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. This thesis presents an investigation on how exactly the presence of the audience influences the propagation of sound. A mathematical model of an audience as an equivalent fluid is constructed with the help of the porous medium theory. The equivalent fluid parameters, such as the wave impedance, the speed of sound and the reflection coefficient from the audience/air boundary, are calculated both for a constant and a variable concentration of an audience. The model is verified by a Boundary Element Method simulation, scale measurements and live concert measurements. A new live measurement method is developed to evaluate the low frequency sound pressure level distribution without disturbing the ongoing event. 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.