Investigation of Methods Based on International Standards to Determine Sound Power of Machinery Noise Sources Under Hemi-anechoic Conditions

Abstract

The importance of reducing the noise exposure of workers to permissible levels via engineering methods is widely recognised in occupational noise criteria and regulations. To facilitate the use of engineering controls, machinery marketed within European Union (EU) countries must, under the EU Machinery Directive, include noise emission declarations. Two fundamental quantities of sound measurements: sound power levels and emission sound pressure levels are involved in noise emission declarations. The ISO 11200 series are basic standards that specify methods for measuring emission sound pressure levels at workstations and other specified positions around machinery and equipment. The ISO 3740 series and ISO 9614 series make up a set of basic standards to determine sound power levels of machinery and equipment. ISO 3741-3747 standards are used to determine sound power levels using sound pressure measurements in specific acoustical environments. The series ISO 9614-1 to ISO 9614-3 describes methods for determining the sound power in almost any environment using sound intensity. Both the series, 3740 and 9614, provide three measurement grades, precision, engineering and survey grades. The intent of this investigation is to identify sound power measurement standards that are both rapid and accurate. The sound power radiated by a source is determined by integrating the scalar product of the sound intensity vector and the associated elemental area vector over a surface fully enclosing the source. In practice, the shape of the enclosing surface, and the number and distribution of sampling points affects the accuracy and speed of measurements. It can take hours to obtain a large number of samples. An efficient measurement will reduce either the number of samples, or the time per sample. In this study a comparison is made of the speed and accuracy of determining sound power levels of noise sources under hemi-anechoic conditions according to ISO 3745 and ISO 3744 using sound pressure measurements and ISO 9614-1 to ISO 9614-3 using sound intensity measurements.