Analysis of Theoretical and Actual Signals in Scanning Acoustic Microscopy

views：97 author：Hiwave source：Hiwave time：2025-07-16 catogory：Frequently Asked Questions
When using a scanning acoustic microscope (SAM) for material inspection, a key issue often arises: there is a significant difference between the theoretical waveform a……
When using a scanning acoustic microscope (SAM) for material inspection, a key issue often arises: there is a significant difference between the theoretical waveform and the actual waveform detected. This discrepancy not only affects data accuracy but also poses challenges for defect identification and localization. So, what causes this phenomenon, and how can it be further analyzed?
The phisical image and cross-section image of stainless steel
To better understand this issue, let’s consider a simple calibration block as an example. This calibration block has the following characteristics:
Structural Composition: A step is located on the top, and a groove is located below.
Material Properties: The entire block is made of stainless steel, which exhibits excellent acoustic transmission performance.
Inspection Environment: The sample is placed in a water tank, with an air cavity beneath it.
In this setup, the transducer emits ultrasonic waves from above. These waves first reach the top surface of the calibration block, where part of the energy is reflected back to the transducer, while the remainder continues to propagate downward. At various interfaces (such as the interface between stainless steel and air), reflections occur. These reflected signals are eventually received and recorded by the transducer as waveform data.
 
According to the basic principles of ultrasonic wave propagation, the expected waveform is as follows:
top signal and bottom signal
Pulse Excitation Signal: This is the initial signal generated when the transducer emits the ultrasonic wave.
Top Surface Echo: When the ultrasonic wave first reaches the top surface of the sample, part of the energy is reflected back, forming the first distinct wave peak.
Bottom Surface Echo: After passing through the top surface, the ultrasonic wave continues to propagate downward and is reflected again at the bottom surface, forming a second major wave peak.
However, in actual testing, the observed waveform is much more complex than this simplified model. Multiple wave peaks are often present, and in some cases, the phenomenon of “multiple wave groups” can be observed.
theorical wave vs actual wave
To understand why the actual waveform appears so complex, we need to take a deeper look into the fundamental propagation characteristics of ultrasonic waves.
  1. Coexistence of Longitudinal and Shear Waves
Ultrasonic waves are mechanical waves, and they propagate primarily in two forms: longitudinal waves (P-waves) and shear waves (S-waves).
Longitudinal Waves (P-waves): The particle vibration direction is parallel to the wave propagation direction. These waves can travel through solids, liquids, and gases.
Shear Waves (S-waves): The particle vibration direction is perpendicular to the wave propagation direction. These waves can only propagate through solids.
In this case:
The reflected wave from the top surface is a longitudinal wave.
Once the wave enters the stainless steel, it propagates as both longitudinal and shear waves.
Different wave types have different propagation speeds. For example, in stainless steel:
The longitudinal wave speed is approximately 5960 m/s,
While the shear wave speed is around 3200 m/s.
As a result, on the waveform:
The first prominent peak corresponds to the longitudinal wave.
This is followed by the shear wave, which travels more slowly and thus arrives at the transducer later.
After the shear wave, a series of evenly spaced echoes appear, caused by multiple reflections.
basic principle
  1. Multiple Reflection Effects
During propagation inside the material, ultrasonic waves do not reflect just once. Instead, they repeatedly reflect between various interfaces, gradually attenuating until they disappear. This phenomenon is similar to hearing multiple echoes when shouting in a valley.
This effect manifests as:
Multiple echoes of the longitudinal wave: Repeated reflections occur between the top and bottom surfaces, forming several equally spaced peaks with gradually decreasing amplitude.
Multiple echoes of the shear wave: These also undergo multiple reflections at different interfaces.
Reflections between the transducer and the sample: Additional reflection signals may be introduced due to the coupling medium (such as water) between the transducer and the sample.
All these factors together contribute to the complex multi-wave structure observed in the actual waveform.
Analysis of Theoretical and Actual Signals in Scanning Acoustic Microscopy
               

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