Studies on Low Frequency and Broadband Anechoic Termination Design

Fan is the main noise source in heating, ventilation and air conditioning (HVAC) duct systems, its sound power determination is thus particularly important for the low-noise design of HVAC system and ISO 5136 is usually recommended for its sound power measurement. The test results based on this international standard are closely related to the sound field in the duct. Due to the end reflection effect at the test duct outlet, an anechoic termination with broadband low and reflection is needed to eliminate the end reflection effect on the sound field measurement inside the duct. In this paper, the finite element (FEM) method is adopted to predict the reflection coefficients of two designed terminations. The effects of the flow resistance of the absorbing material, the thickness of the internal perforated tube and its aperture diameter on the anechoic performance of the terminations are fully discussed. The results show that the designed exponential anechoic termination meets the requirements presented in ISO 5136. Compared with the thickness and aperture diameter of the perforated plate, the flow resistance of the sound-absorbing material has a greater influence on the reflection coefficient of the anechoic termination. The anechoic termination proposed in this paper can provide further support for the sound power determination of the fan by using ISO 5316.