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-Transducer Characteristics

The sound field of an ultrasonic transducer operating at a single frequency is divided into a near field and a far field zone. The transition between these zones occurs at a distance Yo+ (or N) and is sometimes referred to as the "natural focus" of a flat ( of unfocused ) transducer. The near/far distance Yo+ is significant because amplitude variations that characterize the near field (and can make flaw evaluation difficult) change to a smoothly declining amplitude as the distance from the transducer increases.

The tables presented here for unfocused and focused ultrasonic transducers represents a range of typical transducers. An important column in the unfocused tables, Normal Range of Focusing, shows the normal range of focusing for immersion transducers in water for both spherical and cylindrical lenses. The Down Field Focal Zone column shows the start and end of the Yo+ maxima on the transducer axis as the distance from the transducer increases.

Spherical or cylindrical focusing changes the structure of the transducer field by "pulling" the Yo+ point nearer the transducer, to the focal point of the transducer. Again, the Down Field Focal Zone column shows the beginning and end of the focal zone, where the amplitude falls 6 dB below the maxima at the focal point.

It is important to note that the driving excitation normally used in NDT applications are either spike or rectangular pulsars, not single frequency. They can significantly alter the results shown in both tables. Nonetheless, the supporting analysis is widely used because it represents a reasonable approximation and a good starting point.

Unfocused
-

Frequency (MHz)

Diameter (in.)

Focal Length
(in)

6 dB Spot Size
(in)

START
(in)

 

END
(in)
Focal Gain SPHER.
(dB)
CYL.
(in)

1.00

1.00

2.50

.146

1.9

4.10

11.80

9.3

 

 

 

 

 

 

 

 

2.25

.500

1.50

.078

1.10

2.50

10.00

7.5

2.25

.750

2.00

.069

1.50

2.80

24.00

21.5

2.25

.750

3.00

.104

2.20

4.90

13.30

1.8

2.25

1.000

3.00

.078

2.30

3.90

27.90

25.4

2.25

1.000

5.00

.130

3.80

7.90

15.00

12.5

 

 

 

 

 

 

 

 

5.00

.250

.75

.035

.60

1.20

12.90

10.4

5.00

.375

1.00

.031

.80

1.30

26.50

24.0

5.00

.375

2.00

.062

1.50

3.50

8.30

5.8

5.00

.500

1.50

.035

1.20

1.90

30.20

27.7

5.00

.500

3.00

.070

2.20

4.90

12.90

1.4

5.00

.750

2.00

.031

1.60

2.20

39.20

36.7

5.00

.750

4.00

.062

3.10

5.40

40

24.0

5.00

.750

6.00

.094

4.60

9.40

16.20

13.7

5.00

1.000

3.00

.035

2.50

3.30

41.40

38.9

5.00

1.000

4.00

.047

3.20

4.60

37.50

35.0

5.00

1.000

6.00

.070

4.70

7.60

3.20

27.7

 

 

 

 

 

 

 

 

10.00

.125

.38

.018

.30

.60

12.90

10.4

10.00

.250

.75

.018

.60

1.00

30.20

27.7

10.00

.250

1.50

.035

1.10

2.40

12.90

10.4

10.00

.375

1.00

.016

.80

1.10

39.20

36.7

10.00

.375

2.00

.031

1.50

2.70

26.50

24.0

10.00

.375

3.00

.047

2.30

4.70

16.20

13.7

10.00

.500

1.50

.018

1.20

1.60

41.40

38.9

10.00

.500

3.00

.035

2.30

3.80

30.20

27.7

10.00

.500

5.00

.059

3.80

7.60

17.90

15.4

10.00

.750

3.00

.023

2.50

3.20

42.80

40.3

10.00

.750

4.00

.031

3.20

4.50

39.20

36.7

10.00

.750

6.00

.047

4.70

7.40

32.50

30.0

 

 

 

 

 

 

 

 

15.00

.125

.50

.016

.40

.80

16.20

13.7

15.00

.250

.75

.012

.60

.90

37.50

35.0

15.00

.250

1.50

.023

1.20

2.10

23.70

21.2

15.00

.375

1.50

.016

1.20

1.70

39.20

36.7

15.00

.375

2.50

.026

2.00

3.20

30.40

27.9

15.00

.375

4.00

.042

3.10

6.00

19.30

16.8

 

 

 

 

 

 

 

 

20.00

.125

.50

.012

.40

.70

23.70

21.2

20.00

.125

.75

.018

.60

1.20

12.90

10.4

20.00

.250

.75

.009

.60

.80

41.40

38.9

20.00

.250

1.50

.018

1.20

1.90

3.20

27.7

20.00

.250

3.00

.035

2.20

4.90

12.90

10.4

Focused Transducers

Freq. (MHz)

Dia. (in)

Diameter/ wave-length

 

Beam Spread (Degrees)

6dB SpotSize at Yo+
(in)

Near Field Yo+
(in)

Normal Range of Focusing
Min & /Max
(in)

Down Field Focal Zone
Start & End (6dB)
(in)

.50

.750

6.410

8.947

.188

1.202

Yo+

Yo+

.719

2.322

.50

1.000

8.547

6.707

.250

2.137

Yo+

Yo+

1.279

4.128

1.00

.750

12.821

4.470

.188

2.404

1.619

1.923

1.438

4.644

1.00

1.000

17.094

3.352

.250

4.274

2.158

3.419

2.557

8.255

2.25

.250

9.615

5.961

.063

.601

Yo+

Yo+

.360

1.161

2.25

.375

14.423

3.973

.094

1.352

.809

1.082

.809

2.612

2.25

.500

19.231

2.980

.125

2.404

1.079

1.9923

1.438

4.644

2.25

.750

28.846

1.986

.188

5.409

1.619

4.327

3.237

10.448

2.25

1.000

38.462

1.490

.250

9.615

2.158

7.692

5.754

18.574

5.00

.250

21.368

2.682

.063

1.335

.540

1.068

.799

2.580

5.00

.375

32.051

1.788

.094

3.005

.809

2.404

1.798

5.804

5.00

.500

42.735

1.341

.125

5.342

1.079

4.274

3.197

10.319

5.00

.750

64.103

.894

.188

12.019

1.619

9.615

7.192

23.218

5.00

1.000

85.470

.670

.250

21.368

2.158

17.094

12.786

41.276

10.00

.125

21.368

2.682

.031

.668

.270

.534

.400

1.290

10.00

.250

42.735

1.341

.063

2.671

.540

2.137

1.598

5.160

10.00

.375

64.103

.894

.094

6.010

.809

4.808

3596

11.609

10.00

.500

85.470

.670

.125

10.684

1.079

8.547

6.393

20.638

10.00

.750

128.205

.447

.188

24.038

1.619

19.231

14.385

46.436

10.00

1.000

170.940

.335

.250

42.735

2.158

34.188

25.573

82.553

15.00

.125

32.051

1.788

.031

1.002

.270

.801

.599

1.935

15.00

.250

64.103

.894

.063

4.006

.540

3.205

2.397

7.739

15.00

.375

96.154

.596

.094

9.014

.809

7.212

5.394

17.413

20.00

.125

42.735

1.341

.031

1.335

.270

1.068

.799

2.580

20.00

.250

85.470

.670

.063

5.342

.540

4.274

3.197

10.319

Data compiled by Xactex Corporation.  Sources of original data are unknown.

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