This paper presents results of the development of an acoustic device to be utilized as a duct termination with variable reflection coefficient. This study is motivated by the idea to experimentally evaluate the probability of instability of a thermo-acoustic system where combustion acts as an active acoustic element, and this termination acts as a passive acoustic element that can be configured to a desired value of the reflection coefficient at the upstream side of the flame and burner for lab-scaled physical modelling of, for instance, domestic boilers. This termination consists of a cylinder containing a stack of truncated hollow cones with narrow gap in between and a telescopic tube. The gap between the adjacent cones, and sound-absorbing fibrous material (“Acotherm”) placed in the cavity of these cones produce a low reflection coefficient in the frequency range between 40 and 800 Hz. Longitudinal displacement of these cones inside the cylinder generates a reflection coefficient with magnitude ranging from 0.2 to 0.9. The telescopic tube with an adjustable length (between 0.85 - 1.38 m) allows to achieve a wide range of phases of reflection coefficient. The steps taken to optimize the design and performance of this termination in presence of flame are presented here.