Acoustic emission from successive impacts on elastic membranes: The physics of the screaming balloon

International audience ; When a hex nut or a ridged-edge coin, placed inside an inflated rubber balloon and spun vigorously, it emits a surprisingly loud and clear sound as the spinning object impacts the rubber and triggers vibrations of the membrane, a phenomenon known as the screaming balloon. We identify the mechanisms behind the acoustic emission and show that the fundamental frequency of the sound is given solely by the rate of successive impacts of the spinning object onto the membrane as it rolls without slipping. A counter-intuitive observation is that the acoustic power emitted by a given ridged-edge object remains independent of the size of the balloon (over a wide range of volume) in which it spins. This experimental finding is explained by the influence of the tension within the membrane on the acoustic intensity. Finally, we propose a scaling law for the frequency-dependence of the acoustic intensity and show that the sound level depends greatly on the number of ridges on the edge of the spinning object.