the acoustic wave field the following concepts are applied: the front and the beam (Figure 2.1.). Beam waves – a straight line, along which the wave propagates, the wave front is the surface of all the points, which vary in same phase.

Fig. 2.1 – Acoustic wave field
In the process of wave propagation rays diverge, the front area 11 increases. Fig. 2.1 shows that the area of the front surface 2 is greater than 1. Since the front area increases, and, thus the amount of energy remains constant, therefore, the amplitude decreases. This is one of the basic mechanisms of reducing the amplitude of the wave associated with the divergence of rays.

The concepts of the front and beam is widely used for building objects testing schemes.
Fig. 2.2 shows diagram of sounding of a weld seam with angle beam transducer.
The central beam shows acoustic wave.
Control of the weld is carried out in two positions of transducer:
1 – Control the top of the seam with reflected from the bottom surface beam;
2 – Control of the bottom of the seam with straight beam.

Fig. 2.2 Weld seam sounding slanted piezoelectric transducer
Design and analysis of sounding scheme lets you to determine the location and orientation of the detected defects, the control zone, etc.
There are the following wave fronts: flat, cylindrical and spherical. Ultrasonic beams are parallel in a waves with flat front, the front area is not increasing, so the amplitude of the wave remains constant. In waves with cylindrical and spherical fronts beams diverge, and the front increases, so the amplitude of the wave decreases as the wave propagates. Waves in rods have flat front. Surface waves or waves in plates have cylinder front. Longitudinal and transverse waves in bulk samples have cylinder front.

The divergence of the wavelength out from the source leads to reduction of the amplitude of an echo-pulse reflected from defects. Thus, the farther from the transducer is the defect, the smaller the amplitude of the recorded flaw echo-pulse.