Assessing the sound directivity of ducts based on time delay spectrometry

The sound directivity factor is an important parameter for describing the spatial distribution of acoustic energy radiated by duct terminations. Nevertheless, the available standard techniques for measuring the directivity factor rely on the free-field condition, which implies on the availability of a non-reflective environment, such as an anechoic chamber. In this paper, a technique known as time delay spectrometry is used to determine the directivity factor and the sound reflection coefficient of an open unflanged pipe in a normal reflective environment. The results show a good agreement between the experimental data and the theory. Details of the experimental procedures, as well as the discussion on the advantages and shortcomings of the method are presented in this letter.