Abstract
The rich literature on acoustic source localization mostly relies on the assumption of a constant value for the speed of sound. This hypothesis allows establishing simple relations between range differences and time differences and leads to effective estimation algorithms. However, it must be challenged for certain applications of wireless acoustic sensor networks in multizone buildings and outdoor environments. This article revisits the source localization problem for the more general case of an unknown value for the speed of sound. It reviews the physical foundations for the dependence of the speed of sound on the air temperature and presents the essential approaches to acoustic source localization. On this basis, several methods for source localization under uncertain or variable speed of sound conditions from the literature are discussed. Applications from different fields are shown. They comprise the localization of sources, sensors, and reflecting surfaces, time-difference-of-arrival disambiguation, and the direct determination of the speed of sound or the air temperature from acoustic measurements.
Highlights
Acoustic sensor networks attempt to record one or more desired sound sources in the presence of other unwanted sources
Which is a constant value along the propagation path. This analysis shows that the initial assumption of a circular shape of the propagation path is justified since the curvature is constant
The main causes of variations in the speed of sound are temperature differences along the propagation path. This effect is negligible in controlled indoor environments, in particular when air conditioning is in effect
Summary
Acoustic sensor networks attempt to record one or more desired sound sources in the presence of other unwanted sources. This article reviews the effect of temperature variations on sound propagation in air and further on the accuracy of source localization with acoustic sensors. These results are of interest to wireless acoustic sensor networks where synchronization between all sensors is not guaranteed. Unsynchronized communication between wireless acoustic sensors is studied in [11] where bearing-only information is exchanged between the network nodes In all these cases, the conversion from time delay estimates to range estimates (where required) is based on the assumption of a common and known speed of sound.
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