Medical student

Waves

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

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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.

Sound wave characteristics

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:
• Amplitude (A) = maximum oscillation
• Wave length (lambda) = distance from wave top to wave top
• Period (T) = time interval from one wave top at a well defined location until the next wave top gets to the same location
• Frequency (1/T) = number of wave tops per time unit. SI unit for frequency is Hertz (Hz) = s-1
• Velocity (v) = the speed of the wave = lambda/T

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Attenuation

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

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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

Absorption

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.

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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