This thesis proposes a nonlinear differential semblance approach to full waveform inversion as an alternative to standard least squares inversion, which cannot guarantee a reliable solution, because of the existence of many spurious local minima of the objective function for typical data that lacks low-frequency energy. Nonlinear differential semblance optimization combines the ability of full waveform inversion to account for nonlinear physical effects, such as multiple reflections, with the tendency of differential semblance migration velocity analysis to avoid local minima. It borrows the gather-flattening concept from migration velocity analysis, and updates the velocity by flattening primaries-only gathers obtained via nonlinear inversion. I describe a general formulation of this algorithm, its main components and implementation. Numerical experiments show for simple layered models, standard least squares inversion fails, whereas nonlinear differential semblance succeeds in constructing a kinematically correct model and fitting the data rather precisely.

Low frequencies in small rooms are a problem that can affect the sound quality and therefore give an impaired listening experience if not treated. Even the slightest change of the subwoofer position can make a difference. Measurements have been made for two different subwoofer positions in a small control room, called the original and the alternative position. The original position represents the position that is normally used for the chosen control room. The alternative position was selected by measuring the position that had the flattest frequency response. The positions were tested with listening tests by 20 audio engineering students to see what differences existed between the two. The result shows that the original position was perceived to have more bass, which could be connected to the measurements. The subjects also showed that they could agree on the meaning of three of the given attributes used for the listening tests. If a flatter frequency response is wanted for the chosen control room, the alternative position is an option.