User Guide

Sound System Design Reference Manual
increases another 3 dB. Continuing on at D , we
place the sound source in a trihedral (3-sided)
corner, and we note an additional 3 dB increase as
sound power is radiated into one-eighth-space.
We could continue this exercise further, but our
point has already been made. In going from A to D in
successive steps, we have increased the directivity
index 3 dB at each step, and we have doubled the
directivity factor at each step.
We will now define these terms: Directivity
index is the level difference in intensity along a given
axis, and at a given distance, from a sound radiator
compared to the intensity that would be produced at
the same distance by an omnidirectional point source
radiating the same power. Directivity factor is the
ratio of the two intensities. Details are shown in
Figure 3-2. Directivity index (DI) and directivity factor
(Q) are related as follows:
DI = l0 log Q
Q = 10
DI/10
3-2
Figure 3-2. Directivity index and directivity factor
Figure 3-3. Illustration of Molloy’s equation