Cancellation of Radiated Sound Fields by the Use of Multipole Secondary Sources

In this paper the cancellation of sound fields in the region exterior to a primary source by the use of a single, higher order, secondary source is discussed. The nature of the secondary source or sources required to achieve that cancellation, either globally or in particular regions, is a subject of current interest. It has recently been suggested that the sound radiation from coherent, finite size radiators may be represented as a superposition of monopole sound fields or alternatively as the sound field radiated by a single multipole. It has also been observed that the sound field radiated by a monopole source may be represented by an infinite order multipole source placed elsewhere. In the same way, multipole sources of finite order may be represented by displaced multipoles of higher order. In principle, it is thus possible to create a single multipole secondary source that could represent, and then if operated out of phase with respect to the primary source, cancel the sound field generated by an arbitrary coherent radiator. In this paper, the results of a theoretical and experimental study to determine the feasibility of such an approach are presented. Secondary sources of up to octupole order have been realized using loudspeakers and the farfield attenuation they may achieve has been established as a function of frequency, secondary source order and primary-secondary source separation distance. Excellent agreement between measurement and theoretical prediction has been obtained. It will be shown in particular that useful levels of low frequency global farfield attenuation may be obtained using secondary sources truncated at octupole order and positioned at a relatively large fraction of a wavelength from the primary source. In addition, it will be shown that the secondary multipole components’ strengths may be adjusted to achieve cancellation in specific angular sectors. It has thus been concluded that the approach suggested here may find applications in active control of low frequency exterior sound fields

The Cenozoic volcanic province of Tibesti (Sahara of Chad): major units, chronology, and structural features

International audienceUsing both field relationships and some absolute ages, the sequence of volcanic units in the Cenozoic Tibesti Volcanic Province (TVP) (Chad) is established as follows: (1) plateau volcanism, between at least 17 and 8 Ma, consisting of flood basalts and silicic lava plugs, with intercalated ignimbritic sheets in the upper basalt succession increasing in amount upwards. Ages decrease from NE to SW, following the migration of the small NW-SE flexures concentrating the feeding dike swarms; (2) Late Miocene large central composite volcanoes exhibiting diverse and original structures. Some of them (Tarso Toon, Ehi Oyé, and Tarso Yéga) are located along a major NNE fault, representing the main tectonic direction in Tibesti since Precambrian times; (3) construction of three large ignimbritic volcanoes, associated with significant updoming of the basement, ending with the collapse of large calderas: Voon (about 5–7 Ma), Emi Koussi (2.4–1.33 Ma), and Yirrigué (0.43 Ma); (4) basaltic activity, starting at about 5–7 Ma, and essentially consisting of cinder cones and associated lava flows (Tarso Tôh, Tarso Ahon, and Tarso Emi Chi); and (5) final volcanic activity represented by post-Yirrigué caldera activity in the Tarso Toussidé Volcanic Complex, and especially Ehi Toussidé (the only active volcano in Tibesti), plus Ehi Timi and Ehi Mousgou volcanoes, similar to Ehi Toussidé. The two tectonic directions controlling some volcanic features of the province correspond to the major old lithospheric structures delimiting the volcanic province, namely, the great NW-SE Tassilian flexure to the SW and a major NE-NNE fault zone to the E. Unusual conditions of uplift and erosion in the TVP enable exceptional exposure of the internal structure of its volcanoes