Directivity Index

Directivity Index (DI) is the logarithmic expression of the directivity factor, expressed in decibels: DI = 10×log₁₀(Q). An omnidirectional source has DI = 0 dB, a source on a reflective floor has DI = 3 dB, and a highly directional horn loudspeaker might achieve DI = 15–20 dB on-axis at high frequencies. DI is frequency-dependent for real sources: loudspeakers become more directional at higher frequencies as the wavelength becomes smaller relative to the radiating element. In room acoustics, DI is used to calculate the direct sound level at a receiver position and the critical distance. Higher DI values concentrate energy in the forward direction, benefiting speech reinforcement systems where sound needs to reach distant listeners while minimizing ceiling and wall reflections. DI data is provided by loudspeaker manufacturers and is essential for PA and sound reinforcement system design, as well as environmental noise prediction for directional industrial sources.