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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Sunday, January 08, 2012

Human Sound Can be Affected by Helium and Sulfur Hexafluoride, How?

Here is the theory: We produce sound from our vocal cords. To vibrate that vocal cords, we need the air to facilitate it. Air is a mixture of 78.03% nitrogen, 20.99% oxygen, 0.94% argon, 0.03% carbon dioxide, 0.01% hydrogen, 0.00123% Neon, 0.0004% helium, 0.00005% krypton, 0.000006% xenon and the other gases in a very small percentage.

The speed of sound is variable and depends on the properties of the substance through of which the wave is travelling. In solids, the speed of longitudinal waves depend on the stiffness to tensile stress, and the density of the medium. In fluids, the medium's compressibility and density are the important factors.

In gases, compressibility and density are related, making other compositional effects and properties important, such as temperature and molecular composition. In low molecular weight gases, such as helium, sound propagates faster compared to heavier gases, such as xenon (for monatomic gases the speed of sound is about 75% of the mean speed that molecules move in the gas). For a given ideal gas the sound speed depends only on its temperature. At a constant temperature, the ideal gas pressure has no effect on the speed of sound, because pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. In a similar way, compression waves in solids depend both on compressibility and density—just as in liquids—but in gases the density contributes to the compressibility in such a way that some part of each attribute factors out, leaving only a dependence on temperature, molecular weight, and heat capacity (see derivations below). Thus, for a single given gas (where molecular weight does not change) and over a small temperature range (where heat capacity is relatively constant), the speed of sound becomes dependent on only the temperature of the gas. In non-ideal gases, such as a van der Waals gas, the proportionality is not exact, and there is a slight dependence of sound velocity on the gas pressure. Humidity has a small but measurable effect on sound speed (causing it to increase by about 0.1%-0.6%), because oxygen and nitrogen molecules of the air are replaced by lighter molecules of water. This is a simple mixing effect.

The speed of sound in a medium (gas ideal) can be calculated by the equation as follows:

cideal is the speed of sound in an ideal gas.
R (approximately 8.3145 J·mol−1·K−1) is the molar gas constant.
k is the Boltzmann Constant.
γ (gamma) is the adiabatic index (sometimes assumed 7/5 = 1.400 for diatomic molecules from kinetic theory, assuming from quantum theory a temperature range at which thermal energy is fully partitioned into rotation (rotations are fully excited), but none into vibrational modes. Gamma is actually experimentally measured over a range from 1.3991 to 1.403 at 0 degrees Celsius, for air. Gamma is assumed from kinetic theory to be exactly 5/3 = 1.6667 for monoatomic molecules such as noble gases).
T is the absolute temperature in kelvin.
M is the molar mass in kilograms per mole. The mean molar mass for dry air is about 0.0289645 kg/mole (Remember this one only for this topic!).
m is the mass of a single molecule in kilograms.

Now, what is the effect of our sound when we inhale Helium gas or Sulfur hexafluoride gas?
Helium is less dense than the air (Ar = 0.004002602 kg/mole). See the equation and you found that the mass is inversely with the speed of sound. We can analog this problem with the effusion effect (Graham's Law), but that's don't make any sense actually, hehehe.

Because of this weight, Helium can effuse about 7.236 times faster than the normal air, makes our vocal chords vibrate more frequently and makes our sound become lighter.

Sulfur hexafluoride (Ar = 0.145962489920 kg/mole) is denser than the air. It makes this gas effuse about 0.198 times slower than the normal air, makes our vocal chords vibrate less frequently and makes our sound become heavier.

See the video below to prove it (Source: Youtube):

Further Reading . . .

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