LMS CADA-X Acoustic Holography performs a spatial transformation of sound fields – meaning that it takes a measurement of sound pressures along a certain plane and predicts what the readings would be anywhere else. This can be towards the nominal source (back propagation), towards the far field, or in any other direction or plane. Special techniques are available when working in complex multisource environments. Acoustic Holography is a powerful tool that helps to solve noise source identification and quantification problems - it also does this in a fraction of the time taken by more traditional techniques. Furthermore, it can be used to predict the effect of a level reduction at any of the sources on the overall level received at any location. This application notes discusses the underlying theory behind Acoustic Holography, and shows how it is applied to troubleshoot a real-life noise problem on a car engine.
Introduction
Increasing manufacturer emphasis on product sound quality has generated the need for faster (and better) experimental techniques for product refinement. The most critical stage in the product refinement process is to locate and quantify1the various source(s) that contribute to the overall problem. Product characterizations often involve not only the measurement of sound pressure and sound power, but also vibrational velocities of radiating surfaces and energy flow information, such as acoustic intensity vector maps, in order to locate the sources of acoustic energy. For more complex acoustic problems the time required for problem identification using these ‘traditional’ techniques could easily be counted in days, if not weeks. Acoustic Holography can substantially reduce this time. Acoustic Holography has the advantage of being able to determine the vibrational velocity on the radiating surface itself, together with any power descriptor of the near-field (intensity, sound pressure, etc.) by means of Near-Field Acoustic Holography, while the more distant field can be investigated by application of Helmholtz’s Integral Equation. This is much more advanced than the traditional acoustic intensity mapping which only shows what is happening at the measurement plane, and can not be used to predict acoustic fields at any other location.
What is the Principle behind Acoustic Holography?
The basic principle of Acoustic Holography is to measure cross-spectra between a set of reference transducers and the hologram microphones over a plane close to the radiating surface. From these measured cross-spectra a complete three-dimensional description of the sound field can be obtained, ie. sound intensity, particle velocity, sound pres-sure, sound power, radiation pattern, … and the location of the sound source. Once the acoustic source has been located it is a (relatively) straightforward exercise to predict what the sound field would be at any other location, assuming free-field conditions. Additionally, it is possible to simulate changes to the original sound source and predict, for ex-ample, the resulting far-field sound pressure level.
What if there is More than one Source?
Clearly, it is easy to predict the free field produced by a single point source. But the exercise rapidly becomes non-trivial when multiple point sources are present – a phase relation-ship between the sources must be established, assuming that source locations are known, and that they are mutually incoherent. Clearly for complex structures, such as a car engine, where even the number of sources is not initially known, the problem is significant. LMS CADA-X Acoustic Holography solves these problems. The principle is to use Principal Component Analysis to decompose the holographic plane measurements into a set of mutually independent point sources. Starting from these spatially and temporally identified ‘virtual’ sources, one can project onto the target plane using superposition principles. More details on this important subject can be found in this application case.
