Assessing the quality of low frequency audio reproduction in critical listening spaces

The quality of low frequency audio reproduction in small spaces has always been problematic. For some time, methods have been suggested in order to optimise this reproduction. Many such methods have been based upon objective metrics which remain unproven from a subjective perspective. Whilst perception has been studied, this thesis identifies a research gap for more thorough testing. A series of listening tests has been conducted, with virtual rooms auralised and presented over headphones in order to isolate specific modal parameters and allow efficient collection of subjective response from many listening environments. The work presented searches for optimal values and perceptual thresholds of three parameters - modal spacing, density and decay. Results show that optimal spacings and densities may only be defined where assumptions are made which are not valid in realistic listening spaces. Thresholds of modal decay1 have been defined, which are considered valid re- gardless of stimuli or replay level. These are around 0.2 seconds for frequencies above 100Hz, and increase sharply below this point to around 0.85 seconds at 32Hz. Through the testing of these parameters, it is shown that whilst discrimination between two rooms is usually a simple task, this does not reveal the underlying repro- duction quality. The perceived quality of the room response is of great importance, and new experiments assess this quality using a paired comparison method which provides a simpler subjective task than direct scaling methods. A set of descriptors is elicited which can be used to evaluate low frequency audio. These descriptors - articulation, resonance and bass energy - are used to assess the impact of three room parameters on perceived reproduction quality. Room response metrics are also eva- luated for perceived quality. Results reveal that modal decay is a primary indicator of quality, with shorter decays producing rooms with a higher perceived quality of reproduction.

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