Sound transmission analysis by sound intensimetry.

Abstract

This thesis represents the development and evaluation of a theory for sound transmission analysis by sound intensimetry. In the context of this study sound transmission analysis is understood to embrace the following: (1) The measurement of sound reduction indices. (2) Diagnostic analysis of sound transmission through panels and structures. The sound intensity method is examined against the theoretical background of the classic two-room method which forms the basis of currently used international standards. The flanking problem, which is one of the principle limiting factors in the use of the classic method, is analyzed. The standard formulation of the intensity method is expanded to account for leakage error, boundary interference effects and calibration mismatch. It is shown that the commonly observed low-frequency discrepancy between intensity and classic method results is resolved by application of the Waterhouse correction. Sound absorption by the test object on the receiving side is shown to cause an error which increases with the flanking factor and with the fraction of the receiving room absorption located on the surface of the test object. Guidelines are developed for the assessment and control of absorption error in practical situations. Using the common mode rejection index as a performance rating for sound intensity meters, the measurement of sound transmission in reactive fields is investigated. Derivation of a formula for the reactivity near the surface of a transmitting panel surrounded by a flanking structure in a reverberant field, leads to the development of a theoretical framework and criteria for the planning and evaluation of test arrangements for sound transmission analysis. Guidelines are given for the calculation of minimum receiving room absorption and the microphone spacing required in practical situations. A study of the characteristic properties of sound intensity fields in diffuse and non-diffuse environments is used as a basis in formulating a new method of measuring directional diffusivity. Based on the relationship between reactivity and the degree of directional balance in a sound intensity field, this method involves spatial averaging of the pressure level and determination of the magnitude of the total intensity vector at the point under consideration. A direct-reading diffusivity meter has been developed and employed in assessing diffusivity in practical situations. The effect of a lack of directional diffusivity on the accuracy of sound transmission analysis in reactive fields is examined. Criteria for calculating minimum diffusivity requirements in the source and receiving room are developed and evaluated experimentally.