Coherent backscattering of ultrasound without a source

Coherent backscattering is due to constructive interferences of reciprocal paths and leads to an enhancement of the intensity of a multiply scattered field near its source. To observe this enhancement an array of receivers is conventionally placed close to the source. Our approach here is different. In a first experiment, we recover the coherent backscattering effect (CBE) within an array of sources and a distant receiver using time correlation of diffuse fields. The enhancement cone has an excellent spatial resolution. The dynamics ofthe enhancement factor is studied in a second experiment using correlation of thermal phonons at the same ultrasonic frequencies, without any active source.