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This paper reports on an investigation into the effects of varying amounts of background noise on sound power measurements using the two microphone sound intensity approach. Two sets of measurements were taken on machine sources; one set at a distance of approximately 0.5m and the other very close to the sources. Results show that the sound intensity method is a viable technique for measuring the sound power of a source and that indicators can be obtained which show when sound power results are unreliable.

In order to identify noise sources in a diesel engine, specifically exciting forces such as combustion and piston slap, the so-called coherence method which utilizes relationships between the auto and cross spectra of cylinder pressure, cylinder liner acceleration and engine noise has been examined. Also, as an alternative, a multivariable regression analysis in one-third octave band auto spectra of each signal mentioned above has been made. It was shown that the simple coherence model studied does not seem useful for this particular type of noise source identification problem. However, the multivariable regression approach has yielded fairly reasonable results, though some problems have been found in accuracy. From this research, it was found that combustion noise is predominant for heavier engine load conditions, though at lower load and high speed conditions, piston slap noise becomes appreciable.

Existing techniques for identifying noise sources are reviewed. One such technique (the lead-wrapping approach) was used to source-identify a diesel engine and measure the sound power radiated from the major surfaces. Two new advanced techniques (surface intensity and acoustic intensity) were developed and used to measure the sound power radiated from the same major surfaces. The conventional lead-wrapping and new intensity results were compared and agreement was good. The advantages of the new intensity techniques are described and suggestions made for reducing the noise of this and other similar diesel engines.