Ultrasonic Sound: Frequency, Intensity, and Applications

Definition

Ultrasonic refers to sound waves with frequencies above 20,000 Hz (20 kHz), the upper limit of human hearing.

Intensity Measurement

• Decibels (dB): Logarithmic ratio of sound pressure to a reference (20 µPa). Used for audible sound; less intuitive for ultrasound.
• Watts per square meter (W/m²) or W/cm²: Direct measure of acoustic power crossing a unit area—standard for ultrasound dosing.
• Sound Pressure Level (SPL) in dB re 20 µPa: Sometimes used for ultrasonic fields in water or tissue.

Typical Intensity Ranges

Application	Intensity
Diagnostic imaging (B-mode)	10–100 mW/cm² (spatial peak temporal average)
Therapeutic ultrasound (physiotherapy)	0.5–3 W/cm²
High-intensity focused ultrasound (HIFU)	100–10,000 W/cm²
Industrial ultrasonic cleaning	10–100 W/cm²

Safety Note

Biological effects depend on frequency, intensity, exposure time, and tissue type. Thermal and mechanical (cavitation) mechanisms are primary concerns. Always follow ALARA (As Low As Reasonably Achievable) principles in medical use.