A feasibility study on grain noise suppression using baseline subtraction is presented in this paper. Monitoring is usually done with permanently installed transducers but this is not always possible; here instead monitoring is conducted by carrying out repeat C-scans and the feasibility of grain noise suppression by subtracting A-scans extracted from the C-scans is investigated. The success of this technique depends on the ability to reproduce the same conditions for each scan, including a consistent stand-off, angle, and lateral position of the transducer relative to the testpiece. The significance of errors are illustrated and a 3D cross correlation is used which enables the same lateral position to be located within successive C-scans. The experimental results show that a noise reduction of around 15 dB is obtained after baseline subtraction, which will significantly improve the defect detection sensitivity. In practice however, successive C-scans may be conducted at different temperatures and with different transducers of similar specifications but a varying frequency response. Compensation techniques to reduce the impact of such variations are then presented and their effectiveness is verified experimentally. It is shown that it is feasible to obtain an overall improvement of around 10 dB in the signal to noise ratio via baseline subtraction, where a temperature difference of up to 10 $$^\circ $$∘C and a peak frequency shift of as much as ±250 kHz from a baseline value of around 7 MHz can be tolerated. However, this improvement was obtained in laboratory conditions with no changes to the surface of the specimen due to oxidation or corrosion. It is shown that differences in temperature and transducer frequency response are more difficult to compensate for than changes in test geometry and position.

中文翻译：

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颗粒噪声抑制在高散射材料监测中的可行性和可靠性

本文介绍了使用基线减法抑制颗粒噪声的可行性研究。通常使用永久安装的换能器进行监测，但这并不总是可行的；此处通过执行重复 C 扫描进行监测，并研究了通过减去从 C 扫描中提取的 A 扫描来抑制颗粒噪声的可行性。这种技术的成功取决于为每次扫描重现相同条件的能力，包括传感器相对于测试件的一致间距、角度和横向位置。说明了错误的重要性，并使用了 3D 互相关，这使得相同的横向位置能够在连续的 C 扫描中定位。实验结果表明，减去基线后，噪声降低了15 dB左右，这将显着提高缺陷检测的灵敏度。然而，在实践中，连续的 C 扫描可以在不同的温度下进行，并使用具有相似规格但不同频率响应的不同换能器。然后介绍减少此类变化影响的补偿技术，并通过实验验证其有效性。结果表明，通过基线减法获得大约 10 dB 的信噪比总体改善是可行的，其中温差高达 10 $$^\circ $∘C，峰值频移为可以容忍与大约 7 MHz 的基线值相差 ±250 kHz。然而，这种改进是在实验室条件下获得的，试样表面没有因氧化或腐蚀而发生变化。