9,650 research outputs found
A target guided subband filter for acoustic event detection in noisy environments using wavelet packets
This paper deals with acoustic event detection (AED), such as screams, gunshots, and explosions, in noisy environments. The main aim is to improve the detection performance under adverse conditions with a very low signal-to-noise ratio (SNR). A novel filtering method combined with an energy detector is presented. The wavelet packet transform (WPT) is first used for time-frequency representation of the acoustic signals. The proposed filter in the wavelet packet domain then uses a priori knowledge of the target event and an estimate of noise features to selectively suppress the background noise. It is in fact a content-aware band-pass filter which can automatically pass the frequency bands that are more significant in the target than in the noise. Theoretical analysis shows that the proposed filtering method is capable of enhancing the target content while suppressing the background noise for signals with a low SNR. A condition to increase the probability of correct detection is also obtained. Experiments have been carried out on a large dataset of acoustic events that are contaminated by different types of environmental noise and white noise with varying SNRs. Results show that the proposed method is more robust and better adapted to noise than ordinary energy detectors, and it can work even with an SNR as low as -15 dB. A practical system for real time processing and multi-target detection is also proposed in this work
Multiresolution analysis of active region magnetic structure and its correlation with the Mt. Wilson classification and flaring activity
Two different multi-resolution analyses are used to decompose the structure
of active region magnetic flux into concentrations of different size scales.
Lines separating these opposite polarity regions of flux at each size scale are
found. These lines are used as a mask on a map of the magnetic field gradient
to sample the local gradient between opposite polarity regions of given scale
sizes. It is shown that the maximum, average and standard deviation of the
magnetic flux gradient for alpha, beta, beta-gamma and beta-gamma-delta active
regions increase in the order listed, and that the order is maintained over all
length-scales. This study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length-scales in the
active region, and not just those length-scales at which the strongest flux
gradients are found. Further, it is also shown that the average gradients in
the field, and the average length-scale at which they occur, also increase in
the same order. Finally, there are significant differences in the gradient
distribution, between flaring and non-flaring active regions, which are
maintained over all length-scales. It is also shown that the average gradient
content of active regions that have large flares (GOES class 'M' and above) is
larger than that for active regions containing flares of all flare sizes; this
difference is also maintained at all length-scales.Comment: Accepted for publication in Solar Physic
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