Four printed circuit board array microphones of model 130E20 (PCB Piezotronics) are used for measuring the sound pressure inside the duct. These microphones are referred to as "Mic.1," "Mic.2," "Mic.3," and "Mic.4." A two-load method is used to measure the transmission loss (TL) of the duct silencer; one of the advantages of this method is that it does not need an absorption anechoic end,