Sound Beams - Chapter 9

Question 1

what is the basic rule when it comes to sound beams?

Answer 1

narrow beams create better images

Question 2

how is beam width affected as sound travels?

Answer 2

the width of the beam changes: starts at same size as transducer diameter/ aperture; gets narrower until it reaches its smallest diameter; beam diverges

Question 3

where is the focus/focal point found on the beam?

Answer 3

location where the beam reaches its minimum diameter

Question 4

what is the focal depth?

Answer 4

the distance from the transducer to the focus aka focal length or near zone length

Question 5

what is the near zone? aka?

Answer 5

the region or zone in bw the transducer and the focus. sound beams converge here. Fresnel zone

Question 6

what is the far zone? aka?

Answer 6

the region or zone deeper than the focus, beyond the near zone. sound beams diverge here. Franhofer zone

Question 7

what is the focal zone?

Answer 7

the region surrounding the focus where the beam is kinda narrow. Pic is generally good within this zone.

Question 8

Beam diameter changes similar to beam width, what is the diameter of the beam at the focal point?

Answer 8

1/2 of transducer aperture

Question 9

what is focal depth determined by?

Answer 9

1. transducer diameter/aperture
2. frequency of the US

Question 10

what 2 factors result in a shallow focus?

Answer 10

small diameter; low frequency

Question 11

what 2 factors result in a deep focus?

Answer 11

large diameter; high frequency

Question 12

how do beams with a shallow or deep focus differ?

Answer 12

beams with a deep focus have a lower intensity at the focus

Question 13

what does sound beam divergence mean?

Answer 13

describes the spread of the sound beam in the deep far zone.

Question 14

give 4 characteristics of a beam with less divergence

Answer 14

1. larger aperture or larger diameter active element
2. high frequency
3. narrower beam in far field
4. improved lateral resolution in the far field

Question 15

give 4 characteristics of a beam with more divergence

Answer 15

1. smaller aperture or smaller diameter active element
2. low frequency
3. wider beam in far field
4. degraded lateral resolution in the far field

Question 16

what determines frequency in continuous waves? what about pulsed waves?

Answer 16

electronic frequency; thickness of ceramic and speed of sound in ceramic

Question 17

what determines focal length and divergence?

Answer 17

aperture of active element and frequency of sound

Question 18

what is a diffraction pattern? How does it happen? what would happen if sound waves produced by transducers acted in this manner?

Answer 18

V-shaped waves aka Hugens’ wavelet; When produced by a tiny source, with a size near the wavelength of the sound, waves will diverge in this shape as they propagate; the wave would spread broadly as it travels.

Question 19

what is Huygens’ principle?

Answer 19

explains the hourglass shape of an imaging transducer’s sound beam

Question 20

how do these V-shaped waves explain the hourglass shape of the transducer’s sound beam?

Answer 20

each tiny part of the surface of transducer face can be considered an individual sound source, which create a Huygens’ wavelet. The hourglass shape is created as a result of constructive and destructive interference of the many sound wavelets emitted from the “multiple sound sources”