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Opposed Blade Damper - OBD

Determine the sound level which results from the combined effects of several sound sources is not as difficult as it is confusing.

  • The NV data for diffusers given contains an allowance for the sound absorbing properties of the average room and its contents.

  • The Absorption is assumed to be 8 db with sound power level referenced to 10Watts.  (The absorption is 18db referenced to 10Watts)

  • For relatively small spaces - about 76.2m2 or less floor area and ceiling height of 3.0m or less - the following simplified method for estimating NC levels produced by a combination of supply diffusers and return registers or grilles can be used

  1. Determine the difference in NC level between the supply outlets or return intakes having the highest NC and the second highest NC level.

  2. From Table 9 determine the number of decibels to be added to the NV level of the unit having the highest NC level.  This sum is the combined NC level generated by the two units.

  3. If three units serve the space, determine the difference between the combined NC level of the first two units and the NC level of the third unit.  Determine the NC addition as above, and add this to the combined NC level of the first two units.

If the difference between NC levels of two units is 10db or more, the sound generated by the quieter unit will not affect the space NC.


Opposed Blade Damper manufactured from extruded aluminiunm Blades. Blades are held in place by spring wire and starlock push on fix.

The OBD blades are linked and lever or slot operated.

OBD suits all standard grilles and diffusers.

Blade Options:     Standard as per sketch
Finish Options: PR = Primed Black (Optional)
  MF = Mill finish (Standard)
Blade Spacings 25mm    Standard
Ordering Procedure: Example
2 10 381 x 381 OBD - STD MF TO FIT CD

(1)  Dimensions given are for - To ftt ON of mother grille
(2)  If OBD to ftt grille, indicate type of grille, outside neck size.

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