Observation of a coherent backscattering effect with a dipolar source for elastic waves: highlight of the role played by the source.

We report an experimental evidence of the role played by the source on the coherent backscattering effect (CBE) for elastic waves. The experiment is carried out in a chaotic cavity consisting of a silicon plate whose shape is a quarter stadium. With a monopolar source, it has already been shown that the time-integrated squared amplitude at the point source is twice as large as at the other points around the source. Here, by using a dipolar source, we show that we instead obtain two peaks with the same axis as the dipole one's. The shape of this "bicone" is well explained with a modal theory assuming that the source may be modeled by two sources with opposite phases. Then, the theory is generalized to any multipolar emitter and/or receiver. Particularly, we study CBE when emitter and receiver are reciprocal.