Acoustical enclosures are the common arrangements in reducing airborne noise from shipboardmachinery such as engines and generators. In this paper the theoretical models, established based onstatistical energy analysis, are presented for predicting the insertion loss of acoustical enclosures indifferent frequency ranges. In addition to the consideration of resonant modal coupling betweeninternal sound field and enclosure structural vibration, the nonresonant transmission though and theinteraction between enclosure walls in the models are also included. It is shown that the insertionloss of enclosures is mainly controlled by the nonresonant modes in the intermediate frequencyrange. At high frequencies, the insertion loss of enclosures can be improved by increasing the soundabsorption at the internal boundaries of enclosures. Experiments were carried out on two enclosuresmade of different materials. The measured results are compared with the predicted values and thegood agreement between them is the initial demonstration of the validity and feasibility of thetheoretical models. © 2004 Acoustical Society of America.