Unit Infrasound Activity: A Metric for Quantifying Signal Strength, Regularity, and Persistence in the 0.5–10 Hz Band

Conventional acoustic level metrics quantify infrasonic magnitude but provide limited information about how low-frequency activity is organized in frequency and time. This study introduces Unit Infrasound Activity (UIA), a single-channel signal descriptor for characterizing structured activity in the 0.5–10 Hz band through three components: Strength, Regularity, and Persistence. The method was evaluated using controlled synthetic signals, a reproducible Monte Carlo dataset of 240 randomized signals, a component-ablation analysis, and recordings from contrasting real-world environments. In the Monte Carlo analysis, median UIA was monotonically associated with a predefined synthetic activity proxy (Spearman ρ = 0.855), estimated Persistence closely tracked imposed temporal presence, and dominant-frequency estimation remained accurate for stable periodic signals. Ablation analysis showed that the three components are partially dependent and that simpler combinations emphasize different signal properties, while the full formulation retains sensitivity to magnitude, temporal recurrence, and frequency stability. Application to measured recordings further showed that similar aggregate UIA values can arise from substantially different combinations of Strength, Regularity, and Persistence. UIA is therefore proposed as a complementary continuous descriptor of infrasonic signal structure rather than as a conventional sound-level measure, binary detector, source-identification method, or measure of human response.