This paper describes a special mathematical images model to determine the sound level inside a close-fitting sound enclosure. Such\nan enclosure is defined as the internal air volume defined by a machine vibration noise source at one wall and a parallel reflecting\nwall located very close to it and acts as the outside radiating wall of the enclosure. Four smaller surfaces define a parallelepiped\nfor the volume.The main reverberation group is between the two large parallel planes. Viewed as a discrete line-type source, the\nmain group is extended as additional discrete line-type source image groups due to reflections from the four smaller surfaces. The\nimages group approach provides a convergent solution for the case where hard reflective surfaces are modeled with absorption\ncoefficients equal to zero. Numerical examples are used to calculate the sound pressure level incident on the outside wall and the\neffect of adding high absorption to the front wall. This is compared to the result from the general large room diffuse reverberant\nfield enclosure formula for several hard wall absorption coefficients and distances between machine and front wall. The images\ngroup method is shown to have low sensitivity to hard wall absorption coefficient value and presents a method where zero sound\nabsorption for hard surfaces can be used rather than an initial hard surface sound absorption estimate or measurement to predict\nthe internal sound levels the effect of adding absorption.
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