What can cause noisy airflow in a duct?

Noisy airflow in a duct is often caused by high velocity. When air moves rapidly through a duct, it can create turbulence and generate sound waves, leading to noticeable noise. High velocities can result from improperly sized ducts or an excessive flow rate beyond what the duct system can handle effectively. This noise can manifest as a whooshing or whistling sound as the air forces its way through bends, turns, or constricted passages within the ductwork.

The other factors listed may contribute to airflow dynamics but do not directly link to the generation of noise in the same manner. For instance, the airflow direction can influence airflow efficiency or pressure but does not inherently cause noise. The type of material used for the ducts can affect the sound absorption or transmission characteristics, but by itself, it does not determine the noise level caused by airflow. Likewise, the size of the duct might affect velocity, but it is the high velocity that is specifically responsible for creating noise.