High resolution near-field acoustic holography

High resolution Near-field Acoustic Holography (NAH) is not a widely discussed topic in open literature. For instance articles on automotive or naval applications do not discuss resolutions beyond one centimetre. Electronic appliances on the other hand demand a high spatial resolution, since the components of printed circuit boards become smaller and smaller. These applications require a spatial resolution in the order of millimetres. As an example, components such as Surface Mounted Devices (SMDs) carry high frequent switching voltages, which may have a noise generating effect. Even sound power levels as low as 30 dB(A) can be annoying to the customer, requiring a diagnostic tool such as NAH to localise the sound source(s) of relatively low acoustic levels at a high spatial resolution. To obtain high-resolution acoustic images, the acoustic signal processing and measurement set-up have to be tuned carefully. The paper focuses upon the determination of optimal measurement and reconstruction parameters to enable highly accurate source reconstructions, which in this case are limited to stationary, coherent sources only. To illustrate the high-resolution capabilities of Planar Near-field Acoustic Holography (PNAH) a test case is used. The test case is a plate containing three baffled sources, each two millimetre in diameter, half a millimetre apart. The measurements show that the sources are identified with sub-millimetre resolution.