Line Source Directivity

by Jay Mitchell

Just how important is line source directivity. Jay Mitchell provides a graph explaining it.

A recent thread on the listserv raised the issue of the directivity of a continuous line source. Several participants asked why such directivity would be undesirable. The accompanying graphic provides the answer.

The directivity of a ribbon tweeter is quite close to that predicted by the mathematical model of a finite, continuous line source, as the test data below make clear. At off-axis angles, a line source acts like an FIR lowpass filter. Increasing the angle decreases the cutoff frequency and shifts the stopband ripples to lower frequencies. The tweeter tested has an active source length of 5 1/2″. These results may be scaled to accurately predict the behavior of a line source of any length.

Line Source Directivity - Ribbon Tweeter Normalized Responses

Responses at 10-degree intervals normalized to on axis