Directivity Factor

Directivity factor (Q) is the ratio of the sound intensity produced by a source in a specific direction to the intensity that would be produced by an omnidirectional source of equal power at the same distance. Q = 1 for an omnidirectional source in free field, Q = 2 on a reflective plane (half-space), Q = 4 at a wall-floor junction (quarter-space), and Q = 8 in a corner (eighth-space). For directional sources like horns and loudspeakers, Q can be much higher and varies with frequency. Directivity factor determines the direct sound level at a given distance and angle, and consequently the direct-to-reverberant ratio and critical distance. Higher Q extends the critical distance, improving speech intelligibility. Directivity factor appears in the fundamental room acoustics equation and is essential for PA system design, where directional loudspeakers concentrate energy on the audience while minimizing excitation of reverberant surfaces.