Prediction of sound transmission in elongated or compact acoustic cavities

Abstract

The ability to predict sound fields in coupled volumes is important for noise control and acoustic quality with buildings, cars, aircraft and trains. This thesis investigates methods to assess the diffusivity of sound fields in rooms and the prediction of sound transmission between coupled volumes using statistical approaches. Sound fields in a box-shaped room were assessed using ray tracing with the spatial correlation coefficient for instantaneous sound pressure. The results were compared with the theory for a three-dimensional diffuse field and propagating plane waves. Three different options were considered for the measurement lines: (1) pairs of points formed by one fixed point when the other point varies along the same line, (2) pairs of points with fixed spacing and (3) all permutations of points with variable spacing. The general conclusion is that option (1) can lead to conclusions that seem inappropriate. Options (2) and (3) were found to have potential as assessment procedures, but definitively characterising a sound field as diffuse was not possible. Sound transmission between coupled volumes was investigated using an empty cuboid, a cuboid with staggered barriers and a car cabin model based on Statistical Energy Analysis (SEA) and Experimental SEA (ESEA). Experimental work on corridors was used to validate the ray tracing models. For sound transmission along an empty cuboid, the direct field was significant with highly absorptive surfaces such that a propagating two-dimensional model overestimated transmission for low absorption, and underestimated it for high absorption. SEA incorporating coupling loss factors from the general form of ESEA gave improved agreement with ray tracing and showed the importance of indirect coupling between subsystems. For a corridor with staggered barriers, source locations for the Power Injection Method used in ESEA were assessed to ensure accurate predictions of sound transmission along the corridor. For the corridor and car cabin, the general form of ESEA tends to always result in a working SEA model and be more accurate when a source position (point or surface) used for the power injection process is similar to the actual source position. This tends to be more apparent when using a single source rather than multiple sources.