Directivity Index

The directivity index D_Ω (=K₀) is used to account for reflecting surfaces near the source into angles of less than 4π steradians (sr). A point source with no obstacles nearby radiates into an angle of 4π steradians.

\(K_0=10\lg(\frac{4\pi}{\Omega})dB\)

with the angle Ω (sr).

For an angle of 2π sr (e.g. a source near the ground) a correction of 3 dB applies.

In CadnaA, however, the level increase due to ground reflection is calculated automatically for both, for the alternative (A-weighted) as well as for the general method (frequency dependent). The value to be entered on the point source’ dialog is, therefore, the directivity index not considering the ground (thus „without ground“, i.e. ignoring the ground).

So, in order to specify the directivity index all reflecting surfaces besides the ground are relevant for the value of K₀ in CadnaA.

Examples for D_Ω in ISO 9613-2 (resp. K₀ in VDI 2714)

point source above ground: solid angle 2π, K₀ w/o ground = 0 dB

point source in front of a wall: solid angle π, K₀ w/o ground = 3 dB

point source in a corner: solid angle π/2, K₀ w/o ground = 6 dB