Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing (greater than 20KHz) and is a sub-discipline of acoustics.

Ultrasound is not different from normal or audible sound in physical properties, except that humans cannot hear ultrasound. Ultrasound can be further defined as an oscillation in pressure, stress, particle displacement, and particle velocity propagated in a medium with internal elastic or viscous forces. Ultrasound can be viewed as a wave motion in the air or other elastic media.

Ultrasound can propagate through a medium such as air, water, and plasma as longitudinal waves, and also as transverse waves. Ultrasound can be generated by an ultrasonic source, such as the vibrating diaphragm of a transducer which are generally made up of piezo electric material. The ultrasound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the ultrasound wave. At a fixed distance from the source, the pressure, velocity, and displacement of the medium vary in time.

The behavior of ultrasound propagation is affected by the relationship between the density and pressure of the medium, which is affected by temperature and determines the speed of sound within the medium. Motion of the medium such that if the medium is moving can increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, ultrasound moving through fluid can have speed of propagation increased by the speed of the fluid if the ultrasound and fluid are moving in the same direction.

If the ultrasound and fluid are moving in opposite directions, the speed of the sound wave can be decreased by the speed of the fluid. Medium viscosity determines the rate at which sound is attenuated. For many media, such as air or water, attenuation due to viscosity is negligible. But in other media, such as rubber, paper and soft material like cotton, higher viscosity can result in greater acoustic losses.

Although there are many physical complexities relating to the transmission of ultrasound, at the point of reception, such as a microphone or US transducer, ultrasound is simply interpreted as pressure and time with the properties like frequency or wavelength, amplitude, sound pressure or intensity, speed of sound, and direction.

The speed of sound depends on the medium the waves pass through and is a fundamental property of the material. The speed of sound is proportional to the square root of the ratio of the bulk modulus of the medium to the density. These physical properties and the speed of sound change with ambient conditions, such as temperature and humidity.

The Sound Pressure Level, or SPL, is defined as follows,

L_p=10log (p²/pref²)

where

p = rms sound pressure (Pa) pref=reference pressure, 2×10−5Pa

Commonly used reference sound pressures are 20 micro pascals in air and one micro pascal in water. When sound travels through a medium, the sound pressure diminishes with distance. Designed for, a sound pressure is only reduced by the spreading of the wave. However, real world factors further reduce sound pressure.

This additional production results from scattering and absorption. Scattering is the reflection of the sound and directions other than the original direction of propagation. While the absorption is the conversion of sound energy to other forms of energy. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as ultrasonic propagates through a material.

Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedance of the materials on each side of the boundary. The acoustic impedance of a material is defined as the product of the density and acoustic velocity. Acoustic impedance is important in the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedance, the design of ultrasonic transducers, and assessing absorption of sound in the medium.

This difference in acoustic impedance is commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that can be reflected at the interface or boundary between one medium and another

Ultrasonic Sonar Cross Section, or SCS, is a measure of how detectable an object is by sonar. A larger SCS indicates that an object is more easily detectable. An object reflects a limited amount of sonar energy back to the source. The factors that influence this include the material of which the target is made, the size of the target relative to the wavelength of the emitted sonar signal, the absolute size of the target, and the shape of the target.

The shape of the target also affects the incident angle, or the angle at which the sonar beam hits a particular portion of the target and the reflected angle, the angle at which the reflected beam leaves the part of the target hit. The SCS of an object is the cross-sectional area of a designed for reflecting sphere that can produce the same strength reflection as the object in question. Bigger sizes of this imaginary sphere can produce stronger reflections. The SCS of a sonar target is an effective area that intercepts to transmitted sonar power and then scatters that power isotropically back to the sonar receiver.