Overview
This lecture explains the seven parameters of sound waves in ultrasound: their definitions, units, symbols, formulas, relationships, and clinical relevance. The focus is on frequency, period, wavelength, propagation speed, amplitude, power, and intensity.
The Seven Parameters of Sound
- The seven measurable parameters of sound waves are: frequency, period, propagation speed, wavelength, amplitude, power, and intensity.
- Each parameter has specific units, symbols, and formulas.
- Some parameters can be adjusted by the sonographer, while others are determined by the machine or the medium.
Period and Frequency
- Period (T): The time required to complete one cycle.
- Units: seconds (s), milliseconds (ms), microseconds (μs); most commonly μs in ultrasound.
- Symbol: T
- Formula: Period = 1 / Frequency
- Frequency (f): The number of cycles per second.
- Units: hertz (Hz), kilohertz (kHz), megahertz (MHz)
- Symbol: f
- Formulas: Frequency = 1 / Period; Frequency = Propagation Speed / Wavelength
- Relationship: Period and frequency are reciprocals (Period × Frequency = 1). If period increases, frequency decreases, and vice versa.
- Both period and frequency are determined by the transducer and cannot be changed by the sonographer.
Audible Range and Ultrasound Frequency
- Human audible range: 20 Hz to 20 kHz.
- Infrasound: less than 20 Hz (not audible).
- Ultrasound: greater than 20 kHz (not audible); diagnostic ultrasound typically uses 1–17 MHz.
- Higher frequencies provide better resolution but less tissue penetration; lower frequencies penetrate deeper but with less detail.
Propagation Speed and Wavelength
- Propagation Speed (c): The speed at which sound travels through a medium.
- Units: meters/second (m/s), millimeters/microsecond (mm/μs)
- Symbol: c
- Average in soft tissue: 1540 m/s or 1.54 mm/μs
- Determined only by the medium; not affected by frequency.
- Directly related to stiffness (bulk modulus), inversely related to density.
- Wavelength (λ): The physical length of one cycle.
- Units: millimeters (mm), meters (m)
- Symbol: λ (lambda)
- Formula: Wavelength = Propagation Speed / Frequency (λ = c / f)
- In soft tissue: λ (mm) = 1.54 / Frequency (MHz)
- Wavelength is inversely related to frequency and directly related to propagation speed.
- Neither propagation speed nor wavelength can be adjusted by the sonographer; wavelength depends on both the transducer frequency and the medium.
Amplitude, Power, and Intensity (Strength Parameters)
- Amplitude (A): The difference between the average value and the maximum or minimum value of an acoustic variable (e.g., pressure, density, particle motion).
- Units: pascals (Pa), density (g/cm³), distance (mm), or decibels (dB)
- Symbol: A
- Amplitude is measured from the baseline to a peak or trough.
- Power (P): The rate at which energy is transferred.
- Units: watts (W), milliwatts (mW)
- Symbol: P
- Formula: Power ∝ Amplitude²
- Intensity (I): The concentration of energy in the sound beam.
- Units: watts per square centimeter (W/cm²)
- Symbol: I
- Formulas: Intensity = Power / Area; Intensity ∝ Amplitude²; Intensity ∝ Power
- Relationships:
- Amplitude, power, and intensity are all directly related. If amplitude changes by a factor, power and intensity change by that factor squared.
- Power and intensity are directly proportional to each other.
- Intensity is inversely related to the area over which the power is spread.
- All three strength parameters are determined by the machine and can be adjusted by the sonographer (via output power). They decrease as sound propagates through tissue (attenuation).
Key Terms & Definitions
- Period (T): Time for one cycle (s, ms, μs).
- Frequency (f): Cycles per second (Hz, kHz, MHz).
- Propagation Speed (c): Speed of sound in a medium (m/s, mm/μs).
- Wavelength (λ): Length of one cycle (mm, m).
- Amplitude (A): Maximum change from baseline (Pa, mm, dB).
- Power (P): Rate of energy transfer (W, mW).
- Intensity (I): Energy per area (W/cm²).
- Reciprocals: Two values whose product is 1 (e.g., period and frequency).
Action Items / Next Steps
- Complete workbook practice on period, frequency, amplitude, power, and intensity.
- Review and memorize all formulas, units, and relationships for each parameter.
- Practice unit conversions and understand how each parameter affects ultrasound imaging.