Compression Waves Propagation of Sound Sound propagates through solids, liquids and gases as one atom knocks against the next in a chain reaction. The first motion we will discuss is the compression wave. An example is Newton's cradle (Fig. 1) where energy is passed along the line of balls at such a high velocity that the swinging out of the end ball appears to be instantaneous. The speed (or velocity) with which the energy passes through these balls is approximately 6 kilometres per second, and can be considered an analogy to the compressive wave form of sound through solid metals. Fig. 1 The compression wave is the most efficient way of transmitting sound mechanically through any medium gases, liquids or solids. Moving in the line of the wave motion, each atom travels only a small distance before striking the next atom(s) and passing the energy on. This enables the waveform to travel at high speeds with minimal loss of energy. In their natural state, the atoms can be considered stationary and equally spaced (Fig. 2). Fig. 2
In ultrasonics the crystal strikes the atoms, and displaces them (Fig. 3). The atoms will take the path of least resistance as they move out of the way as quick as possible and will compress into their neighbours, passing on the vibrations at about 6 kilometres per second (KPS) in steel. This compression wave will radiate out from its point source in all directions, creating a wave hemispherical in shape when viewed from the surface. Shear Waves Fig. 3 Fig. 4 The elastic properties of the material will allow the atoms to vibrate, setting up a secondary wave form, known as shear wave, due to the shearing motion in relation to the direction of propagation (Fig. 4). In a solid, it is impossible to create a compression wave without a secondary shear wave form being generated, and it is likewise impossible to generate a shear wave without creating a compression wave first. Because the atoms are linked in all 3 dimensions in a solid, both the compression and shear waveforms will radiate in all directions with a hemispherical wave front.
Fig. 5 Figure 5 shows two spherical wave forms one compression (red) and one shear (blue) radiating out from the point of impact (the front top left hand corner). Lobes, Cones, and Other Myths Ultrasonic textbooks usually have diagrams such as the one below (Fig. 6) that show the UT beam firing straight out of the probe in a "flame" configuration. This type of diagram can be misleading and dangerous to the technician. First of all, sound radiates from its source as a hemisphere regardless of frequency. The "flame like" model makes it look like all the sound propagates in one direction. One wave motion cannot be generated without generating another. Transducer Sonic Lobes Fig. 6 Beam Spread Psychedelic pictures from the 70 s and 80 s showed interference patterns generated in a liquid or jelly that were very misleading. There are no such things as lobes. Beam spread is a complete misnomer; there is no conical effect of sound motion.
The angle and the receptive angle range of probes are down to the design and efficiency of the transducer and cannot be calculated by the beam spread formula. The above, though obvious, contradicts what technicians are taught. T R This style of diagram is often used by manufactures showing a beam of sound coming from the receiver transducer. Experiment to demonstrate the hemispherical principle To show that sound will radiate (propagate) hemi-spherically regardless of frequency, a simple demonstration can be done with two 0 probes. 1) The flaw detector is to be calibrated with a single 0 probe, with a test range of 100mm across full screen. 2) Connect the second 0 probe to the flaw detector, and set the control to dual or through transmission. 3) Place the transmitter probe (shown in blue) onto the IIW V1 (Fig. 7) at the side of the 100mm radius notch. 4) Place the receiver probe (shown in red) on the opposite side, directly underneath the transmitter. 5) The first signal on the screen will be a compression wave that will appear at 50mm on the time base. 6) Move the receiver probe to the 91mm land and the signal will appear from 45.5mm, increasing as the probe is slid along the 40mm long land to the corner of the land to the 100mm radius until the signal is at 50mm on the screen. Add 20 db to compensate for the loss of probe contact area. 7) Place the receiver probe on the 100mm radius bottom corner. The signal will be at 50mm on the screen. As the receiver is moved around the 100mm radius, the signal will remain at 50mm with minimal drop in amplitude, until 85, contrary to the "flame like" model.
Fig. 7 IIW V1 Calibration Block As the receiver probe is slid from the bottom corner around the 100mm radius, a second low amplitude signal will break off of the first 50mm signal. This is the echo of the first back wall echo compression wave, which travels across the screen, increasing in distance as the receiver probe moves towards the top of the 100mm radius. To confirm that the second signal on the screen is the shear wave Repeat the above steps 1, 2, 3, 4, and 5. Then: 1) With the first signal at 50mm on the screen, move the gate to the second signal on the time base. 2) Increase the gain until the signal is 80% screen height. 3) Adjust the velocity until the sound path distance digital display shows 50mm. 4) Note that the velocity of this sound wave is that of a shear wave. The experiment can now be repeated with shear waves to show that this waveform is also hemispherical.