The Propagation of Low Frequency Sound through an Audience. Part 1

Authors: Shabalina, Elena
Publication Date: 01.08.2015

Frequent concert or open-air festival visitors might have noticed that at large events subwoofers, the dedicated low frequency loud- speakers, are often placed in a row in front of the stage as an evenly spaced array (Fig. 1). The reason for this is the directivity control that can be achieved by using beamforming techniques. Opposite to the standard left/right set-up which creates strong interference throughout the listening area, a carefully designed subwoofer array provides an even sound pressure level distribution across the listening area, and keeps the sound away from the stage and the neighbours.
However, at large outdoor events the audience often stands tightly packed in front of the stage (Fig. 2) and the sound from the subwoofer array propagates partly through the crowd and partly above it. Live sound engineers notice the difference in the tonal balance in an empty venue and with the audience present, but they have different opinions on the topic. Some say the bass becomes louder when the audience is present, some say it is quieter, so we’ve decided to find out how much of the low frequency energy is actually coming through the crowd. The question is: does it make any sense to work on the beamforming algorithms for subwoofers, if all the sound is absorbed by the audience anyway?
In the part 1 of this article (Acoustics Bulletin July-August 2015) we discussed the diffuse field absorption of the human body at low frequencies and a mathematical model of an audience as a porous medium. Part 2 covers a Boundary Element simulation of an audience as a set of hard cylinders along with a scale meas- urement, and a series of live concert measurements of the sound propagation through a crowd of people.