Acoustic Double Holography Method and its Application to Engine Noise Research

The acoustic holography (AH) method with single measuring plane has been well known as the conventional method and can be implemented by far field measurement with simple instruments. However, the noise source resolution of the AH is not sufficient. In order to improve the resolution in the noise source identification, several kinds of the acoustic holography methods have been so far proposed. For example, the near field acoustic holography (NAH) can provide high and accurate resolution of the holography by the nearfield measurement. However, the nearfield measurement within one wave length is sometimes impossible in the actual circumstances. The Acoustic Double Holography (A D H) proposed in this paper is a simplified approach with higher resolution of the noise source locations than that of the conventional AH methods. The ADH method basically uses dual measuring planes and does not require nearfield measurement. The sound pressure data detected on the rear plane are transformed into the virtual pressure data on the front plane taking into account of the distance between the plane and the object. Comparing the virtual pressure data with the actual data measured on the front plane, resolution on holography can be improved significantly. Computer simulation and an experiment with two loud speakers were executed in order to confirm the fundamental feature of the proposed method. Several advantages on the method with respect to resolution over the conventional AH method were discussed. Furthermore, the ADH measurement was carried out on running engine under the full load operation. Through these results, the highly noise radiating areas on the engine surface were detected and reduced with noise shielding material. The overall engine noise level was reduced by 1.5dBA as the first stage in this noise control work.