Abstract
This paper presents the state of art of acoustical holography and its two applications in engine component design:
- The quantification of the contribution of different parts on an engine in running condition,
- The help for design of components by tests with artificial excitation.
Acoustical holography allows the representation of acoustical fields (maps) in the same way that intensity method. After measurement on a planar array, its backpropagation processing allows computing acoustical field (pressure or intensity) near the source. The advantage of this processing is to increase resolution, i.e. separation of two sources. This technique has the following capabilities:
- Compute partial power of different components,
- Estimate reduction of the noise emission of a component (after modification of this component) without disruption of the others sources,
- Measure in a room without acoustic treatment (measurement in nearfield) and compute the noise level at different distances (for example at 1 meter for test in an engine cell, or at 7.5 meters for test on a rolling drum).
This technique can be used for tests on engine in running conditions (engine cell or vehicle rolling on a drum). It is possible to:
- Compute the acoustic power,
- Visualise holograms for different frequency bands. An hologram shows noise radiation of the oil pan and the extremity of the crankshaft in a specific frequency band,
- Compute the contribution of acoustical power for different components.
At the end of the test, the technique gives an acoustical model of the engine.
Acoustical holography can also help to design a component in an acoustical point of view. It requires a test bench with artificial excitation (shaker or loudspeaker). This can be considered as representative of the real behaviour. Take the example of an air inlet system (air filter + pipes). An acoustic excitation at the end of the pipe can be considered as representative. In that case, the acoustical holography allows to:
- Compute the efficiency of the component,
- Localise the acoustic weakness.
This test can compare different solutions for these components and reduce the number of tests in real conditions, that require the whole engine.