Nature of Ultrasound & Properties of Waves

These vocabulary flashcards cover the fundamental properties of sound waves, acoustic variables, classifications of sound, and parameters determining ultrasound propagation as discussed in DMU 3313 Unit 2.

Sound

A form of mechanical energy produced by vibrations that cause the molecules of a medium to move back and forth as a pressure wave.

Electromagnetic waves

Energy that can propagate through a vacuum, including light (visible, infrared, ultraviolet), radio waves, gamma rays, X-rays, and microwaves.

Mechanical waves

Disturbances that require a medium to travel through, transferring energy through matter by causing particles to bump into each other.

Acoustic Variables

Variations caused by sound as it propagates through a medium, which include pressure, particle motion (displacement, velocity, acceleration), density, and temperature.

Cycle

One complete variation of an acoustic variable from its resting value to both extremes and back to resting.

Compression

The point in a wave where a variable reaches its highest value, also known as the crest.

Rarefaction

The point in a wave where a variable reaches its lowest value, also known as the valley or trough.

Sinusoidal wave

A wave where oscillations occur at a constant rate or speed.

Longitudinal Waves

Waves in which variations of acoustic variables occur parallel to the direction of sound propagation.

Transverse Waves

Waves in which variations of acoustic variables occur perpendicular to the direction of sound propagation.

Continuous Wave (CW)

The constant emission of sound that is always on.

Pulsed Wave (PW)

The intermittent emission of sound, consisting of a few cycles of sound followed by a gap.

Frequency ($f$)

The number of complete cycles that occur in one second, measured in Hertz ($Hz$).

Infrasound

Sound with a frequency less than $20\,Hz$, which is below the range of human hearing.

Audible sound

Sound within the range of human hearing, typically between $20\,Hz$ and $20,000\,Hz$ ($20\,kHz$).

Ultrasound

Sound with a frequency greater than $20,000\,Hz$ ($20\,kHz$), beyond the range of human hearing.

Period ($T$)

The time it takes for one complete cycle to occur, measured in units such as seconds ($s$), milliseconds ($ms$), or microseconds ($\mu s$).

Amplitude ($A$)

The difference between the maximum value of an acoustic variable and its normal, undisturbed value.

Power ($P$)

The rate at which work is done or the rate at which energy is transferred.

Intensity ($I$)

The rate at which energy passes through a unit area, directly related to power and equal to the amplitude squared ($I \propto A^2$).

Wavelength ($\lambda$)

The length of space over which one complete cycle occurs, commonly measured in millimeters ($mm$) in DMU.

Wave Equation

The formula relating propagation speed ($c$), frequency ($f$), and wavelength ($\lambda$), expressed as $c = f \times \lambda$.

Acoustic Velocity ($c$)

The speed at which a sound wave travels through a medium, also known as propagation speed; for soft tissue, this averages $1540\,m/s$.

Density ($\rho$)

The concentration of matter in a unit of volume, measured in $kg/m^3$ or $g/cm^3$. Increased density typically results in slower acoustic velocity.

Acoustic impedance ($Z$)

The resistance of a material to compression, also known as stiffness or bulk modulus, measured in Rayls ($R$).

Compressibility ($K$)

The fractional decrease in volume when pressure is applied; stiffness is inversely related to compressibility.

Elasticity ($e$)

The ability of an object to return to its original shape and volume after force is removed.

Constructive Interference

Occurs when waves with the same frequency are transmitted in phase and combine to form a wave with a larger amplitude.

Destructive Interference

Occurs when waves with the same frequency are transmitted $180^{\circ}$ out of phase and combine to form a wave with a smaller amplitude.

Huygens’ Principle

Theory stating that all points on a wave can be considered as point sources for the production of three-dimensional spherical waves.

Non-linear Propagation

Propagation where the speed of sound depends on pressure, causes the wave shape to change (becoming saw-tooth), and allows crests to advance faster than valleys.

Fundamental Frequency

The original frequency of the sound wave sent into the medium.

Harmonics

New frequencies that are multiples of the fundamental frequency, produced by nonlinear propagation to improve image quality and reduce artifacts.