Conventional UT

Ultrasonic field

 

Ultrasonic field

 

Transducer

Parameters:

D = Diameter, f = frequency V = velocity, λ=wavelength

Near field

z<No

(Circular Transducer)

 

The field intensity is irregular and the beam width is smaller than the transducer diameter.

Near field

z<No

(Rectangular Transducer)

where “a” is the shorter size of the transducer and “b” the largest size of the transducer

Beam spread

The beam spread can be reduced by selecting a transducer with a higher frequency, a larger element diameter or both

For flat transducers, the pulse-echo beam spread angle is given by:

where:  α/2 = Half angle spread.

”k” =  constant value which depends on where the beam edge is defined

“k”  = 0.51 gives the half beam width at -6dB drop in pulse-echo mode.

 

“k”  for Transmission mode

Drop % dB

Circular transducer

Rectangular transducer

10% (20 dB)

1.08

0.60

50% (6 dB)

0.54

0.91

 

“k” value for Pulse-echo

10% (20 dB)

0.87

0.74

50% (6 dB)

0.51

0.44

 


 

Focused sound fields

The beam width can be reduced by focusing in the near-field zone using a lens

 

  => zfoco: actual focal depth

 

 

The focus position (zfoco) for a given lens radio is:

 

                                               => VM: means de sound velocity in the specimen

                                               => VL: sound velocity in the lens material

                                               => R: lens curvature radius

Focusing factor

A focused beam is characterized by:

A focused beam can be classified by  Sac as:

 

0.1   ≤ Sac ≤ 0.33 => strong focusing.

0.33 ≤ Sac ≤ 0.67 => medium focusing.

0.67 ≤ Sac ≤ 1.0 => weak focusing.

 

Most of the industrial applications use: Sac < 0.6

Focusing Depth

The formula is only valid for Sac < 0.6

Focused beam diameter

The beam diameter in mm at -6dB drop

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