Mode Conversion (3)

Question 1

What is Mode Conversion?

Answer 1

when an incident beam strikes an interface at an angle other than 0°

splitting the sound beam into different particle displacements (different angles within the material), a beam of longitudinal waves and a beam of shear waves
- both new waves travel at different velocities

Question 2

What happens when a longitudinal wave strikes a surface at an angle other than 0°?

Answer 2

splits into two reflected waves; reflected longitudinal (2.67km/s) = to the angle of incidence, reflected shear (1.12km/s) calculated using Snell’s Law.

Question 3

What happens when a longitudinal wave enters a medium with a back-wall angle other than 0°

Answer 3

splits into two reflected waves; reflected longitudinal and reflected shear
(loss of back surface signal and longitudinal wave energy)

Question 4

Magic 61° angle

Answer 4

angle of slope in steel is 61°
angle in aluminium is 64°

splits into two reflected waves; reflected longitudinal and reflected shear, the 61° causes the shear wave to travel 90° parallel to the surface (at 3.24km/s), when it hits the wall it returns via 90° re-hits the slop and becomes a longitudinal wave moving straight up to the transducer (at 5.94km/s).

Question 5

Mode Conversion of Refraction

Answer 5

splits into two waves; refracted longitudinal and refracted shear.

Calculated via Snell’s Law

Question 6

First Critical Angle

Answer 6

Refracted longitudinal sound beam.

increase the incident angle the longitudinal refracted wave will be 90° with the material surface , leaving only the shear wave in the material

Question 7

Second Critical angle

Answer 7

Past the First Critical Angle continuing to increase the incident beam angle causing the refracted shear wave to be parallel with the material surface (no waves in material, only surface waves)

Question 8

Lucite shoes are angled to create which critical angle?

Answer 8

First Critical Angle, leaving only a shear refracted wave in the material.