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.S.
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.S.
The directivity index determined in this manner for a hearing aid under simulated real-ear conditions is referred to as the simulated real-ear aided directivity index, and the directivity index determined in this manner for the open ear configuration of the manikin is referred to as the manikin unaided directivity index.
Toole introduces the directivity index: the difference between a speaker's direct frequency response to the response over an averaged window of measurements across the horizontal and vertical planes of the speaker.
To get the directivity index, the loudspeaker is enclosed a spherical shell with microphones placed at regular intervals.
--Directivity index #1 is the first reflection response directivity index: The difference between the direct sound and the first reflections.
--Directivity index #2 is the directivity index: the difference between the direct sound and the total power response