Abstract
The acoustic behavior of a double annular resonator with flow is studied as a special case to illustrate the possibilities, but also the difficulties, offered by flow on the acoustic performance of metamaterials. Compared with the associated single resonator, the double resonator exhibits – in the lossless case – a transmission peak near the resonant frequency. This peak is associated with an evanescent out-of-phase coupling between the two resonators and is known in literature as “Autler-Townes splitting”. In measurements with a double resonator, this peak is strongly attenuated by viscous effects, to such an extent that it almost disappears. When a flow is added, even a very small one, a gain is created and the peak reappears close to the resonator frequency. As the average flow velocity increases, this gain can become sufficiently large for a whistling to appear.