Ultrasound
Ultrasound refers to sound waves with frequencies above the upper limit of human hearing, typically above 20,000 Hz (20 kHz), extending to several gigahertz in specialised applications. Because of its high frequency and corresponding short wavelength, ultrasound can resolve fine structural details and is strongly absorbed or reflected by tissue boundaries, making it invaluable in medical diagnostics (obstetric scans, echocardiography), industrial non-destructive testing, sonar navigation, and the sonication used in cleaning and chemical processing.
Key Formula
d = (v × t) / 2
LaTeX: d = \frac{v \cdot t}{2}
| Symbol | Meaning | Unit |
|---|---|---|
| d | Distance to the reflecting surface | m |
| v | Speed of ultrasound in the medium | m/s |
| t | Round-trip travel time of the pulse | s |
Worked Example
Problem
An ultrasound pulse is emitted into tissue and its echo returns after 0.000120 s. The speed of ultrasound in soft tissue is 1540 m/s. How deep is the reflecting surface?
Solution
Step 1: Use d = (v × t) / 2 (the factor of 2 accounts for the two-way journey). Step 2: d = (1540 × 0.000120) / 2 = 0.1848 / 2 = 0.0924 m.
Answer
Depth of the reflecting surface = 0.0924 m ≈ 9.24 cm
Ultrasound Frequency Ranges and Applications
| Frequency Range | Wavelength in Tissue | Application | Penetration Depth | Resolution |
|---|---|---|---|---|
| 20 kHz – 1 MHz | 1.5 mm – 75 mm | Industrial sonar, cleaning | Deep | Low |
| 1 – 5 MHz | 0.3 – 1.5 mm | Abdominal/obstetric imaging | Moderate (10–20 cm) | Medium |
| 5 – 15 MHz | 0.1 – 0.3 mm | Cardiac, vascular imaging | Shallow (3–8 cm) | High |
| 15 – 50 MHz | 0.03 – 0.1 mm | Skin and eye imaging | Very shallow (<3 cm) | Very high |
| >100 MHz | <0.015 mm | Acoustic microscopy | Microscopic | Microscopic |
Interactive Tools
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Related Terms
Doppler Effect
The Doppler effect is the apparent change in frequency (and thus pitch or colour) of a wave perceived by an observer when the source of the wave and the observer are moving relative to each other. When a source approaches, the observed frequency increases; when it recedes, the frequency decreases. The effect is named after Austrian physicist Christian Doppler (1842) and applies to all wave types including sound, light, and radar, with applications in medical ultrasound, police speed guns, weather radar, and astronomical redshift measurements.
Sound Intensity
Sound intensity is the power carried by a sound wave per unit area perpendicular to the direction of propagation, measured in watts per square metre (W/m²). It quantifies how much acoustic energy passes through a given surface each second and decreases with the square of the distance from a point source — the inverse square law. Sound intensity is the physical basis for the decibel scale and is central to audiology, architectural acoustics, and occupational noise exposure standards.
Infrasound
Infrasound refers to sound waves with frequencies below the lower limit of human hearing, typically below 20 Hz, including frequencies as low as 0.001 Hz for geophysical phenomena. Although inaudible to humans without specialised equipment, infrasound can travel enormous distances through air, water, and the earth's crust with very little attenuation, making it valuable for monitoring nuclear explosions, volcanic eruptions, earthquakes, and severe weather. Many large animals — including elephants, whales, rhinos, and alligators — communicate via infrasound over hundreds of kilometres.
From Latin "ultra" (beyond) + "sonus" (sound). The prefix "ultra-" denotes exceeding a normal limit. The generation of ultrasound using piezoelectric crystals was pioneered by Paul Langevin during World War I (1917) for submarine detection.