3: Sound Wave Form Flashcards
When does mode conversion happen?
When an incident beam strikes the interface at an angle other than 0 degrees.
What is mode conversion?
Splitting of the sound beam into different particle displacements (respective to propagation of the wave).
2 conditions required for mode conversion:
- Incident beam must hit an interface between materials of different acoustic properties.
- Incident angle must not be 0 degrees.
When mode conversion splits the reflected waves, the angle of reflection is different. Why?
They travel at different velocities.
Refraction only occurs when…
The wave crosses an interface.
We have reached the first critical angle when…
The incident angle has become large enough that the longitudinal wave moves to the surface of the part.
What is the first critical angle?
The minimum INCIDENT ANGLE required to produce 90 degree refraction angle of the longitudinal wave.
Why are lucite shoes generally cut at an angle that is past the first critical angle?
To avoid having two different waves in the material.
What is the second critical angle?
When we continue to increase the incident angle, and the shear wave moves to the surface.
When calculating the first critical angle, which velocity do you use?
Longitudinal
When calculating the second critical angle, which velocity do you use?
Shear
When calculating critical angles, what is the refracted angle?
90 degrees
What do theta3 and V3 represent in the formula?
Shear wave angle and velocity
What happens to the refracted waves when the incident angle is smaller than the first critical angle?
Longitudinal and shear refracted waves are present in the second medium.
What happens when the incident angle is between the first and second critical angle?
Only shear refracted waves are present in the second medium.
What happens when the incident angle is at or past the second critical angle?
Only surface waves are present.
What is the definition of a longitudinal wave?
The particles move in the same direction as the wave propagation.
What is a normal incidence wave?
A wave entering the material at 90 degrees to the front surface.
What is an oblique incidence wave?
When the wave enters the material at a slight angle.
What is meant by refraction?
Angular change in direction of ultrasonic beam as it passes between materials of different velocities.
What is meant by mode conversion?
A phenomenon where sound waves hit an interface at an angle other than 0 degrees and partially transform into a different mode of vibration.
What is the only condition required to produce reflection of sound at an interface?
A change of acoustic impedance between two materials
What are 3 uses of longitudinal waves?
Measure material thickness, detect discontinuities parallel to front surface and to detect lack of bond.
What is the most common use of shear waves?
To detect defects at an angle from front surface.
What is the name of the formula used to produce refraction of sound?
Snell’s Law.
