Overview
This lecture covers the seven essential parameters of sound waves—key concepts for understanding ultrasound physics—including definitions, units, formulas, relationships, and their clinical significance.
The Seven Parameters of Sound
- The seven sound parameters are: frequency, period, propagation speed, wavelength, amplitude, power, and intensity.
- These parameters define measurable aspects of sound waves and are essential for ultrasound.
Frequency and Period
- Frequency is the number of cycles per second (Hz, kHz, MHz); in ultrasound, typically 1–17 MHz.
- Period is the time taken to complete one cycle, usually measured in microseconds.
- Frequency and period are reciprocals: frequency = 1/period, period = 1/frequency.
- Both are determined by the transducer and cannot be adjusted by the sonographer.
- Audible sound ranges from 20 Hz to 20 kHz; ultrasound is above 20 kHz.
Propagation Speed and Wavelength
- Propagation speed is how fast sound travels through a medium (measured in m/s or mm/μs); average in soft tissue is 1540 m/s.
- Propagation speed depends only on the medium’s stiffness (direct) and density (inverse).
- Wavelength is the distance a cycle occupies (measured in mm); λ = c/f.
- Wavelength is inversely related to frequency and directly related to propagation speed.
- Wavelength in soft tissue: λ = 1.54 mm/μs ÷ frequency (MHz).
Strength Parameters: Amplitude, Power, Intensity
- Amplitude: difference from baseline to peak (commonly in pressure, e.g., megapascals); amplitude = peak minus baseline.
- Power: rate of energy transfer (watts or milliwatts); directly related to amplitude squared.
- Intensity: power per unit area (watts/cm²); intensity = power/area.
- All three are directly related; determined by the machine, adjustable by the sonographer, but decrease with propagation.
- Changes in amplitude cause squared changes in both power and intensity.
Key Terms & Definitions
- Cycle — One complete oscillation in a wave.
- Frequency (f) — Cycles per second, unit: Hz, MHz.
- Period (T) — Time for one cycle, unit: seconds, μs.
- Propagation speed (c) — Speed of sound in a medium, unit: m/s.
- Wavelength (λ) — Length of one cycle, unit: mm.
- Amplitude (A) — Maximum value minus average value of the wave variable.
- Power (P) — Rate of energy transfer, unit: watts.
- Intensity (I) — Power per area, unit: W/cm².
Action Items / Next Steps
- Complete period and frequency practice problems in your workbook.
- Fill in charts for missing sound parameter values.
- Review charts for amplitude, power, and intensity changes.
- Work through the "nerd check" knowledge review for all seven parameters.