Acoustic holography techniques for characterizing noisy industrial machinery

Outline of reasons and objectives
Acoustic holography is a technique that makes it possible to map the acoustic field at the surface of a noisy mechanical system on the basis of measurements taken in a parallel plane situated a small distance away. This mapping makes it possible to locate the sound sources and thus to define a strategy for reducing noise at source. Depending on the structure studied and on the type of sound signals emitted by the structure, various different holography methods can be implemented.
Approach
Three methods were initially considered, namely: measuring sound pressure (which is the usual method); measuring particle velocities; and Broadband Acoustic Holography based on Intensity Measurements (BAHIM), which, unlike the others, does not require reference signals (which are chosen empirically). Furthermore, the first two methods can apply only when the noise sources in the structures under study are fully correlated, which is not generally the case. This limitation has led to a fourth method being developed and implemented, namely Multi-Reference Near-field Acoustic Holography (MRNAH). Finally, since the BAHIM holography method was unsatisfactory, a fifth and new method, called CIBNAH (Complex Intensity Based Near-field Acoustic Holography), was developed.
All of these methods were applied to:
1) an academic structure having two well-known sources whose parameters are adjustable;
2) a table saw with several sources (motor, blade, radiating plates, air turbulence, exhaust, etc.);
3) an electricity generator set having several sources.
The results are presented in the form of maps of the sound pressure fields and of particle velocity Application to the academic structure made it possible to adjust and assess each method and to compare them. Application to the machinery made it possible to test the accuracy of the results by comparison between measurements and calculations, in the presence of complex acoustic fields.
Main results
With the academic structure, the MRNAH multi-reference method was accurate in showing the sources, their positions, and their levels. The BAHIM method proved to be highly inaccurate. The CIBNAH method showed the same effectiveness as the MRNAH method. Comparison of the calculations for reconstructing the sound fields a small distance away from the surface of the saw by MRNAH and CIBNAH with direct measurements at the same distance away, showed that both approaches worked, CIBNAH offering greater accuracy at the high frequencies. Another positive point is that, when it is performed by measuring particle velocity, which decreases faster than sound pressure, holography allows the measurement plane and thus the measurement effort to be smaller than when measuring sound pressure.
Discussion
Among the holography methods developed and applied to an academic structure and then to machinery, two methods (MRNAH and CIBNAH) proved to be satisfactory. In addition, CIBNAH offers the advantage of being simple mathematically, and of not requiring reference signals.