Damper influence

Volume dampers are used to adjust the required volume for wall grilles.

These use impact losses to eliminate the excess pressure between the transfer grille and the ducting. For practical reasons the volume dampers are fitted just behind the diffuser, so that they are easily accessible. However, this means that closing the volume damper effects both the noise produced by the diffuser and the throw.
If it is unacceptable to use a volume damper behind the diffuser there are various other solutions available.