Bodytext1 A medium is the substance or material that carries the wave

The medium carries the wave from one point in the medium to another point

The wave medium is not the wave and it doesn’t create the wave; it merely carries or transports the wave from its source to other locations

The medium is made of particles, which are temporarily displaced and then return to the point of origin without transport of particles

A sound wave is transmitted by the vibration of a medium. The medium can be air, water, wood, or any other material

Fact text: When a stone is dropped into a pond, the water molecules are disturbed
The molecules start oscillating around their original position – they vibrate

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Absorption is the major cause of energy loss (attenuation) of ultrasound in biological tissue and is due to friction which converts kinetic energy to heat energy (thermal relaxation)

Absorption depends on the tissue type (e.g. the absorption is high in bone and low in fluids) and on the frequency of the sound wave

High frequency causes more absorption

Absorption accounts for 80% of the attenuation of sound in soft tissues.

Fact text: Absorption is not a safety problem. The heat energy is relatively low and dissipates in the tissue
Absorption is only a concern in ophthalmological and obstetrical sonography
The power output from the ultrasound transducer is kept as low as required to generate adequate clinical images

Bodytext1 A sound wave is a mechanical wave

Mechanical waves require a medium in order to transport their energy from one location to another

They cannot propagate in vacuum

Mechanical waves can be longitudinal or transverse

A sound wave is a longitudinal wave

In a longitudinal wave the oscillating disturbance is parallel to the direction of travel

Sound waves are always longitudinal waves: The air molecules vibrate in the same direction as the sound wave travels and form a series of compressions (high pressure) and rarefactions (low pressure), where the molecules are squeezed together and pulled apart respectively

Fact text: A vibrating tuning fork creates a longitudinal wave. As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a low-pressure area allowing the air particles to move back to the left.

Bodytext1 Transmission

When the sound wave propagates in a medium without striking any interfaces, it passes through the interface without any reflection, scattering or refraction

In a homogeneous medium the transmission is only reduced by absorption

Even in a heterogeneous medium some of the sound wave is usually transmitted when it strikes an interface

Penetration

The ability of a sound wave to penetrate through tissue depends on the attenuation. This loss of penetration capacity is proportional to frequency

Penetration expresses how deep the ultrasound wave can penetrate down into the tissue

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As a sound wave propagates through a medium, the sound wave loses energy proportional to distance travelled from the source of sound

This energy loss or weakening of the sound wave amplitude is called attenuation of sound. It is mainly due to absorption but also to reflection and scattering at tissue interfaces

Attenuation of a sound wave is proportional to the frequency of the sound wave and differs among body tissues

Fact text: The figure shows that low frequencies are less attenuated than higher frequencies. This means that lower frequencies can penetrate deeper into e.g. soft tissues

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Amplitude, wave length, period, frequency, velocity

Waves can be described mathematically with a location-amplitude-coordinate system and a time-amplitude-coordinate system with a sinus curve

The basic parameters of a harmonic wave are: