Apologies for the ADHD induced tangent. Has anyone else noticed that regular little party balloons seem to have a passive noise cancelling effect? If you bring them close to your ear there's a zone of 'dead air' when they are maybe an inch away. My theory was that there's something in passing through the rubber envelope that creates a phase delay or inversion, but it could just all be in my head lol.
edit: Steve Mould's video "I Made a Lens, But for Sound" demonstrates how balloons filled with gasses of different density than the surrounding air, act as a lens on sound waves. Helium filled balloons will scatter sound because the helium is less dense than air. He shows how a balloon filled with carbon dioxide can focus the sound.
A balloon filled with a gas that has a different sound speed than that of air has been used as an
acoustic lens. One purpose of the lens is to show refraction of sound waves in an analogy to
geometric optics. We discuss the physics of the balloon lens demonstration. To determine the
validity of a gas-filled balloon as a classroom demonstration of an acoustic lens and to understand
the corresponding phenomena, its physics is considered analytically, numerically, and
experimentally. Our results show that although a geometric analogy is a good first-order
approximation, scattering theory is required to fully understand the observed phenomena. Thus this
demonstration can be adapted to a wide range of students, from those learning the basic principles
of refraction to advanced students studying scattering
I doubt it would be cheaper than foam, but this is similar to gas filled windows. Argon or Krypton gas is pumped in-between the window panes to provide another layer of insulation.
At the old Exploratorium in the Palace of Fine Arts there was an exhibit that had a large 3-4m balloon filled with something heavy (Argon or maybe SF6?) and two points on the floor at the foci of the balloon. You could whisper at one focus and hear it easily at the other. I think it has been replaced with a more durable pair of concrete parabolic reflectors with similar effects.
Do other smooth surface spherical objects have the same effect?
Sound reflects off smooth surfaces. The ballon is probably just acting like any simple physical obstruction, because the surface does a lot all by itself even if theres almost no substance.
The air inside the ballon is also at a different density than outside, without helium, because of the elastic tension in the rubber. The air inside is always slightly compressed vs outside. I have no idea how much the two densities must differ to make the accoustic lense effect. I din't think it's this, just everyone seemed to be overlooking that even plain air will also have a different density.
Density does not affect the speed of sound of a gas. Temperature and molar mass does. So increasing the pressure of a given composition of gas won’t change anything, assuming you can bring the temp back down. Helium is obviously a different molar mass. But also dry air vs moist air can have an effect.
just the humidity level in the gas used to fill a balloon is going to have a significant effect on its properties, human breath will have more c02 that air and s higher humidity.
Any canned pressurised gas, will be pure
with zero humidity.
Heating exchange works through convection
cells, the greater the number and the smaller they are, the lower covective heat
transfer will happen, so filling a room with balloons, and you have made foam,
and at that point the acoustical properties must be pronounced
fun stuff
The activity of silk working as a sound absorber is its property of bieng
one of(the) best heat conductive substances, and as a basic fact, all sound
is eventualy turned into heat
The silk is presumably working to convert the mechanicsl motioninduced by sound into heat, quickly disapating it
and releasing it to the air.
