Contact center agents often work in close proximity to other agents. The diameter of the acoustic transducer is selected such that it is smaller than the diameter of the delay paths so that diffracted waves generated in the area of the edge of the acoustic transducer generate fewer parasitic echoes and surface waves on the boundary surfaces resulting in less corruption of the measured result. In one embodiment, the first working fluid comprises CO2. The radially extendable filter members have a circumferential dimension that is less than a longitudinal dimension thereof.