Sound Waves and Acoustic Properties

Sound Waves in Diagnostic Sonography

Introduction to Sound Waves

• In diagnostic sonography:
  • Sound pulses are created by a transducer.
  • These pulses travel through biologic tissue, referred to as the medium.
  • Reflections from boundaries between structures in the body return to the transducer and are processed into images.
• Two main effects are noted:
  • Acoustic Propagation Properties: Effects of the medium upon sound waves.
  • Biological Effects: Effects of sound waves upon the biologic tissue.

Characteristics of Sound Waves

• Sound is a mechanical wave that transfers energy.
  • Particle Movement: Particles in the medium vibrate back and forth from a fixed position.
• Sound consists of:
  • Compressions:
  • Areas of increased pressure and density.
  • Rarefactions:
  • Areas of decreased pressure and density.
• Additional properties of sound waves:
  • Type of Wave: Longitudinal waves.
  • Propagation Path: Sound travels in a straight line.
  • Medium Requirement: Sound requires a medium to travel (cannot propagate in a vacuum).

Types of Waves: Longitudinal vs. Transverse

• Transverse Waves:
  • Particle movement is perpendicular to the propagation path.
• Longitudinal Waves:
  • Particle movement is in the same direction as the wave propagation.
  • Sound waves are categorized as longitudinal waves.
• For illustration purposes, sound waves may be depicted as transverse waves.

Visual Examples

• Transverse Wave (Fig. 2.1):
  • Waves move from left to right (black arrow), while particles move up and down (red arrow).
• Longitudinal Wave (Fig. 2.2):
  • Waves move from left to right (black arrow), with particles moving back and forth in the same direction.

Key Concepts in Describing Sound Waves

• Acoustic Variables:
  • Sound waves are identified by their rhythmic oscillation in acoustic variables.
  • Three acoustic variables define sound waves:
  • Pressure:
    • Concentration of force in an area.
    • Unit of measurement: Pascal (Pa).
  • Density:
    • Concentration of mass in a volume.
    • Unit of measurement: g/cm³ (or kg/m³).
  • Distance:
    • Measure of particle motion.
    • Units include cm, mm, m, mile, foot, etc.

Acoustic Parameters

• After identifying a wave as a sound wave, its features can be described using seven acoustic parameters:
  1. Period
  2. Frequency
  3. Amplitude
  4. Power
  5. Intensity
  6. Wavelength
  7. Propagation Speed

Wave Interference

• In-Phase Waves:
  • A pair of waves is considered in-phase when their peaks occur at the same time and location.
• Out-of-Phase Waves:
  • Peaks occur at different times, leading to different interference effects.
• Interference:
  • When more than one sound beam arrives at the same location simultaneously, they combine to form a single wave.
  • Types of interference:
  • Constructive Interference:
    • Occurs with in-phase waves, enhancing the amplitude.
  • Destructive Interference:
    • Occurs with out-of-phase waves, potentially canceling each other out.

Visual Examples of Interference

• Constructive Interference (Fig. 2.5):
  • Demonstrated through in-phase waves enhancing amplitude.
• Destructive Interference (Fig. 2.6):
  • Illustrated with out-of-phase waves that may cancel each other out (Fig. 2.7).
• Alternative Interference Patterns (Fig. 2.8):
  • Waves with different frequencies result in a combination of constructive and destructive interference patterns.