Ch 9 Sound Beams

at the starting point, the beam width is exactly the same as the transducers

diameter

transducer diameter, AKA

aperture

a sound beam narrows progressively until it reaches the smallest diameter and then it

diverges

there are ____ terms to describe the anatomy of a sound beam

5

- focus

- near zone

- focal length

- far zone

- focal zone

transducer anatomy terms

the location where the beam is the narrowest

focus

for a disc shaped crystal, the width of the beam at the focus is _______ the width of the beam leaving the transducer

1/2

AKA near field or fresnel zone

near zone

the region from the transducer to the focus

near zone

the beam gradually narrows, or ______ within the near zone

converges

in the near zone, the diameter as the sound beam leaves the transducer is _______ as the diameter of the active element

the same

at the end of the near zone, the beam narrows to _____ the width of the active element

1/2

the ______ is at the end of the near zone

focus

AKA focal depth or near zone length

focal length

the distance from the transducer to the focus

focal length

AKA far field or Fraunhofer zone

far zone

the region that starts at the focus and extends deeper

far zone

at the beginning of the far zone, the beam is only _____ as wide as the transducer

1/2

when the beam is ____ near zone lengths from the transducer, it is again the same size as the active element

2

at depths more than 2 near zone lengths, the beam is _____ that the active element

wider

the region around the focus where the beam is relatively narrow

focal zone

reflections arising from the _____ create images that are more accurate than reflections from other depths

focal zone

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

focal depth

___ factors combine to determine focal depth

2

1. transducer diameter

2. frequency of sound

factors that determine focal depth

transducer diameter and focal depth are ______ related

directly

a smaller diameter will have a ________ focus

shallower

frequency and focal depth are _____ related

directly

a lower frequency will have a _______ focus

shallower

_______ frequency creates a deeper focus

higher

focal depth =

diameter (mm)^2 x frequency (MHz) / 6

the gradual spread of the US beam in the far field

beam divergence

___ factors combine to determine beam divergence

2

1. transducer diameter

2. frequency of sound

(same as focal depth)

factors that determine beam divergence

crystal diameter and beam divergence are _______ related

inversely

a smaller diameter will have _____ divergence in the far field

increased

frequency and beam divergence are ________ related

inversely

lower frequency will have _____ divergence

increased

sound waves produced by small sources diverge in the shape of a

V

a V shaped wave is created when the sound source is about the size of the sounds

wavelength

- diffraction patterns

- Huygens waves

AKA for spherical waves

US transducers with large PZT crystals create sound beams shaped like an

hourglass

states that 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 wavelet with a V shape

Huygen's principle

Huygens wavelets interfere both __________ and __________

constructively , destructively