Low-Frequency Sound Wave Propagation at Open-Air Concerts: Acoustic Metamaterial’s Audience Phenomenon

Over the past decade, there has been growing concern about how the audience affects sound wave propagation at open-air concerts, particularly in the low-frequency range. The key lies in the difference between the conditions when the system engineer sets up the mid-high units and the subwoofers before the show, without the audience, and when a dense crowd occupies the audience area during the performance. In this paper it will be shown that the audience behaves as an equivalent medium resulting in similar phenomena to those found in some acoustic metamaterials, delaying the sound propagation compared with the air above the audience. This delay affects the entire sound wave, modifying its phase and magnitude as it propagates. The front-of-house engineer can experience these phenomena, and the analysis of how it affects the front-of-house position, whether raised or on the ground, is required to properly set up the sound system within the varying conditions.