Radiologic & Imaging Physics: Acoustic Parameters of Sound Waves

Radiologic & Imaging Physics: Acoustic Parameters of Sound Waves

Seven Acoustic Parameters of Sound Waves

1. Frequency (f)

• Definition: Frequency refers to the number of cycles in a sound wave per second.

  • Unit: Measured in hertz (Hz), articulated as:

  • 1 Hz = 1 cycle per second.

• Example: A 3 MHz transducer emits a frequency of 3 million cycles per second.

• Determined by:

  • The Sound Source (Transducer)

• Relationships:

  • Frequency and Period: Inversely related (as one increases, the other decreases).

  • Frequency and Wavelength: Inversely related.

• Effects:

  • Beam Penetration:

  • Frequency and Penetration: Inversely related (higher frequency = lower penetration).

  • Axial Resolution:

  • Frequency and Resolution: Directly related (higher frequency = better resolution).

  • Note: Not changed by the sonographer.

2. Period (T)

• Definition: The period is the amount of time taken to complete one cycle of the sound wave.

  • Unit: Any measure of time, typically seconds (s) or microseconds (µs).

• Determined by:

  • The Sound Source (Transducer).

• Relationship:

  • Inversely related to frequency (as frequency increases, period decreases).

• Additional Info:

  • A complete compression and rarefaction of the wave is referred to as a cycle (one period).

3. Wavelength (λ)

• Definition: The wavelength is the distance traveled by sound in one cycle, or the distance between two identical points in the wave cycle.

  • Unit: Measured in any unit of length, typically millimeters (mm).

• Determined by:

  • The Sound Source (Transducer) and the Medium.

• Relationships:

  • Wavelength and Frequency: Inversely related (higher frequency = shorter wavelength, lower frequency = longer wavelength).

• Effects:

  • Beam Penetration:

  • Wavelength and Penetration: Directly related (short wavelength = low penetration).

  • Axial Resolution:

  • Wavelength and Resolution: Inversely related (short wavelength = higher resolution).

  • Note: Not changed by the sonographer.

4. Propagation Speed (v)

• Definition: Propagation speed is the rate at which sound waves travel through a medium.

  • Unit: Typically expressed as distance divided by time, such as meters per second (m/s) or millimeters per microsecond (mm/µs).

• Determined by:

  • The Medium.

• Factors Influencing Speed:

  • Stiffness of the Medium:

  • Speed is directly related to stiffness (increased stiffness = faster speed).

  • Stiffness is inversely related to compressibility/elasticity.

  • Density of the Medium:

  • Speed is inversely related to density (increased density = slower speed).

• Note: Speed is not changed by the sonographer, and stiffness has a greater influence on speed.

5. Amplitude (A)

• Definition: Amplitude refers to the difference between the average value and the maximum or minimum value of the wave, indicating the size or magnitude of the wave.

  • Unit: Usually expressed in decibels (dB).

• Determined by:

  • The Sound Source.

• Relationships:

  • Amplitude and Power: Directly related.

  • Amplitude and Intensity: Directly related.

• Characteristics:

  • Amplitude decreases as sound travels through a medium; the rate of decrease depends on both the wave and the medium characteristics.

  • Measured as:

  • Baseline to Peak (either positive or negative portion of sine wave).

  • Peak to Peak (half the peak to peak amplitude).

• Changeability: Amplitude can be changed by the sonographer.

6. Power (P)

• Definition: Power is the rate at which energy is transferred or the rate at which work is performed.

  • Unit: Expressed in Watts (W).

• Determined by:

  • The Sound Source.

• Relationships:

  • Power and Amplitude: Directly related.

  • Power and Intensity: Directly related.

• Characteristics:

  • Power decreases as it travels through a medium; the rate of power decrease is dependent on wave characteristics and medium.

• Changeability: Power can be changed by the sonographer.

7. Intensity (I)

• Definition: Intensity is defined as the concentration of energy within the sound wave over a specified area.

  • Unit: Expressed in watts per square centimeter (W/cm²), calculated as power divided by beam area.

• Determined by:

  • The Sound Source (Transducer Diameter).

• Relationships:

  • Intensity is greatest where the beam is narrowest.

  • Intensity and Beam Width: Inversely related.

  • Intensity and Power: Directly related.

  • Intensity and Amplitude: Directly related.

• Characteristics:

  • Intensity decreases as it travels through a medium; the rate of intensity decrease is influenced by the characteristics of both the wave and the medium.

• Changeability: Intensity can be changed by the sonographer.