Reverberation: models, estimation and application

The use of reverberation models is required in many applications such as acoustic measurements, speech dereverberation and robust automatic speech recognition. The aim of this thesis is to investigate different models and propose a perceptually-relevant reverberation model with suitable parameter estimation techniques for different applications. Reverberation can be modelled in both the time and frequency domain. The model parameters give direct information of both physical and perceptual characteristics. These characteristics create a multidimensional parameter space of reverberation, which can be to a large extent captured by a time-frequency domain model. In this thesis, the relationship between physical and perceptual model parameters will be discussed. In the first application, an intrusive technique is proposed to measure the reverberation or reverberance, perception of reverberation and the colouration. The room decay rate parameter is of particular interest. In practical applications, a blind estimate of the decay rate of acoustic energy in a room is required. A statistical model for the distribution of the decay rate of the reverberant signal named the eagleMax distribution is proposed. The eagleMax distribution describes the reverberant speech decay rates as a random variable that is the maximum of the room decay rates and anechoic speech decay rates. Three methods were developed to estimate the mean room decay rate from the eagleMax distributions alone. The estimated room decay rates form a reverberation model that will be discussed in the context of room acoustic measurements, speech dereverberation and robust automatic speech recognition individually

High Frequency Reproduction in Binaural Ambisonic Rendering

Humans can localise sounds in all directions using three main auditory cues: the differences in time and level between signals arriving at the left and right eardrums (interaural time difference and interaural level difference, respectively), and the spectral characteristics of the signals due to reflections and diffractions off the body and ears. These auditory cues can be recorded for a position in space using the head-related transfer function (HRTF), and binaural synthesis at this position can then be achieved through convolution of a sound signal with the measured HRTF. However, reproducing soundfields with multiple sources, or at multiple locations, requires a highly dense set of HRTFs. Ambisonics is a spatial audio technology that decomposes a soundfield into a weighted set of directional functions, which can be utilised binaurally in order to spatialise audio at any direction using far fewer HRTFs. A limitation of low-order Ambisonic rendering is poor high frequency reproduction, which reduces the accuracy of the resulting binaural synthesis. This thesis presents novel HRTF pre-processing techniques, such that when using the augmented HRTFs in the binaural Ambisonic rendering stage, the high frequency reproduction is a closer approximation of direct HRTF rendering. These techniques include Ambisonic Diffuse-Field Equalisation, to improve spectral reproduction over all directions; Ambisonic Directional Bias Equalisation, to further improve spectral reproduction toward a specific direction; and Ambisonic Interaural Level Difference Optimisation, to improve lateralisation and interaural level difference reproduction. Evaluation of the presented techniques compares binaural Ambisonic rendering to direct HRTF rendering numerically, using perceptually motivated spectral difference calculations, auditory cue estimations and localisation prediction models, and perceptually, using listening tests assessing similarity and plausibility. Results conclude that the individual pre-processing techniques produce modest improvements to the high frequency reproduction of binaural Ambisonic rendering, and that using multiple pre-processing techniques can produce cumulative, and statistically significant, improvements

An Automated Study of Antioxidant Potentials of Polar Extract of Turmeric as Influenced by Ultraviolet Radiation

Turmeric polar extract (TPE) was obtained by dielectric-precipitation of turmeric slurry and found to contain three proteins with two in the 10-11 KDa range being dominant. Antioxidative activity and persistence (AP) of TPE (5%, w/v) respectively showed 87% and 85% greater generation of alkoxy- and peroxyl radicals compared the non-redox-active buffer alone showing significant (p\u3c0.05) pro-oxidative behavior. Conversely, purified curcumin (CU) (0.1% w/v) was dramatically antioxidative with AA and AP values of 2,828 and 1,129%, respectively, compared to the blank. However, a combination of the two at the same concentration dropped these values to 590 and 389%, respectively, reflecting dramatic dampening of the efficacy of CU. Ultraviolet radiation significantly modulated the efficacy of CU where UVB (300 nm) exposure gave the highest enhancement when limited to five min. Data showed that turmeric contains highly pro-oxidant polar proteins that significantly dramatically diminishes the beneficial antioxidative efficacy of its principal phytochemical, CU

An Automated Study of Antioxidant Potentials of Polar Extract of Turmeric as Influenced by Ultraviolet Radiation

Turmeric polar extract (TPE) was obtained by dielectric-precipitation of turmeric slurry and found to contain three proteins with two in the 10-11 KDa range being dominant. Antioxidative activity and persistence (AP) of TPE (5%, w/v) respectively showed 87% and 85% greater generation of alkoxy- and peroxyl radicals compared the non-redox-active buffer alone showing significant (p\u3c0.05) pro-oxidative behavior. Conversely, purified curcumin (CU) (0.1% w/v) was dramatically antioxidative with AA and AP values of 2,828 and 1,129%, respectively, compared to the blank. However, a combination of the two at the same concentration dropped these values to 590 and 389%, respectively, reflecting dramatic dampening of the efficacy of CU. Ultraviolet radiation significantly modulated the efficacy of CU where UVB (300 nm) exposure gave the highest enhancement when limited to five min. Data showed that turmeric contains highly pro-oxidant polar proteins that significantly dramatically diminishes the beneficial antioxidative efficacy of its principal phytochemical, CU

Towards the Perceptual Optimisation of Virtual Room Acoustics

In virtual reality, it is important that the user feels immersed, and that both the visual and listening experiences are pleasant and plausible. Whilst it is now possible to accurately model room acoustics using available scene geometry in real time, the perceptual attributes may not always be optimal. Previous research has examined high level control methods over attributes, yet have only been applied to algorithmic reverberators and not geometric types, which can model the acoustics of a virtual scene more accurately. The present thesis investigates methods of perceptual control over apparent source width and tonal colouration in virtual room acoustics, and is an important step towards and intelligent optimisation method for dynamically improving the listening experience. A review of the psychoacoustic mechanisms of spatial impression and tonal colouration was performed. Consideration was given to the effects early of reflections on these two attributes so that they can be exploited. Existing artificial reverb methods, mainly algorithmic, wave-based and geometric types, were reviewed. It was found that a geometric type was the most suitable, and so a virtual acoustics program that gave access to each reflection and their meta-data was developed. The program would allow for perceptual control methods to exploit the reflection meta-data. Experiments were performed to find novel, directional regions to sort and group reflections by how they contribute to an attribute. The first was a region of in the horizontal plane, where any reflection arriving within it will produce maximum perceived apparent source width (ASW). Another discovered two regions of and unacceptable colouration in front of and behind the listener. Any reflection arriving within these will produce unacceptable colouration. Level adjustment of reflections within either region should manipulate the corresponding attributes, forming the basis of the control methods. An investigation was performed where the methods were applied to binaural room impulse responses generated by the custom program in two different virtual rooms at three source-receiver distances. An elicitation test was performed to find out what perceptual differences the control methods caused using speech, guitar and orchestral sources. It was found that the largest differences were in ASW, loudness, distance and phasiness. Further investigation into the effectiveness of the control methods found that level adjustment of lateral reflections was fairly effective for controlling the degree of ASW without affecting tonal colouration. They also found that level adjustment of front-back reflections can affect ASW, yet had little effect on colouration. The final experiment compared both methods, and also investigated their effect on source loudness and distance. Again it was found that level adjustment in both regions had a significant effect on ASW yet little effect on phasiness. It was also found that they significantly affected loudness and distance. Analysis found that the changes in ASW may be linked to changes in loudness and distance