Textbook Sound Waves and Interference Notes

Sound in Diagnostic Ultrasonography

• Sound pulses are created by a transducer and travel through biologic tissue.
• Reflections from boundaries between structures return to the transducer and are processed into images.

Sound Waves

• Waves carry energy from one location to another.
• Sound is a mechanical wave where particles in the medium move.
• Molecules in the medium vibrate back and forth from a fixed position.
• Sound cannot travel through a vacuum; it requires a medium.
• In a medium, molecules are compressed (squeezed) and rarefied (stretched).
• Sound travels in a straight line.
• Sound waves are longitudinal waves.

Acoustic Propagation & Biologic Effects

• Acoustic propagation properties: Effects of the medium on the sound wave.
• Biologic effects: Effects of the sound wave on the biologic tissue.

In-Phase and Out-of-Phase Waves

• In-Phase Waves: Peaks (maximum values) occur at the same time and location; Troughs (minimum values) also occur at the same time and location.
• In-phase waves are "in step."
• Out-of-Phase Waves: Peaks occur at different times, as do troughs.
• Out-of-phase waves are "out of step."

Interference

• Multiple sound beams may travel in a medium and arrive at the same location simultaneously.
• Waves combine to form a single wave, known as interference.
• Both in-phase and out-of-phase waves undergo interference but combine differently.

Acoustic Variables

• Sound waves are identified by oscillations in acoustic variables.
• Three acoustic variables: pressure, density, and distance.
• If pressure, density, or distance rhythmically oscillates, it's a sound wave.
• Sound waves are also known as acoustic waves.

Acoustic Variables

• Pressure: Concentration of force in an area.
  • Units: Pascals (Pa)
• Density: Concentration of mass in a volume.
  • Units: kg/cm^3
• Distance: Measure of particle motion.
  • Units: cm, feet, mile.

Acoustic Parameters

• Acoustic parameters describe the characteristics of a sound wave.
• Seven acoustic parameters are used.

Seven Acoustic Parameters

• Period
• Frequency
• Amplitude
• Power
• Intensity
• Wavelength
• Propagation speed

Constructive Interference

• Interference of in-phase waves results in a single wave of greater amplitude.
• The resulting wave is larger than either of its components.

Destructive Interference

• Interference of out-of-phase waves results in a single wave of lesser amplitude.
• The resultant wave is smaller than at least one of its components.
• Complete destructive interference occurs when two out-of-phase waves are of equal amplitude.

Interference of Waves with Different Frequencies

• When waves of different frequencies interfere:
  • At some moments, the waves are in phase, leading to constructive interference.
  • At other moments, the waves are out of phase, leading to destructive interference.
• Both constructive and destructive interference occur.