Sound Beams - Chapter 9 Flashcards
what is the basic rule when it comes to sound beams?
narrow beams create better images
how is beam width affected as sound travels?
the width of the beam changes: starts at same size as transducer diameter/ aperture; gets narrower until it reaches its smallest diameter; beam diverges
where is the focus/focal point found on the beam?
location where the beam reaches its minimum diameter
what is the focal depth?
the distance from the transducer to the focus aka focal length or near zone length
what is the near zone? aka?
the region or zone in bw the transducer and the focus. sound beams converge here. Fresnel zone
what is the far zone? aka?
the region or zone deeper than the focus, beyond the near zone. sound beams diverge here. Franhofer zone
what is the focal zone?
the region surrounding the focus where the beam is kinda narrow. Pic is generally good within this zone.
Beam diameter changes similar to beam width, what is the diameter of the beam at the focal point?
1/2 of transducer aperture
what is focal depth determined by?
- transducer diameter/aperture
- frequency of the US
what 2 factors result in a shallow focus?
small diameter; low frequency
what 2 factors result in a deep focus?
large diameter; high frequency
how do beams with a shallow or deep focus differ?
beams with a deep focus have a lower intensity at the focus
what does sound beam divergence mean?
describes the spread of the sound beam in the deep far zone.
give 4 characteristics of a beam with less divergence
- larger aperture or larger diameter active element
- high frequency
- narrower beam in far field
- improved lateral resolution in the far field
give 4 characteristics of a beam with more divergence
- smaller aperture or smaller diameter active element
- low frequency
- wider beam in far field
- degraded lateral resolution in the far field
what determines frequency in continuous waves? what about pulsed waves?
electronic frequency; thickness of ceramic and speed of sound in ceramic
what determines focal length and divergence?
aperture of active element and frequency of sound
what is a diffraction pattern? How does it happen? what would happen if sound waves produced by transducers acted in this manner?
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.
what is Huygens’ principle?
explains the hourglass shape of an imaging transducer’s sound beam
how do these V-shaped waves explain the hourglass shape of the transducer’s sound beam?
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”
