When a hex nut is made to rotate in an elastic rubber balloon, a "screaming" sound can be heard. This is easily reproducible by hand using commercial nuts and balloons. In this article, we aim to understand the origin of that sound, and how it depends on the parameters of the system.
We first focus on the frequential components of the sound, then on the amplitude components. Once the whole sound has been analyzed, we provide an explanation for the "screaming" behavior of the sound.
Experimental setup
Our experiments can be divided into two parts, by hand and mechanized.
Each experiment is done using a new (non inflated before) balloon, in which a hex nut is put before inflating it by breathing in.
The sound produced by the experiments in recorded by a microphone at a fixed distance from the center of the balloon at rest. Videos of the experiments are tracked by a high-speed camera.
Experiment by hand
A human operator translates the balloon in a circular motion at a constant angular speed, maintaining a trajectory as close to the horizontal as possible. The translation speed is afterwards extracted from the sound as explained in [XX].
The human operator holds the balloon as shown in [FIG], the only contact being between his fingers and the top of the balloon to minimize modifications to the boundary conditions.
Mechanized experiment
We use a lab shaker to control the translation speed and trajectory of the balloon. The balloon has to be held in place by {metal bits} attacked to the shaker, only lightly touching the balloon.
{Données add => comment initialiser le mvt dans le shaker}
General tools used in our experiments
- Audacity 2.2.1, Fourier transforms of size 65536 using a Hanning window
- Calibrated-response microphone SM57
- Phantom high-speed camera [Ajouter modèle précis]
- [XX] lab shaker
Young modulus measurement
Commercial balloons come without any information on their elastic properties, which we therefore measured.
The measurement is done by cutting a stripe of known dimensions from a new balloon and analyzing its elongation for a given stress. Thickness was measured at recurrent intervals. From the deformation curve and the thickness variation we deduce the Young's Modulus and Poisson ratio of the balloon. Since the measure destroys the balloon and all experiments were made with new balloons, we assume that the Young's Modulus and Poisson ratio are constant among a batch of balloons.