1. Near-field area

Due to the sharp fluctuations in sound pressure near the source, multiple maximum values ??and minimum values ??appear. The last maximum value of the sound pressure is at the distance from the sound source. The distance becomes the near field length, represented by N, and the area within the N value is called the near field area.

When the measurement distance r=λ/2π≈λ/6, the intensity of the induced field is equivalent to the intensity of the radiation field. In places relatively close to the radiation source (r<λ/6), the intensity of the induced field is greater than the intensity of the radiation field.

2. Far field area

Generally when r is greater than 3λ, the components of the induction field can be ignored and it is considered to be in the far field (area).

A field area in which the angular distribution of radiation field intensity is basically independent of the distance from the antenna. In the radiation far field area, the lines connecting each point on the antenna to the measurement point are regarded as parallel, and the introduced The error is less than a certain limit. If the antenna size is D, the distance in the far field area should be greater than 2D2/λ.

Extended information:

Ultrasound is an elastic mechanical vibration wave. It has some characteristics compared with audible sound:

The direction of propagation is stronger and can It gathers into a narrowly oriented line beam; the acceleration of the particle vibration in the propagation medium is very large; in the liquid medium, when the ultrasonic intensity reaches a certain value, cavitation will occur.

Beam characteristics

The sound waves emitted from the sound source propagate directionally in a certain direction (other directions are very weak), which is called beaming. Because ultrasonic waves have a short wavelength, when they pass through a small hole (a hole larger than the wavelength), they will appear as a concentrated beam of rays moving in a certain direction.

Because ultrasound is highly directional, it can collect information in a directional manner. Similarly, when there is an obstacle with a diameter larger than the wavelength in the direction of ultrasonic wave propagation, an "acoustic shadow" will be produced behind the obstacle. These are like light passing through small holes and obstacles, so ultrasonic waves have beam characteristics similar to light waves.

The quality of the ultrasonic beam is generally measured by the size of the divergence angle (customarily expressed by the half emission angle). Taking the flat circular piston sound source as an example, its size determines the basic principle of ultrasonic waves, the appropriate diameter (D) of the sound source and the wavelength (λ) of the sound wave.

Reference source:?Baidu Encyclopedia-Basic Principles of Ultrasound

Baidu Encyclopedia-Ultrasound