Air registers are a necessity in any home or business. However, depending on the air flow volume coming through the register, they may be prone to excessive noise. In a home theater or office space, this noise can approach unacceptable levels. Choosing the proper registers for a given space mitigates this noise problem, making the environment quieter and more enjoyable.
Why Lowering Noise is Important
Regardless of the type or style of vent, the proper size vent must be chosen for the space. This is determined by adding air volume (in cubic feet per minute, or CFM) with the diameter of the duct leading to the vent. Once this is determined, you can find the airflow and select the right vents accordingly. Too small, and the vent with vibrate from air pressure. Too large, and it will simply look out of place and won't bolt properly to the ductwork. Since heating and cooling systems run all year long in most climates, the noise will never abate if the improper register vents are selected.
Linear-style vents are available that can direct air to the desired location. However, specialized vents are available that are adjustable to control the airflow volume entering the space. The volume of air relative to the opening of the vent's grille is in direct proportion to the amount of noise the vent generates. Louvered vents are especially adept at this, since they will only open in proportion to the amount of air passing through it. Slot diffuser-style vents are manually adjustable for maximum control.
Calculating Register Size
Register vent sizing is dependent on the amount of air passing through it. For example, a 12-by-12-inch register should see no more than 329 CFM of airflow, or the effectiveness of it is reduced. In terms of noise, adding more air adds extra pressure, causing the louvers to vibrate. Additionally, the constrained air will cause the duct to rattle and cause a "whooshing" noise, present with increased air velocity caused by the inadequate duct. Placing your thumb over a running water hose is an example of how constraining flow increases pressure, and noise at the exit point.
Choosing the quietest register vents has the added benefit of lowering energy costs. The proper register lowers the demands placed on the HVAC system's blower, since the proper air pressure throughout the ducting is maintained. Proper pressure at the register openings equals reduced noise. Additionally, since an even temperature is maintained in the space, the heating/cooling system lowers its on/off cycles, since the system only runs at the right times. It is important that the right air return registers are selected in relation to the vent registers, allowing the proper cycle of air to pass through the system.