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GRD Terminology

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

Part Number(s):

Air FlowSupply/ReturnSupply OnlySupply Only
DamperNoYesMost of the Time
MarketResidential/CommercialResidential OnlyResidential/Commercial
  1. NC Criteria
  2. Velocity Limitations

Communication EnvironmentTypical Occupancy
< NC 25Extremely quiet environment; suppressed speech is quite audible; suitable for acute pickup of all sounds.Broadcasting studios, concert halls, music rooms.
NC 30Very quiet office; suitable for large conferences; telephone use satisfactory.Residences, theaters, libraries, executive offices, directors rooms.
NC 35Quiet 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 40Satisfactory for conferences at a 6-to 8-foot table; normal voice 6 to 12 feet; telephone use satisfactory.General office, labs, dining rooms.
NC 45Satisfactory 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 50Unsatisfactory 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.
ApplicationRecommended Face Velocities
Broadcasting Studios<500 FPM
Residences500 to 750 FPM
Apartments500 to 750 FPM
Churches500 to 750 FPM
Hotel Guestrooms500 to 750 FPM
Legitimate Theaters500 to 1000 FPM
Private Offices, acoustically treated500 to 1000 FPM
Private Offices, not treated1000 to 1250 FPM
Motion Picture Theaters1000 to 1250 FPM
General Offices1250 to 1500 FPM
Stores, upper floors1500 FPM
Stores, main floors1500 FPM
Industrial Buildings1500 to 2000 FPM

Introduction to Grilles, Registers and Diffusers