Noise protection in mobile compressor units

Abstract

The author has developed a classification of sound-lnsulation hoods based on the design features. The statistical theory of acoustics has been used to derive analytical formulae which describe the effectiveness of sound-insulation hoods. A method has been developed for calculating the values with higher accuracy than was feasible earlier [2]. The theoretical and experimental studies have led to the following conclusions. One of the main factors which determine effectiveness of sound-insulation hoods is the equivalent sound-absorption area. The use of a single sound-absorbing lining can increase the effectiveness of the hood by a factor of more than 2 in the relevant frequency range. It is to be noted that the effectiveness depends primarily on the area of the lining and the nature of the material, whereas the thickness of the lining has a much smaller effect. The effectiveness of the hood depends on the design. Thus, the sound insulation of a hood with holes is 7-14 dB higher than that of a hood with openings and slots of equal area. The noise in a MCU can be most effectively reduced by means of a sound-insulation hood with acoustic screens and mufflers at the openings. This reduces the sound pressure by 8-25 dB in the relevant frequency range. Sound-insulation hoods have been developed on the basis of theoretical and experimental studies, as well as the recommendations regarding selection and design. These hoods are intended for three main types of mobile compressor units: with diesel engines, with gasoline engines; and with electric motors. The approach to noise protection in compressor units is described for the typical case of a MCU with diesel engine. Let us first determine the average sound pressure at a distance L from the perimeter of the uninsulated unit according to the requirements in national and international standards. The acoustic characteristics of the MCU when working under load are shown in Table i.