This paper presents analyses of the 3D acoustic fields generated by motorcycles at very low frequencies ( < 150 Hz) at the idle speed and during sudden acceleration. Diagnosis and analyses of sound sources at low frequencies have always been a significant challenge because the directivity of low-frequency sound is very poor. To date, there are no research papers and/or reports that have demonstrated low-frequency sources localization and radiation patterns of any kind at high spatial resolution in 3D space. This study shows that by using sound viewer technologies, which include the passive sonic detection and ranging (SODAR), the Helmholtz equation least squares (HELS) method, advanced signal processing, denoising, etc., the locations of sound sources and visualization of the sound fields can be determined with high spatial resolution, even at frequencies below 150 Hz. In particular, the HELS method allows for reconstructing all the acoustic quantities, including the acoustic pressure, time-averaged acoustic intensity, time-averaged acoustic power on the source surfaces and in 3D space. The hardware needed consists of a 3D array with six free-field precision microphones with pre-amplifiers, a miniature wide-angle camera, an eight-channel digital signal processor and a laptop computer.

中文翻译：

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摩托车低频噪声分析

本文介绍了摩托车在极低频率下产生的 3D 声场的分析（ < 150Hz）在怠速和突然加速期间。低频声源的诊断和分析一直是一项重大挑战，因为低频声音的指向性很差。迄今为止，还没有研究论文和/或报告证明了在 3D 空间中的高空间分辨率下的任何类型的低频源定位和辐射模式。这项研究表明，通过使用声音查看器技术，包括被动声波探测和测距（SODAR）、亥姆霍兹方程最小二乘法（HELS）方法、先进的信号处理、去噪等，声源的位置和可视化即使在低于 150 Hz 的频率下，声场也能以高空间分辨率确定。特别是，HELS 方法允许重建所有声学量，包括声压、时间平均声强、声源表面和 3D 空间中的时间平均声功率。所需的硬件包括一个 3D 阵列和六个带有前置放大器的自由场精密麦克风、一个微型广角相机、一个八通道数字信号处理器和一台笔记本电脑。