Basic Principles Of Ultrasound - Knowledge

Ultrasonic is part of the sound waves, is the human ear can't hear the sound waves, frequency is higher than 20 KHZ, it and sound waves have in common, which are produced by material and vibration, and is only transmitted in the medium; At the same time, it also widely exist in nature, many animals can transmit and receive ultrasonic, of which the most bats is outstanding, it make use of the ultrasonic echo of weak flight and catch food in the dark. But ultrasound also has special properties, such as higher frequencies and shorter wavelengths, so it is similar to light waves with shorter wavelengths.

features

Ultrasonic wave is an elastic mechanical vibration wave, which has some characteristics compared with audible sound. The acceleration of vibration at the mass point of the transmission medium is very large. Cavitation occurs in liquid media when the ultrasonic intensity reaches a certain value.

Beam characteristics

Sound waves from a sound source travel in a direction (weak in other directions) called a beam. Because of its short wavelength, ultrasonic waves show a concentrated beam of radiation moving in a certain direction as they pass through the hole, which is larger than the wave length. Because of the strong direction of ultrasound, the information can be collected. Also, when the diameter of an obstacle is larger than the wavelength in the direction of ultrasonic propagation, the "sound shadow" will be generated behind the obstacle. These are like light passing through holes and obstacles, so ultrasonic waves have beam characteristics similar to light waves.

The beam quality of ultrasonic wave is generally measured by the size of the divergence Angle (customarily)

This is shown as a semi-transmitting acetabulum. Taking plane circular piston type sound source as an example, its size determines

Basic principles of ultrasound

Basic principles of ultrasound (4 photos)

The suitable diameter (D) of the sound source and the wavelength of the sound wave are shown below. Thus, to make the sound body emits a directional good ultrasound, must make theta Angle is small, as far as possible direct spasm, D emitter (source) must be large or frequency f also must be high to get fired, otherwise will backfire. As the wavelength of ultrasound, shorter than the wavelength of audible sound, so it has better than audible sound wave beam characteristics, the higher the frequency of ultrasound, the shorter the wavelength is, the propagation characteristics is significant to a certain direction.

Absorption characteristics

When ultrasonic waves travel in various media, with the increase of the propagation distance, the ultrasonic intensity will gradually weaken and the energy will gradually be consumed. This kind of energy is absorbed by the media, which is called sound absorption. 1845 Stoke. G. G.) Found: when sound waves through the liquid, due to the liquid particle relative motion and the internal friction (that is, the viscous effect) lead to sound absorption, thus deduced caused by internal friction of the medium or viscous liquid in sound absorption formula. Also, when sound waves travel through liquid media, the temperature of the compression zone will be higher than the average temperature. On the contrary, the temperature is lower than the average temperature of sparse area, therefore, due to the heat transfer between the compression and sparse part of the sound waves to heat exchange, thus the decrease of the acoustic energy in 1868 Kirchhoff (Kirchhoff g.) caused by the sound absorption of heat conduction formula are deduced.

It can be seen that the absorption coefficient a is proportional to the square of the sound wave frequency, and when the frequency increases by 10 times, the absorption coefficient increases by 100 times. That is, the higher the frequency, the greater the absorption, so the sound wave propagation distance is smaller. In gas, Einstein proposed in 1920 by the audio frequency dispersion to determine the reaction rate of associated gas, thus promote the uptake of gas molecular thermal relaxation mechanism extends to the liquid, because the molecules in medium is obtained by the collisions between molecules absorb thermal relaxation. So low frequency sound waves can travel a long distance in the air, and high frequency sound waves decay rapidly in the air.

In solids, sound absorption depends largely on the actual structure of solids.

Caused by the above to see some of the reasons for different medium on the sound absorption, but the main reason is that the medium viscosity, heat conduction, the actual structure of the medium and the medium of microscopic dynamics caused by the relaxation effect, etc., in the process of sound absorption of the medium changes with the frequency of the sound. Ultrasonic wave is a high frequency sound wave, when propagating in the same medium, as the frequency increases, the energy absorbed by the medium increases. For example, the frequency is

The ratio of energy absorbed by Hz ultrasound in the air is

The sound waves of Hz are 100 times larger. For the same frequency of ultrasonic transmission due to different media. For example, when propagating in gas, liquid and solid, its absorption is the strongest, weaker and smallest respectively. So ultrasonic waves travel the shortest distance in the air.

When ultrasonic waves are propagated in a uniform medium, the acoustic intensity is weakened with the increase of distance due to the absorption of the medium, which is the attenuation of sound waves.

When the initial intensity of ultrasonic wave is J0, after a distance of x meters, its intensity is