Ultrasound is a vital concept in medical physics, enabling clinicians to generate internal body images using high-frequency sound waves. This topic explores what ultrasound is, why its frequency matters, and how its fundamental properties make it suitable for diagnostic applications.

Ultrasound refers to sound waves above human hearing, enabling detailed imaging of internal structures. Its high frequency and longitudinal wave behaviour underpin medical diagnostic technologies.

Ultrasound as a Form of Sound

Ultrasound belongs to the broader family of mechanical waves, which require a medium such as air, tissue, or water to propagate. Like all sound, ultrasound involves oscillations of particles about fixed equilibrium positions. However, it operates at frequencies too high for the human ear to detect.

These waves are generated and detected using specialist equipment, but their defining characteristic is simply their frequency relative to human auditory limits. Humans typically detect sound up to about 20 kHz; everything above this is categorised as ultrasound regardless of source, amplitude, or application.

This diagram shows the frequency ranges of infrasound, audible sound, and ultrasound, highlighting the 20 kHz threshold that defines the start of the ultrasound region. Extra examples of applications are included but are optional enrichment beyond the OCR requirements. Source.

Ultrasound waves propagate by compressing and rarefying particles in the medium. Because the oscillations occur parallel to the direction of propagation, they differ fundamentally from transverse waves, such as electromagnetic waves, in which oscillations occur perpendicular to propagation.

The diagram illustrates compressions and rarefactions in a longitudinal sound wave, showing how particle density varies as the wave propagates. This directly reinforces the definition of ultrasound as a longitudinal wave. Source.

Key Properties of Ultrasound

Frequency

The most important defining feature of ultrasound is its frequency exceeding 20 kHz. In medical diagnostics, the frequencies used are far higher—commonly between 1 MHz and 15 MHz—because increasing frequency increases image resolution. However, this also increases attenuation, limiting penetration depth.

Wavelength

The wavelength of ultrasound depends on both frequency and the speed of sound in the medium. For a given medium, higher-frequency ultrasound yields shorter wavelengths. Short wavelengths enable better discrimination of small anatomical features, contributing to the detailed imaging possible in clinical practice.

Speed of Propagation

The speed of ultrasound varies between tissues because sound travels faster in denser or less compressible media. For example, ultrasound typically travels at around 1540 m s⁻¹ in soft tissue, a value often used for calibration purposes. Differences in sound speed underpin techniques such as echolocation within the body, since reflected waves return at times dependent on the properties of the tissues they encounter.

Longitudinal Wave Behaviour

Particle Motion

Because ultrasound is a longitudinal wave, particles in the medium oscillate parallel to the direction of wave travel. This behaviour produces regions of compression—where particles are pushed close together—and rarefaction—where they are further apart. These alternating regions form pressure fluctuations that propagate through the medium.

Pressure Variation

The oscillating pressure pattern is essential for medical imaging because ultrasound devices detect the returning pressure waves reflected from tissue boundaries. The magnitude of pressure variation also affects the intensity of the wave: greater pressure amplitude corresponds to higher intensity, which can influence image brightness and clarity.

Why Ultrasound Differs from Audible Sound

While the only strict definition of ultrasound is “frequency greater than 20 kHz,” its practical behaviour differs markedly from audible sound. Several features arise due to its high frequency:

• Higher attenuation in biological tissues, limiting depth of penetration.

• Increased resolution due to shorter wavelengths.

• Stronger reflection at boundaries where acoustic properties change rapidly.

These characteristics make ultrasound particularly suitable for medical imaging, where detailed information about soft tissue structures is required.

Generation and Detection of Ultrasound (Conceptual Overview Only)

Though the detailed technology is covered in later subsubtopics, it is helpful to understand that ultrasound used in medical imaging is generated using specialised devices known as piezoelectric transducers. These devices convert electrical energy into mechanical vibrations at ultrasound frequencies and also detect returning echoes by converting mechanical vibrations back into electrical signals. The ability of the transducer to both emit and receive ultrasound is essential for producing real-time, two-dimensional images.

Normal sound sources such as speakers cannot generate medical ultrasound frequencies effectively, which is why piezoelectric crystals or ceramics are required. Their ability to oscillate rapidly under an applied voltage allows precise control over wave frequency.

Applications of the Definition

Understanding that ultrasound is a longitudinal wave above 20 kHz provides the foundation for appreciating how it interacts with biological tissues. The definition immediately connects to important clinical behaviours:

• Reflection, as ultrasound meets boundaries between tissues of different acoustic properties.

• Refraction, where ultrasound changes direction when entering a medium with a different sound speed.

• Attenuation, due to absorption and scattering processes that increase with frequency.

• Transmission, referring to ultrasound passing through tissue with minimal reflection.

All these behaviours depend fundamentally on ultrasound's frequency and longitudinal wave nature. For this reason, the definition of ultrasound is not merely a classification but a crucial starting point for understanding its diagnostic value in medicine.