Ch.3 Describing Sound waves powerpoint Notes

Describing Sound Waves

Parameters of Sound

• Parameters describe the feature or attributes of the sound wave.
• There are seven parameters used.
• Each parameter has appropriate units.
• Units give context.

Parameter Categories

• System
• User Controls
• Medium
• Sound source/ medium
  • Period
  • Amplitude
  • Speed
  • Wavelength
  • Frequency
  • Power
  • Intensity

Sound Source Parameters

Period

• Time from the start of one cycle to the start of the next.
• Units of time: 0.1 to 0.5 microseconds
• Determined by the sound source.

Frequency

• Number of cycles during a time frame.
• Cycles per second (Hertz).
• 1 cycle per second = 1 Hz
• Ultrasound is greater than 20 kHz.

How Period and Frequency Relate

• Period and frequency are inversely related (high frequency = low period).
• Reciprocal Relationship: (period * frequency = 1)

Frequency Spectrum of Sound

• Infrasound: Less than 20Hz
• Audible sound (human): 20Hz - 20kHz
• Ultrasound: Greater than 20kHz

Bigness Parameters - Amplitude

• Amplitude: Difference between max/min and baseline.
• Expressed in pascals, g/cm3, distance.
• 1 MPa - 3 MPa
• May be adjusted by the user (gain and TGC).
• Key points:
  • Max or peak
  • Minimum (trough)
  • Baseline (mean)
  • Peak to peak amplitude

Bigness Parameters - Power

• Power (sound source): Rate of energy transfer or rate at which work is performed.
• Watts (4-90 milliwatts).
• Directly related to amplitude (power is proportional to amplitude squared).
• Power or gain.

Bigness Parameters - Intensity

• Intensity: Concentration of energy in the sound beam.
• Divide beam’s power by beam’s cross-sectional area.
• Watts per area: 0.01 to 300 W/cm^2
• Intensity is proportional to power.
• Intensity is proportional to amplitude squared.

Wavelength

• Distance of one complete cycle.
• Units of length (0.15 - 0.8 mm).
• Determined by the sound source and medium; cannot be changed by the sonographer.
• Inversely related to frequency.

Propagation Speed

• Distance the sound wave travels through a medium in one second.
• Meters per second (distance/ per time).
• Depends on the tissue it is traveling through.
• Not adjustable.
• Speed of sound in soft tissue: 1540 m/s.

Acoustic Propagation Properties

• Describe the effects of the medium on a sound wave.
• Pressure: Force per area.
• Density: Concentration of mass in a volume.
• Stiffness: Ability of an object to resist compression.

Stiffness vs. Density

Stiffness

• Stiffness and speed are directly related.
• As stiffness increases, speed increases.
• Opposite of elastic or compressible.

Density

• Relative weight of medium.
• Density and speed are inversely related.