= Acoustic directivity
{title2=$p\sim r^{-(d-1)/2}\mathcal D(\widehat{\mathbf x})e^{i(k_0r-\omega t)}$}
Acoustic directivity describes the angular dependence of radiation in the <acoustic far field>. For a time-harmonic source in $d$ spatial dimensions, separating the geometric spreading and oscillatory phase leaves a complex amplitude pattern $\mathcal D$. The angular intensity pattern is proportional to $|\mathcal D|^2$. Directivity can be normalized by a reference direction or total radiated power; a stated convention is needed when comparing absolute directivity factors.
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