Textbook Sounds notes (Ch. 2)

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.

Review questions:

1. How is diagnostic ultrasound preformed? A transducer emits sound pulses and reflects are processed into images.

2. What is sound? A mechanical wave in which particles in the medium move/vibrates

3. Can sound travel through a vacuum? No it must travel through a medium

4. What type of waves are sounds? Longitudinal waves

5. What are acoustic propagation properties? The effects of the medium upon the sound wave

6. What are biological effects in ultrasound? Effects of sound waves on biological tissue.

7. When  are waves considered in-phase? When their peaks occur at the same time and location

8. When are waves considered out-of phase? Their peaks occur at different times.

9. What are the three acoustic variables? Pressure, density, and distance.

10. Name the seven acoustic parameters. Period, frequency, amplitude, power, intensity, wavelength, and propagation speed.