In the prediction and measurement of sound transmission through partitions, errors and discrepancies between predicted and measured data have been observed. Because the sound transmission loss was introduced as a characteristic property of the partition, the measured data for the same partition should be always identical within the meaningful statistical range. However, it has been known that the repeatability and reproducibility of measurement are not guaranteed in general. This is due to the basic assumptions adopted in the definition of the sound transmission loss. The effect of partition size is one of the causes of discrepancies, which has not been considered seriously in spite of its practical importance. The partition size has been usually assumed to be infinite according to the definition of transmission loss although the real partitions obviously have finite area not withstanding to the fact that some panels can be considered infinite. For noise control engineers, it is of interest to know what special features of a finite panel influence the sound transmission characteristics, which is the motive and main objective of the present work.
In this study, the sound transmission through a finite partition in an infinite rigid baffle is analyzed by using the classical modal approach. Partition is classified into three basic configurations: single panel, double panel with an air-gap, and foam-lined double panel. It is assumed that the edges of all panels are simply supported and excited by a planar acoustic wave having a fixed incidence angle. For each configuration, a prediction model for random incidence sound transmission is suggested by averaging over the incidence angle, in which the finiteness such as size and radiation factor is included as an analytical parameter. By comparing with the measured result, the validity of each prediction model is verified. It is shown that results from the present analysis differ from those of an analysis where the panel is assumed to be infinite. For the single and the double panel, approximated expressions for sound transmission loss are suggested, which can be used for rough prediction in the field.
The effect of finiteness on the sound transmission characteristics is investigated from the viewpoint of four features, which are distinguishable from the infinite partition: finite size including area and aspect ratio, vibration response, sound radiation characteristics, and additional energy loss from the finite boundaries. From the calculated results, the effect of each feature on the sound transmission was investigated. Then, the possible error of the infinite panel theory was suggested quantitatively.
