He (and SF6)

As the video demonstrates, the amusement potential of helium remains undiscovered within the latex of the pretty balloons at your little sister's birthday. The symbol "He" might as well stand for "Hours of Entertainment". But how does this rising gas bring your voice along for the ride?

The essential characteristic of helium that enables this effect is its lower density, specifically compared to that of normal air. Imagine bringing an empty, sealed water bottle to the bottom of a pool. Intuitively, we all know that the bottle will rapidly rise to the surface. This is because the bottle weighs less than the medium (water in this case) that the bottle has displaced, or taken the place of. Similarly, helium is less dense than plain air and therefore weighs less than it in the same area, causing the balloon to experience buoyancy, albeit in a medium of air.

Another result of lower density, apart from floating felines, is the way in which sound moves through it. The speed of sound varies with the square root of the density of the gas. In layman's terms, the denser the gas, the slower the sound. In helium, sound moves 2.7 times as fast as in plain air, and with sulfur hexafluoride 0.44 times as fast. With a faster speed comes a higher frequency and pitch, and vice versa. 

In honor of The Martian, I found that the composition of Mars' atmosphere is mostly carbon dioxide, which is denser than "Earth air". That would mean that if I somehow breathed the poisonous air and lived, I would speak in a lower pitch. Cool, huh?

Sources:

Landsberg, Randy. "Balloon Laboratory." (2002): n. pag. Kavli Institute for Cosmological Research. University of Chicago. Web. 22 Oct. 2015.

Sprott, Clint. "Sound." Sprott's Gateway. University of Wisconsin - Madison, n.d. Web. 22 Oct. 2015.

Your Voice on Mars. Perf. Dr. Bruce Betts. The Planetary Society. N.p., 23 Oct. 2014. Web. 26 Oct. 2015.

Good Vibes

Even prior to the annual science units on the moon phases, the identity of sound as a wave has been ingrained into every child's grey matter. Sound is a wave. Sound is a wave. Sound is a ...wait, what's a wave?

The ripples of a pond disturbed by a pebble are an example of waves. The oscillating rope below is an example of a wave. A wave is an energy-transferring disturbance in a medium, or the substance through which the wave moves. For instance, in the first example the disturbance is the rippling of the water surface and the medium is the water.

To no one's surprise, a dichotomy between types of waves exists. A sound wave would be classified as a mechanical wave, in which the particles of the medium experience motion. This is in contrast to an electromagnetic wave which requires no medium and deals with light, not sound, so we shall pay it no more heed.

Furthermore, a sound wave is further categorized as a longitudinal wave, as opposed to a transverse wave. A transverse wave is one in which the particles of the medium move perpendicular to the direction of the wave. Since a picture is worth about $300 million, a graphic is probably appropriate.

As you can see in the above GIF, the motion of the wave is quite apparently moving to the right. However, the individual components of the medium (people) are moving vertically. The water ripples and the twanged rope are both examples of transverse waves.

However, sound is not a transverse wave but a longitudinal wave in which the particles of the medium travel parallel to the wave's motion. Cue another fabulous visual by Daniel A. Russell.

Similar to the "people-wave", the motion of this wave is to the right. Unlike the previous wave, however, the motion of each individual particle is side-to-side (parallel to the motion of the wave!). Applying this model to sound, we can imagine all the black specks as various molecules in the air. The red bar on the left is a speaker which compresses the air, creating a disturbance that is propagated through the medium. This disturbance eventually reaches our eardrums, where it is captured (much like a wild gazelle) and sent to the brain to be analyzed. Therefore, sound is simply a transmitted vibration of the air.

Sources:

Knight, Randall Dewey, Brian Jones, and Stuart Field. "Traveling Waves and Sound." College Physics: A Strategic Approach. 3rd ed. N.p.: Pearson Education, n.d. N. pag. Print.

Russell, Daniel A. Multiple Wave GIFs. Digital image. Acoustics and Vibration Animations. N.p., 18 Feb. 2015. Web. 20 Oct. 2015.

"What Is a Wave?" What Is a Wave? The Physics Classroom, n.d. Web. 20 Oct. 2015.