The abbreviation GRD is commonly used for the grille, register, & diffuser (air distribution products) category.
It seems that over time, the terminology has become confused - for example, the term “vent” quite often used as a substitute for any one of the above.
Please note that Grilles are primarily used for return air, while Registers and Diffusers are strictly for supply air.
A Grille simply covers an opening - allowing air to pass through. It does not have an attached damper to restrict or allow air movement. However, a grille can be used for both supply air and return air. Both
A Register is a damper-equipped grille used for supply air. Generally, Registers are used in residential applications only.
A Diffuser is an air flow device designed to discharge air in a spreading pattern, specific path, or particular direction.
GRD Cross Reference
|Air Flow||Supply/Return||Supply Only||Supply Only|
|Damper||No||Yes||Most of the Time|
|Communication Environment||Typical Occupancy|
|< NC 25||Extremely quiet environment; suppressed speech is quite audible; suitable for acute pickup of all sounds.||Broadcasting studios, concert halls, music rooms.|
|NC 30||Very quiet office; suitable for large conferences; telephone use satisfactory.||Residences, theaters, libraries, executive offices, directors rooms.|
|NC 35||Quiet office; satisfactory for conference at a 15-foot table; normal voice 10 to 30 feet; telephone use satisfactory.||Private offices, schools, hotel guestrooms, courtrooms, churches, hospital rooms.|
|NC 40||Satisfactory for conferences at a 6-to 8-foot table; normal voice 6 to 12 feet; telephone use satisfactory.||General office, labs, dining rooms.|
|NC 45||Satisfactory for conferences at a 4- to 5-foot table; normal voice 3 to 6 feet; raised voice 6 to 12 feet; telephone use occasionally difficult.||Retail stores, cafeterias, lobby areas, large drafting and engineering offices, reception areas.|
|> NC 50||Unsatisfactory for conference of more than two or three persons; normal voice 1 to 2 feet; raised voice 3 to 6 feet; telephone use slightly difficult.||Computer rooms, stenographic pools, print machine rooms, process areas.|
|Application||Recommended Face Velocities|
|Broadcasting Studios||<500 FPM|
|Residences||500 to 750 FPM|
|Apartments||500 to 750 FPM|
|Churches||500 to 750 FPM|
|Hotel Guestrooms||500 to 750 FPM|
|Legitimate Theaters||500 to 1000 FPM|
|Private Offices, acoustically treated||500 to 1000 FPM|
|Private Offices, not treated||1000 to 1250 FPM|
|Motion Picture Theaters||1000 to 1250 FPM|
|General Offices||1250 to 1500 FPM|
|Stores, upper floors||1500 FPM|
|Stores, main floors||1500 FPM|
|Industrial Buildings||1500 to 2000 FPM|
Introduction to Grilles, Registers and Diffusers