Near field area: Due to the sharp fluctuations in sound pressure near the source, multiple maximum values ??and minimum values ??appear. The distance between the last maximum sound pressure value and the sound source becomes the near field length, expressed in N means that 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. At 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.

Far field area: When r is greater than 3λ, the components of the induced field can be ignored and it is considered to be in the far field (area). The angular distribution of the 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 error introduced 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.

The quality of the ultrasonic beam is 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.

Extended information

Researchers believe that focused ultrasound changes the balance of excitation and inhibition when processing sensory stimuli at the brain area it targets. This change prevents the stimulation of excitement. Expand to enhance awareness.

This discovery leads to a new, non-invasive way to modulate human brain activity with greater spatial resolution than any existing method. Based on relevant research results, researchers believe that ultrasound transcranial magnetic stimulation and transcranial direct current stimulation have higher spatial resolution.

Ultrasound provides technical and theoretical proof for accurately grasping neural circuit activity, helping to develop potential treatments for neurodegenerative disorders. It also provides a way to explore normal human brain function and understand cognition, decision-making and thinking. Here comes a powerful new tool.

Baidu Encyclopedia-Ultrasound