Ultrasonic air couplingTechnology is widely used in fields such as non-destructive testing and distance measurement because it does not require direct contact with the object being measured. However, the significant difference in acoustic impedance between air and solid media (approximately 413 Rayl for air and 10 ⁷ Rayl for metal) can cause ultrasound waves to be reflected at over 99% of the interface, with only a very small amount of energy transmitted, resulting in the problem of "acoustic impedance mismatch". To solve this problem, efforts need to be made in three dimensions: impedance transition, energy enhancement, and signal optimization, to construct a full chain solution and improve the efficiency of ultrasonic coupling.

The design of gradient acoustic impedance matching layer is the core breakthrough path, which essentially constructs a smooth impedance transition channel of "air matching layer solid". The matching layer shall be made of materials whose acoustic impedance is between the air and the measured medium, such as polyurethane foam, silicone rubber composite, etc. The acoustic impedance of the matching layer shall be equal to the geometric mean value of the acoustic impedance of the air and the solid by adjusting the density and sound velocity of the materials, so as to increase the energy transmission. In practical applications, double-layer or multi-layer matching structures are often used. The first layer matches air with the middle layer, and the second layer connects the middle layer with the solid. For example, when detecting steel, a double-layer structure of "silicone rubber (acoustic impedance 1.5 × 10 ⁶ Rayl)+epoxy resin (3 × 10 ⁶ Rayl)" can be used to improve the transmission efficiency to over 30%.

Optimize the structure and excitation method of the transducer to enhance effective sound energy output from the source. The preferred material for the transducer oscillator is piezoelectric ceramics or piezoelectric composite materials. By improving the shape of the oscillator (such as an arc-shaped oscillator), the beam focusing ability is enhanced, and energy is concentrated in the detection area to reduce diffusion losses. The excitation circuit adopts wideband pulse excitation technology to generate short pulse signals, reducing the energy attenuation of signals in interface reflection; Simultaneously increase the excitation voltage (controlled within a safe range), enhance the vibration amplitude of the oscillator, and compensate for the energy loss caused by interface reflection. In addition, the use of a transceiver integrated transducer design shortens the signal transmission path and improves the receiving sensitivity.

Improve the coupling environment and auxiliary medium to reduce impedance interference in the air layer. In high-precision detection scenarios, inert gases (such as argon) or aerosols can be injected into the air coupling gap, and the acoustic impedance can be fine tuned by adjusting the medium density to reduce the difference with solids; Acoustic lens focusing technology can also be used to focus the ultrasonic beam on the measured interface, reducing energy diffusion in the air layer. For outdoor or complex environments, it is necessary to design a sealed coupling chamber, control the temperature (20-25 ℃) and humidity (40% -60%) inside the chamber, avoid changes in air acoustic impedance caused by temperature and humidity fluctuations, and ensure coupling stability.

The optimization of signal processing technology is a key supplement to improving detection performance. Adopting adaptive filtering algorithm to filter the clutter signal generated by interface reflection and extract effective transmitted waves; By using phase compensation technology, the phase shift of the signal propagating in the air and matching layer is corrected to improve the signal resolution. For weak signals, low-noise amplifiers and signal superposition techniques can be used to enhance the signal-to-noise ratio by superimposing multiple detection signals. For example, in the detection of defects in metal sheets, signal superposition and filtering processing can make the defect signals that were originally submerged in noise appear clearly.

crackUltrasonic air couplingThe problem of acoustic impedance mismatch requires a deep integration of material innovation, structural optimization, and signal processing technology to reduce energy loss at the source and strengthen effective signals through backend technology. This process requires targeted design solutions based on specific application scenarios, such as detection accuracy and environmental conditions, in order to enable ultrasonic air coupling technology to break through performance bottlenecks and achieve efficient and reliable detection applications in more fields.