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computational auditory scene analysis
Computational auditory scene analysis (CASA) is the study of auditory scene analysis by computational means.〔Wang, D. L. and Brown, G. J. (Eds.) (2006). ''Computational auditory scene analysis: Principles, algorithms and applications''. IEEE Press/Wiley-Interscience〕 In essence, CASA systems are "machine listening" systems that aim to separate mixtures of sound sources in the same way that human listeners do. CASA differs from the field of blind signal separation in that it is (at least to some extent) based on the mechanisms of the human auditory system, and thus uses no more than two microphone recordings of an acoustic environment. It is related to the cocktail party problem.
==Principles==
Since CASA serves to model functionality parts of the auditory system, it is necessary to view parts of the biological auditory system in terms of known physical models. Consisting of three areas, the outer, middle and inner ear, the auditory periphery acts as a complex transducer that converts sound vibrations into action potentials in the auditory nerve. The outer ear consists of the external ear, ear canal and the ear drum. The outer ear, like an acoustic funnel, helps locating the sound source.〔Warren, R.(1999). ''Auditory Perception: A New Analysis and Synthesis''. New York: Cambridge University Press.〕 The ear canal acts as a resonant tube (like an organ pipe) to amplify frequencies between 2-5.5 kHz with a maximum amplification of about 11dB occurring around 4 kHz.〔Wiener, F.(1947), "On the diffraction of a progressive wave by the human head". ''Journal of the Acoustical Society of America'', 19, 143–146.〕 As the organ of hearing, the cochlea consists of two membranes, Reissner’s and the basilar membrane. The basilar membrane moves to audio stimuli through the specific stimulus frequency matches the resonant frequency of a particular region of the basilar membrane. The movement the basilar membrane displaces the inner hair cells in one direction, which encodes a half-wave rectified signal of action potentials in the spiral ganglion cells. The axons of these cells make up the auditory nerve, encoding the rectified stimulus. The auditory nerve responses select certain frequencies, similar to the basilar membrane. For lower frequencies, the fibers exhibit “phase locking”. Neurons in higher auditory pathway centers are tuned to specific stimuli features, such as periodicity, sound intensity, amplitude and frequency modulation.〔
There are also neuroanatomical associations of ASA through the posterior cortical areas, including the posterior superior temporal lobes and the posterior cingulate. Studies have found that impairments in ASA and segregation and grouping operations are affected in patients with Alzheimer's disease.〔Goll, J., Kim, L. (2012), "Impairments of auditory scene analysis in Alzheimer's disease", ''Brain'' 135 (1), 190–200.〕
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