LF (Lateral Fraction)

Lateral Fraction (LF or LF80) is the ratio of laterally arriving early sound energy (0–80 ms) to total early energy, measured using a figure-of-eight microphone oriented to reject frontal sound and an omnidirectional microphone. Defined in ISO 3382-1:2009 §4.6, LF quantifies the spaciousness and envelopment perceived by listeners in concert halls. Values of 0.10–0.35 are typical, with higher values indicating greater apparent source width (ASW) and spatial impression. LF is a critical design parameter for concert halls because it correlates strongly with listener preference: halls rated as having "good acoustics" typically have LF values of 0.20–0.30 at mid-frequencies. Lateral reflections from side walls are the primary contributors to LF. Wide halls tend to have lower LF than narrow halls. LF measurement requires specialized equipment (coincident omnidirectional and figure-of-eight microphones) and is primarily used in performance space assessment and design verification.