it maybe acting as an acoustic lens.

https://www.youtube.com/watch?v=RLBmWF9Xo10

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.

Somebody wrote a paper on it in 2008: https://physics.byu.edu/docs/publication/644

  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

Here's Harvard demonstrating it too: https://sciencedemonstrations.fas.harvard.edu/presentations/...

So would a wall of helium balloons work as a sound dampener? Or perhaps diffuser? A lot cheaper than fiberboard & foam

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.

Makes sense

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.

Oh dang! I bet that's it! Nice find, thank you!

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.

My wife makes balloon arches and I've never noticed but this sounds fascinating. Maybe I'll get her to work, for "science". :P

Note: Her balloons are not typically helium filled, so the other users question about air/helium might make a difference.

The latex from which the balloon is made is a decent soundproofing material, so what you're hearing is likely just that.

Having filled a room with balloons, this is very noticeable. It also make heating the room difficult.

heating difficult? Why would that be?

Pretty much kills convection. Each balloon is like an insulation cell.

Holy cow, I read your comment and thought, "Wait, I didn't write that!" Nice to meet you, doppleganger.

Could it be an interaction between static electricity from the balloon and tiny elements in the ear?

Do you see a difference regardless of helium or air? I assume from your description that it has to be inflated to work.

Helium is apparently great for soundproofing: https://www.youtube.com/watch?v=jxOzpPJbnTI (a great demonstration of sound suppressing properties of helium)

I am a light sleeper and would love to use it somehow in my bedroom, but keeping it contained is tricky.

im curious about replicating this

It seems like the effect would be very frequency specific.

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.

Definitely

Balloon hats for autistic and sensitive people.