Overview
This lecture covers the seven parameters of sound waves—frequency, period, propagation speed, wavelength, amplitude, power, and intensity—key for understanding ultrasound physics. It explains each parameter's definition, units, relationships, and relevance to sonography.
The Seven Parameters of Sound
- The seven parameters are: period, frequency, propagation speed, wavelength, amplitude, power, and intensity.
- These characteristics define and differentiate sound waves used in ultrasound.
Period & Frequency (Section 3.1)
- Period: Time to complete one cycle; units: microseconds (μs); symbol: T.
- Frequency: Number of cycles per second; units: hertz (Hz), kilohertz (kHz), megahertz (MHz); symbol: f.
- Period and frequency are reciprocals (Period × Frequency = 1).
- If period increases, frequency decreases and vice versa.
- Both are set by the machine/transducer and cannot be adjusted by the sonographer.
- Audible sound for humans: 20 Hz to 20 kHz; ultrasound: >20 kHz, usually 1–17 MHz in clinical imaging.
- Use unit pairs: seconds & Hz, milliseconds & kHz, microseconds & MHz for calculations.
Propagation Speed & Wavelength (Section 3.2)
- Propagation speed: Speed of sound through a medium; units: m/s or mm/μs; symbol: c.
- Average soft tissue speed: 1540 m/s or 1.54 mm/μs.
- Determined only by the medium’s stiffness (directly related) and density (inversely related).
- Wavelength: Distance one cycle occupies in space; units: millimeters; symbol: λ (lambda).
- Wavelength = Propagation speed / Frequency.
- High frequency = shorter wavelength; low frequency = longer wavelength.
- Wavelength is set by both transducer (frequency) and tissue; cannot be changed by sonographer.
Strength Parameters: Amplitude, Power, Intensity
- Amplitude: Difference between acoustic variable’s average and its max/min; units: pascals (Pa), decibels (dB); symbol: A.
- Power: Rate of energy transfer; units: watts (W), milliwatts (mW); symbol: P.
- Intensity: Concentration of energy in the sound beam; units: W/cm²; symbol: I.
- All three are set by the machine, can be adjusted by the sonographer (via output power), and attenuate as they pass through tissue.
- Relationships: Power ∝ Amplitude², Intensity ∝ Amplitude², Intensity ∝ Power.
- Changes by a factor: If amplitude doubles, power and intensity increase by four (2²); if amplitude halves, power and intensity decrease by four (½)².
Key Terms & Definitions
- Cycle — One complete oscillation in a sound wave.
- Period (T) — Time for one cycle to occur.
- Frequency (f) — Number of cycles per second.
- Wavelength (λ) — Physical length of one cycle.
- Propagation Speed (c) — Speed a sound wave travels in a medium.
- Amplitude (A) — Maximum variation from average value in a wave.
- Power (P) — Rate of energy transfer by the sound wave.
- Intensity (I) — Power per unit area (energy concentration).
Action Items / Next Steps
- Complete workbook practice: fill charts with missing period/frequency/wavelength/propagation speed values.
- Review matching units for calculations (e.g., seconds with Hz).
- Memorize soft tissue speed (1540 m/s) and basic formulas for exams.
- Work through "nerd check" activities to self-test knowledge of all seven parameters.