The propagation of low frequency sound through an audience

• Die Schallausbreitung durch Publikum bei tiefen Frequenzen

Shabalina, Elena D.; Vorländer, Michael (Thesis advisor)

Berlin : Logos-Verl. (2013, 2014)
Dissertation / PhD Thesis

In: Aachener Beiträge zur technischen Akustik 17
Page(s)/Article-Nr.: III, 71 S. : Ill., graph. Darst.

Zugl.: Aachen, Techn. Hochsch., Diss., 2013

Abstract

In this thesis it is investigated, how low frequency sound (30-100Hz) propagates through an audience. At large outdoor concerts or festivals subwoofers, the dedicated low frequency loudspeakers, are often placed in a row in front of the stage as an evenly spaced array in order to control the directivity of sound at low frequencies. The audience itself often stands tightly packed in front of the array, so that sound waves propagate partly through the audience and partly over it. The purpose of the present research is to investigate how exactly the presence of the audience influences the propagation of sound. A new live measurement method was developed to evaluate the low frequency sound pressure level distribution without disturbing the ongoing event. It was experimentally shown that there is a measurable difference between the sound pressure level distribution in an empty venue and in the presence of an audience. The sound decay with the distance tends to be less in the presence of an audience. A mathematical model of an audience as a porous medium was constructed. This model allows to calculate the wave impedance of an audience from its concentration, the modelling results correspond to Boundary Element Method (BEM) modelling of an audience as a set of hard upright cylinders. It was shown both analytically and with the use of a BEM-simulation that the finite height of an audience leads to a modal field within the audience as well as evanescent waves. The study reaches the conclusion that at low frequencies an audience forms a medium with the impedance significantly different from the impedance of the air, which leads to the reflection of sound waves from the boundaries back into the audience and increase of the sound pressure level as well as interference effects. Possible applications of the findings are mainly sound system design and event planning.