Applications of Ultrasound

A piezoelectric transducer converts an electrical pulse into a mechanical vibration, launching a brief ultrasound pulse into the medium. When the pulse reaches a boundary between two materials with different acoustic impedances (Z = ρv, where ρ is density and v is wave speed), part of the wave reflects and part transmits. The transducer then 'listens' for the returning echo. The distance d to the boundary is calculated from d = vt/2, where t is the round-trip time. This principle underlies all pulse-echo systems, from medical scanners to SONAR.

In medical ultrasound, transducers operating at 1–20 MHz scan across the body. A coupling gel eliminates air gaps between the probe and skin, because air has a very different acoustic impedance to tissue and would reflect almost all the energy. The scanner builds a 2-D or 3-D image by firing pulses along many parallel lines and mapping the depth and intensity of each echo. Soft-tissue boundaries — such as the edge of an organ or a foetal outline — each produce a distinct reflection. Doppler ultrasound additionally measures blood-flow velocity by detecting the frequency shift of waves reflected from moving red blood cells.

SONAR applies the pulse-echo principle underwater, where the speed of sound is approximately 1500 m s⁻¹ (compared with 340 m s⁻¹ in air). Active SONAR emits a pulse and analyses the returning echo to determine the range and direction of submarines, shoals of fish, or the seabed. Passive SONAR simply listens to sounds already present in the water, such as engine noise. The large wavelengths used in deep-sea SONAR (typically 1–10 kHz) give long range but coarser resolution than medical scanners. Many marine animals — dolphins and bats use biological SONAR (echolocation) based on precisely the same physics.

Ultrasonic cleaning baths operate at 20–400 kHz. A transducer attached to the base of a liquid-filled tank creates alternating pressure waves. During the low-pressure (rarefaction) phase, millions of microscopic bubbles form in the liquid — a process called cavitation. When these bubbles collapse violently during the high-pressure phase, they release tiny jets of fluid with enormous local energy, dislodging dirt, grease, and biological material from the surface of submerged objects. This technique cleans everything from surgical instruments and jewellery to electronic circuit boards, reaching into crevices that brushes cannot.

In industry, ultrasound locates hidden cracks, voids, or inclusions inside metal components — aircraft wings, pipelines, welds — without cutting them open. A flaw creates a partial reflection where there should be none; its depth and size are inferred from the echo timing and amplitude. In medicine, 'A-scan' diagnostics measure eye axial length for cataract surgery planning, and 'B-scan' imaging checks for retinal detachment. Bone densitometry uses the speed and attenuation of ultrasound through bone to estimate mineral density, helping diagnose osteoporosis.

• d — Distance to boundary (metres (m)): The one-way distance from the transducer to the reflecting surface or object.
• v — Speed of sound in medium (metres per second (m s⁻¹)): The speed at which ultrasound travels through the specific medium; approximately 1500 m s⁻¹ in soft tissue and seawater.
• t — Round-trip time (seconds (s)): The total time elapsed between emitting the pulse and detecting the returning echo. Divided by 2 because the wave travels to the boundary and back.

Problem. A ship's SONAR emits an ultrasound pulse. The echo from the seabed is detected 0.24 s later. If the speed of sound in seawater is 1500 m s⁻¹, calculate the depth of the seabed beneath the ship.

Answer. The seabed is 180 m below the ship.

• Wave Speed, Frequency and Wavelength — The relationship v = fλ determines which frequencies are needed to achieve a given resolution or penetration depth in ultrasound applications.
• Reflection and Refraction of Waves — Ultrasound imaging depends entirely on partial reflection at boundaries between materials; understanding why and how much reflects requires knowledge of wave reflection principles.
• Properties of Waves — Core wave properties — amplitude, frequency, wavelength, and period — underpin every quantitative aspect of ultrasound technology.
• The Doppler Effect — Doppler ultrasound uses the frequency shift of reflected waves to measure blood-flow velocity, directly applying the Doppler effect to medical diagnostics.