Sound beams Flashcards Preview

Physics > Sound beams > Flashcards

Flashcards in Sound beams Deck (53):
0

What happens to the width of the beam as sound travels

it changes

1

At the starting point what size is the beam width as compared to the transducer diameter?

They are exactly the same

2

What are the five terms that describe the shape and regions of a sound beam?

focus

near zone

focal length or near zone length

far zone

focal zone

3

What is the focus or focal point?

the location where the beam is the narrowest

4

for a disc-shaped crystal, the width of the sound beam at the focus is ________ the width of the beam as it leaves the transducer.

one-half

5

What are all of the names for the near zone?

near zone

near field

fresnel zone

focal depth

focal length

near zone length (NZL)

6

What is the near zone?

the region from the transducer to the focus

7

what happens to the beam as it get closer to the focus?

the beam gradually narrows or converges

8

where is the focus located?

at the end of the near zone

9

What is the focal length?

the distance from the transducer to the focus

10

what is the far zone?

far field
Fraunhofer zone

the region that starts at the focus and extends deeper

within the far zone the beam diverges (spreads out)

11

How far does the beam have to be from the transducer to be the same size as the active element again?

two near zone lengths

12

what happens to the beam when it is farther than two near zone lengths from the active element?

the beam is wider than the active element

13

What is the focal zone?

a region around the focus where the beam is relatively narrow, resulting in superior image detail

14

what kind of images are reflected back from the focal zone?

more accurate than from other depths

15

How is the focal zone split up between near field and far field?

half in the near, half in the far

16

At these locations:
transducer, focus, 2 near zone lengths, deeper than 2 near zone lengths...
what is the beam diameter?

transducer = beam diameter equals transducer diameter

focus = 1/2 transducer diameter

2 near zone lengths = beam equals transducer diameter

deeper than 2 nzl's = beam is wider than transducer diameter

17

what is an adjustable focus system called?

phased array

18

with a fixed focus transducer, what two factors combined determine the focal depth?

transducer diameter

frequency of sound

19

How does transducer diameter affect the focal depth?

a larger diameter results in a deeper focus

20

what is the relationship between transducer diameter and focal depth?

they are directly related

21

how does frequency affect focal depth?

higher frequency sound results in a deeper focus

22

what is the relationship between frequency and focal depth?

directly related

23

so what do you need if you want a shallow focus?

smaller diameter PZT

lower frequency

24

what do you need if you want a deep focus?

larger diameter PZT

higher frequency

25

higher frequency sound creates a ________ focus

deeper

26

What is the equation to find focal depth?

focal depth (mm) = diameter (mm)2 x frequency (MHz) / 6

or

focal depth (mm) = diameter (mm)2 / 4x wavelength (mm)

27

what is the formula for wavelength?

wavelength = speed/frequency

28

what is aperture?

diameter of the PZT or single element

29

active element diameter and near zone length are ___________related

directly

30

frequency and near zone length are ________ related

directly

31

wavelength and near zone length are _________related

inversely

32

Where is the diameter of the sound beam the smallest

at the end of the near zone (the focus)

33

What is beam divergence?

the gradual spread of the ultrasound bean in the far field

34

what two factors combine to determine beam divergence?

transducer diameter

frequency of the sound

35

how does transducer diameter affect beam divergence in the far field?

smaller diameter crystals produce beams that spread out or diverge more in the deep far (Fraunhofer) zone.

36

crystal diameter and beam divergence are __________ related

inversely related

37

What happens to the divergence of a beam when it has larger diameter crystals?

it diverges less in the far field

38

since larger diameter crystals create sound beams that diverge less in the far field, larger diameter crystals do what to lateral resolution in the far field?

improve it!

39

how does frequency alter beam divergence in the far field?

lower frequency sound beams spread out or diverge more in the deep far (fraunhofer) zone.

40

how are frequency and beam divergence related

inversely

low frequency more divergence

41

if you want less divergence what do you need?

larger diameter

higher frequency

42

what do you need if you want more divergence

smaller diameter

lower frequency

43

what improves lateral resolution in the far field?

higher frequency sound because it diverges less in the far field.

44

Transducer frequency and beam divergence are ________ related

inversely

45

active element diameter and beam divergence are _______ related

inversely

46

what are spherical waves?

sound waves produced by very small sources (tiny pieces of PZT) diverge in the shape of a "V"

47

When is the V shaped wave created?

when the source is about the size of the sound's wavelength

48

What are the other names for V-waves?

spherical waves

diffraction patterns

Huygens' wavelets

49

What are huygens' sources

the small sources of sound producing these wavelets

50

What does Huygens' principal state?

a large active element may be thought of as millions of tiny, distinct sound sources.

each of these tiny particles is a Huygens' source and creates a Huygens' wavelet with a V-shape.

they interfere creating an hour-glass shaped beam

51

what produces the hourglass shape of the beam?

produced by a large crystal and is the result of interference of the many Huygens' sound wavelets emitted from these numerous sound sources

some are IN PHASE and interfere CONSTRUCTIVELY

52

When does destructive interference occur?

where the wavelets are OUT-OF-PHASE and the sound beam is CANCELLED