Directivity Factor (Q)

Directivity factor quantifies how focused a sound source is. It compares the on-axis intensity to what would exist if the same total power were radiated uniformly in all directions. The related Directivity Index (DI) expresses the same concept in decibels: DI = 10 × log10(Q). For a source radiating into full space (suspended in air, away from boundaries), omnidirectional radiation gives Q=1, DI=0 dB. Place that source on a reflective floor (half-space) and Q doubles to 2, DI=3 dB. In a corner (one-eighth space), Q=8, DI=9 dB. This boundary loading effect is why subwoofers placed near walls and corners produce more output. Horn-loaded loudspeakers achieve high directivity through waveguide control. A typical constant-directivity horn with 90° × 40° coverage might have Q=10 (DI=10 dB) at frequencies above 2 kHz. This means the on-axis SPL is 10 dB higher than the average SPL, and almost all of that energy reaches the audience rather than hitting walls and ceilings. High directivity provides three major benefits. First, it increases the critical distance, placing more listeners in the direct-sound-dominant zone. Second, it reduces excitation of the reverberant field, improving intelligibility in reverberant spaces. Third, it improves gain before feedback because less energy reaches microphones that are off-axis. SonaVyx's speaker measurement tool calculates directivity index from multi-angle measurements, showing how Q varies with frequency. This data is essential for predicting coverage and critical distance in real venue installations.