<p>Transfer function method impedance tube measurement is a well-known approach used for determining acoustical properties of mostly porous materials. This paper presents a prototype of impedance tube together with the software solution published under open-source license. The prototype is developed specifically for experimental design of Helmholtz resonators. For room acoustic purposes, these are difficult to sufficiently characterize by other methods because of their highly frequency dependent complex-valued pressure reflection factor resulting in large phase shifts happening upon reflection. The main benefits of the presented method are the movable rigid termination operated by a stepper motor and the use of open-source libraries as well as widely available hardware for the whole measurement.</p>