Ultrasonic immersion transducers are the core equipment for industrial non-destructive testing. Its working principle relies on the propagation characteristics of sound waves in different media. This paper systematically analyzes the core principle of this technology from three aspects: physical mechanism, signal processing and system structure.

The core component of the transducer is a piezoelectric ceramic crystal. When an external electrical pulse (typically 50–100 V) is applied across the crystal, mechanical vibration occurs via the inverse piezoelectric effect. This vibration propagates as longitudinal waves through the surrounding medium, usually within the 1–20 MHz frequency range. Frequency selection directly impacts performance:

High-frequency waves (>5 MHz) offer higher resolution, suitable for thin-walled components.

Low-frequency waves (<2 MHz) provide higher penetration, ideal for thick materials.

Critical Importance of Water Coupling

The unique feature of immersion testing is the use of water as the coupling medium. After being emitted from the transducer, the sound wave first enters the water layer. Due to the significant difference in acoustic impedance between water (≈1.5×10⁶ kg/(m²-s)) and air (≈415 kg/(m²-s)), more than 99% of the sound wave is reflected at the water-air interface. Precise control of the thickness of the water layer (typically 5-100 mm) ensures efficient energy transfer to the test sample.

Acoustic Propagation and Defect Detection Mechanism

Sound waves enter the material and propagate according to the Huygens principle. Defects (e.g., cracks, voids) cause interface reflections and scattering. The echo signal received by the transducer consists of two main components:

Back wall echo: Reflection from the bottom surface of the material.

Defect echoes: Early reflections from defect interactions.

By measuring the time difference (Δt) between these echoes and the wave speed "c" in the material (steel ≈ 5900 m/s, aluminum ≈ 6300 m/s), the depth of the defect can be calculated.

Signal Processing and Imaging Techniques

Modern immersion systems incorporate a digital signal processor (DSP) for the following functions

Gain compensation: Automatic adjustment of signal strength to attenuation.

Noise filtering: Elimination of ambient noise by means of a bandpass filter.

Waveform analysis: Extraction of defect features using the FFT algorithm.

The advanced system can visualize defect location and morphology through B-scans (2D cross-section mapping) and C-scans (3D volumetric mapping).