Attenuation (4)

Question 1

Define Attenuation

Answer 1

The loss of energy of a sound beam, from the time it is sent to the time it is received

Question 2

Causes of Attenuation

Answer 2

• Absorption
• Scattering
• Beam Divergence
• Coupling Losses
• Geometrical Losses

Question 3

Absorptions impact on Attenuation

Answer 3

Particles vibrate, causing friction, transforming mechanical energy into heat.

• Dependant on elasticity and density of material
• heavier particles don’t vibrate as easy and increase attenuation
• crystalline defects increase attenuation

Question 4

Define Dislocations

Answer 4

Crystalline defects

- defects in a crystalline structure

Question 5

Scatterings impact on Attenuation

Answer 5

When material contains reflections, large in relation to half the wavelength (porosity, precipitates, grain boundary) causing sound to be reflected into all directions instead of returning directly to the transducer.

• reduces back wall signal
• large grain structure can be seen as an interface if not oriented correctly (long waves better than shear)
• causes grass/hash
• can be reduced by lowering frequency (increase wavelength) but also will decrease sensitivity (not recommended)

Question 6

Beam Divergence’s impact on Attenuation

Answer 6

When any part of the sound beam hits an interface at an angle other than zero = no reflection back to transducer

• larger transducer and/or higher frequency will reduce beam spread

Question 7

Coupling Losses impact on Attenuation

Answer 7

From any two materials that have an acoustic mismatch.

reflection is greater than transmission

• includes rough surfaces

Question 8

Define Dispersion

Answer 8

When the angle at the entry surface causes, scattering, refraction and mode conversion.

• loss of backwall amplitude
• loss of reflector amplitude
• unwanted generation of surface waves

Question 9

Rough surfaces impact on Attenuation

Answer 9

• dispersion
• widen initial pulse
• coupling losses
• rough backwall will cause a wide and rough backwall indication

Question 10

Geometrical Losses impact on attenuation

Answer 10

• causes reflection, refraction and mode conversion.
• produce non-relevant indications
• decrease backwall height
• widen initial pulse

Question 11

Non-Parallel Surfaces impact on Attenuation

Answer 11

Front and back surfaces aren’t parallel, sound beam will hit the surface at an angle other than 0°.

• reflection several times until the energy dies out
• mode conversion (shear waves)

Includes:

• Blindholes
• Convex surfaces
• Concave surfaces
• Long and thin specimens