It is seen that the directivity index increases as frequency increases.
Directivity patterns showed that for the simple planar speaker, directivity index increases as frequency increases.
One parameter commonly used to quantify the directionality of an electroacoustic device that receives sound (sound receiver), such as a hearing aid, is the directivity index D.
The directivity index quantifies the directivity by effectively comparing the reference-axis sensitivity of the device to the sensitivity of the device to sound arriving from all directions.
Directivity index measurements of hearing aids are made at NIST according to the general method specified in the relevant U.
When the directivity index measurement is implemented with the semi-aligned zone array sampling method that utilizes the forty eight sound source locations specified in the relevant U.
Some focused on the importance of getting a good directivity index between the direct sound and an average of only vertical dispersion curves taken over wide angles around the speaker.
The curve is then used to generate a second directivity index again using the direct sound measurement as the other parameter.
To get the directivity index, the loudspeaker is enclosed a spherical shell with microphones placed at regular intervals.
Two metrics characterize narrow-band arrays: the directivity index
and noise sensitivity.