This study focuses on acoustic silencers on indoor-use doors positioned in an array. In this paper, we discuss our studies of side branch silencers which are built in the ventilation door. By designing and fabricating a special attachment, we were able to attach silencers to the measurement tube that had larger cross sections than the impedance measurement tube. Using this attachment, we measured acoustic transmission losses for each of six different prototype silencers. By adding orifice silencer effects, we obtained transmission losses across a wide frequency range. However, attenuation decreased in frequency ranges below the attenuation peak of the side branch tubes. A wedge-shaped longitudinal cross section for the side branch tubes can double the length of the silencer and develop lower-frequency attenuation effects, while maintaining the same volume. This wedge-shaped side branch tube is suitable for a long silencer placed side by side in an array formation We performed theoretical analyses of acoustic transmission losses of silencers using the transfer matrix method based on a one-dimensional wave equation. We performed element decomposition on the transfer matrix for silencers with side branch tubes with a wedge-shaped longitudinal cross section. We obtained calculated results that were sufficiently valid compared to our experimental measurements.