When HVAC systems are in operation, they can be quite noisy. Various electrical and mechanical devices in the HVAC system, such as fan, transmit noise when in operation. In addition, vibration of the system and pressure drops occuring at various points, such as when air passes through filters or dampers, also cause noise. Both circular and rectangular sound attenuators, also known as duct silencers, introduce sound absorbing media in the air stream, thus removing acoustic energy from the air as it passes. The result is a reduced overall HVAC noise.