Noise attenuation characteristics of in-duct acoustic screens are studied. Transmission loss was measured for various acoustic screens with different geometry. It was observed that sound path asymmetry - which arises when an acoustic screen is installed so as to divide the original duct into two asymmetric channels and recombine - gives rise to large narrowband sound attenuation. Particular attention was paid to understanding the narrowband sound attenuation mechanism. Based on the physical insight gained from the experimental and numerical investigations, an analytical model which accurately describes the narrowband noise attenuation was developed.