Dependency of threshold and loudness on sound duration at low and infrasonic frequencies

Many environmental sounds contain significant energy in the infrasonic and low-frequency (ISLF) ranges that have been associated with cases of annoyance and noise complaints. This study assessed the effect of sound duration on audibility and loudness of ISLF sounds. A first experiment evaluated detection thresholds for tones of 4, 16, and 32 Hz with durations up to 4000 ms. Furthermore, equal-loudness-level contours (ELCs) were obtained as function of duration up to 2000 ms. Tones of 1000 Hz were also included here. Results displayed the known pattern of general sound level decrease with increasing duration up to several hundred milliseconds. ELCs stabilized slightly earlier than thresholds, but after 1000 ms, levels remained roughly constant for both measures except for 4-Hz tones, where the decrease continued up to the longest durations tested. As 4-Hz cycles are perceptually resolved as separate pressure pulses, the authors hypothesized their duration dependence would resemble that of pulse trains. Hence, a second experiment evaluated pulse-train thresholds (1000-Hz carrier) for durations up to 4000 ms. For both pulse repetition rates of 4 and 32 Hz, threshold stabilized after 1000 ms as for tones ≥16 Hz, suggesting the continuing threshold decrease for a 4-Hz tone is specific to infrasound.