The Nearfield Error In Determining the sound power – A Survey Of Present State Of Knowledge Together With Some Latest Findings

摘要

The realization of GUM (Guide to the expression of Uncertainty in Measurement) among others requires the elimination of systematic errors (bias) occuring if one physical quantity is measured according to different measurement procedures. This is the case for the sound power quantity when using sound pressure (ISO 3745, 3744, 3746) or sound intensity (ISO 9614) measurements. As known this nearfield bias is defined as the ratio of the sound powers measured on the enveloping surface either by sound pressure squared or by the intensity technique. An extensive investigation project dealing with the nearfield error was finished at the end of 2003 with the aim to summarize and supplement the relevant present state of knowledges and to discuss the questions whether relevant corrections can be introduces into the sound power measurement procedures. The paper will present some main topics of this report. The investigation dealt with a wide span of technical sound sources, several different models and practical machines and equipment, giving data in functions of frequency and measurement distance. One general rule was developed for the correlation of the near field error with the sound radiation factor σ: sound sources in ranges with small radiation factor (σ≤1) have large near field errors unfortunately. The possibility introducing a general near field correction in sound pressure frame documents such as ISO 3745, 3744, 3746 is regarded as not very succesful but its use in machinery specific test codes have a good and reasonable chance.