Ch.9 Sound Beams

Question 1

What are the 5 areas of a sound beam?

Answer 1

1) Focus
2) Near zone
3) Focal length or near zone length
4) Far zone
5) focal zone

Question 2

What is the focus?

Answer 2

is the location where the beam is the narrowest.

Question 3

For a disc-shaped crystal, the width of the sound beam at the focus is…

Answer 3

1/2 the width of the beam as it leaves the transducer

Question 4

What is the near zone?

Answer 4

(Near field or Fresnel zone)
is the region from the transducer to the focus

Question 5

The beam gradually narrows, or converges, within the ___ ____

Answer 5

near zone

Question 6

Where is the focus or focal point located?

Answer 6

1) at the end of the near zone
2) at the beginning of the far zone
3) the middle of the focal zone

Question 7

What is the focal length?

Answer 7

(focal depth or near zone length)
is the distance from the transducer to the focus

Question 8

What is the far zone?

Answer 8

(far field or Fraunhofer zone)
is the region that starts at the focus and extends deeper
-within the far zone, the beam diverges, or spreads out.

Question 9

When the beam is two near zone lengths from the transducer, the beam is ?

Answer 9

again the same size as the active element

Question 10

At depths more than two near zone lengths, the beam is ?

Answer 10

wider than the active element

Question 11

what is the focal zone?

Answer 11

is a region around the focus where the beam is relatively narrow. Half is located in the near field and the other half is in the far field.

Question 12

Reflections arising from the _____ _____ create images that are more _____ than those from other ____ .

Answer 12

focal zone
accurate
depths

Question 13

Location Beam diameter
1) at transducer
2) at 2 near zone lengths
3) at the focus
4) deeper than 2 near zone lengths

Answer 13

1) beam diameter = transducer diameter
2) beam diameter = transducer diameter
3) beam diameter is 1/2 transducer diameter
4) beam diameter is wider than transducer diameter

Question 14

The focal depth

Answer 14

is the distance from the transducer to the narrowest part of the beam (the focus).

Question 15

Ultrasound systems that adjust the focus location are called what?

Answer 15

phased array

Question 16

The characteristics of what determine the depth of the focus?

Answer 16

active element

Question 17

What characteristics of a fixed focus transducer determine the focal depth?

Answer 17

1) Transducer diameter
2) Frequency of the sound

Question 18

How does the transducer diameter affect the focal depth?

Answer 18

a larger diameter results in a deeper focus

Question 19

What is the relationship between the transducer diameter and focal depth ?

Answer 19

directly related

Question 20

How does frequency affect focal depth?

Answer 20

higher frequency sound results in a deeper focus

Question 21

What is the relationship between frequency and focal depth ?

Answer 21

are directly related

Question 22

What are the two characteristics of shallow focus?

Answer 22

smaller diameter PZT
lower frequency

Question 23

What are the two characteristics of deep focus?

Answer 23

larger diameter PZT
higher frequency

Question 24

PHYSICS: Higher frequency sound creates a

Answer 24

deeper focus

Question 25

How do transducer manufacturers overcome the rule that higher frequencies create deeper focus?

Answer 25

by making very small diameter, high frequency crystals

Question 26

Focal depth formula

Answer 26

focal depth (mm) = diameter (mm)^2 x frequency (MHz) / 6
or
focal depth (mm) = diameter (mm)^2 / 4 x wavelength (mm)

Question 27

what characteristics of a transducer determine the spread of the beam in the far field?

Answer 27

1) transducer diameter
2) frequency of sound

Question 28

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

Answer 28

crystal diameter and beam divergence are inversely related
(small crystals produce beams that spread out or diverge more in the deep far zone (Fraunhofer) )

Question 29

Larger diameter crystals create sound beams that diverge ____ in the far field, larger diameter crystals improve ______ _______ in the far field.

Answer 29

less
lateral resolution

Question 30

How does frequency alter beam divergence in the far field?

Answer 30

frequency and beam divergence are inversely related

Question 31

High frequency sound beams diverge ____ in the far field, higher frequency sound improves ____ ______ in the far field.

Answer 31

less
lateral resolution

Question 32

Less Divergence in far field have 2 characteristics?

Answer 32

1) larger diameter
2) higher frequency

Question 33

More divergence in far field have 2 characteristics?

Answer 33

1) smaller diameter
2) lower frequency

Question 34

Larger transducer diameters produce a beam with ____ ______

Answer 34

less diverge

Question 35

Sound waves produced by very small sources (tiny pieces of PZT) diverge in the shape of a ?

Answer 35

V

Question 36

The V shaped waves are known as ?

Answer 36

spherical waves
diffraction patterns
Huygen’s wavelets

Question 37

Huygens’ Principle

Answer 37

states that a large active element may be thought of as millions of tiny, distinct sound sources. Each tiny particles is a Huygens’ source and creates a Huygens’ wavelet with a V shape