Abstract: An apparatus for damping acoustic waves that includes a volume of fluid having actively excited acoustic waves, an enclosure disposed in the volume of fluid including delimiting surfaces which define a cavity, at least one of the delimiting surfaces being permeable to the fluid, such that fluid may be exchanged between the cavity and the volume, and at least one sound producer disposed proximate the cavity to actively excite the actively excited acoustic waves in the fluid disposed in the cavity such that acoustic waves are damped in the volume.

Abstract: An apparatus for damping acoustic waves that includes a volume of fluid having actively excited acoustic waves, an enclosure disposed in the volume of fluid including delimiting surfaces which define a cavity, at least one of the delimiting surfaces being permeable to the fluid, such that fluid may be exchanged between the cavity and the volume, and at least one sound producer disposed proximate the cavity to actively excite the actively excited acoustic waves in the fluid disposed in the cavity such that acoustic waves are damped in the volume.

Abstract: In the optical measurement of the flow vectors in gas flows, two substantially parallel light beams (11a, 11b) are focussed at separate focussing spots. The particles passing the focussing spots light up and thereby generate a start pulse and a stop pulse, respectively. Therewith, the component of the flow vector extending in the normal plane to the optical axis (OA) is detected by transit time measurement. The flow component pointing in the direction of the optical axis (OA) is detected independent thereof according to the Doppler principle by supplying the scattered light generated by the particles to an optical frequency/amplitude converter (22).