Directivity

The directivity in acoustics is a measure used to define the directional characteristics of a sound source or sound receiver. Sound sources usually produce different sound intensity in each spatial direction, while microphones may have different sensitivity in each spatial direction. Both of these devices can be therefore seen as spatial filters and the directivity describes that filter's characteristics.

In practice the far field directivity is most often used. In the far field it is adequate to only measure the sound pressure and omit the particle velocity.

The directivity is a 3-dimensional complex valued function and can be described with two 3D surfaces at a given frequency or frequency band: the first one is the magnitude-directivity and the second is the phase-directivity. In practice only a 2D plot of the magnitude directivity is given plotted at a fixed vertical angle in only usually wide-band. Usual measurement densities are 1 or 5 degress on the horizontal plane.

The peak point of the directivity is usually called the main direction or main lobe. The N dB angle range of this main lobe is used to measure how direct the source is which is called the beamwidth. A parameter Q is also often used as the ratio of the intensity (power density) of the main lobe and the total intensity (power density) of all directions.

Commonly used omni-directional microphones are omni-directional only up to a certain frequency from which they become more and more spatially selective.