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Sound Field Visualization in Noise Hazard Control
Published in Dariusz Pleban, Occupational Noise and Workplace Acoustics, 2020
Acoustic holography is a method offering the possibility of obtaining a complete description of a sound field in three-dimensional space [Maynard et al. 1985]. As opposed to the beamforming method, acoustic holography measurements are taken with the use of two-dimensional microphone arrays or special acoustic transducers enabling direct measurement of the vector component of the sound field—the particle velocity (known also as the acoustic particle velocity). The measurements are typically carried out in the sound nearfield, but taking measurements in the farfield is also possible. The result of application of acoustic holography may not only create a sound pressure level distribution image but also a depiction of particle velocity and sound intensity vector fields in three-dimensional space [Williams 1999; Hayek 2008].
Instrumentation for Noise Measurement and Analysis
Published in David A. Bies, Colin H. Hansen, Engineering Noise Control, 2017
David A. Bies, Colin H. Hansen
Acoustic holography involves the measurement of the amplitude and phase of a sound field at many locations on a plane at some distance from a sound source, but in its near field so that evanescent waves contribute significantly to the microphone signals. The measurements are than used to predict the complex acoustic pressure and particle velocity on a plane that approximates the surface of the source (the prediction plane). Multiplying these two quantities together gives the sound intensity as a function of location in the prediction plane, which allows direct identification of the relative strength of the acoustic radiation from various areas of a vibrating structure. The theoretical analysis that underpins this technique is described in detail in a book devoted just to this topic (Williams, 1999) and so it will only be summarised here.
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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[acoustics] Using both amplitude (pressure) and phase of the mechanical waves collected by specific sensors (seeacoustic intensity method), an image can be formed from dispersed and reflected sound waves similar to optical holography. Generally, acoustical holography is a near-field technique with resolution and accuracy directly dependent on the number of sensors. A reconstruction algorithm uses finite element methods and computational physics as required for the complexity of the diagnostic design. The three-dimensional representation of the energy distribution yields the geometric and material information of the polymorphic construction under investigation. Time-domain analysis can additionally provide motion artifacts such as friction (and other density and contact surface interaction related characteristics) (see Figure A.31).
Improved integral formulae for supersonic reconstruction of the acoustic field
Published in Inverse Problems in Science and Engineering, 2018
Array measurements that consist of microphones (or velocity probes) sampling the fields simultaneously at a spatially distributed set of points are common in acoustics. Array measurements are ideal for beamforming, very common in the acoustic measurement industry which is often used to locate sources in space. Although intensity arrays also exist providing a direct measurement of the intensity at each sensor location, there are elegant mathematical formulations that allow for an indirect measurement of the intensity based on the distributed measurement of the pressure/velocity fields acquired from arrays with pressure/velocity probes. For example, for separable surfaces of the wave equation (planes, spheres or cylinders), the field has a spectral eigenfunction decomposition that allows the localization of sources on the vibrator. One such method is near-field acoustical holography (NAH) [1]. Similarly this decomposition can be explicitly used to the localization of hot spots by minimizing the bipolar nature of the intensity field. This approach is known as supersonic intensity (SSI) [2,3]. The SSI technique has been used extensively to locate the radiation sources on naval ships with great success.
Noise source identification of high-speed motion mechanism of textile equipment based on equivalent source method
Published in The Journal of The Textile Institute, 2020
Yang Xu, Angang Li, Xiaowei Sheng, Ziyu Zhang
The basic idea of near-field acoustic holography based on ESM is to superimpose and replace the sound field generated by a series of equivalent sources distributed near the boundary surface inside the sound source. The intensity of a series of equivalent sources can be calculated from the sound pressure measured by the holographic plane, so that the whole sound field can be reconstructed and predicted. The positions of the equivalent source plane, the sound source plane, the reconstruction plane and the holographic plane are shown in Figure 1.