question: Why does sound travel faster through helium if it is a less dense gas than air since sound travels faster through solids and liquids surely the speed of sound should be less in helium?
comment: Helium is much lighter than the nitrogen and oxygen that makes up the majority of air. So, the sound waves will cause the helium molecules to move faster than they would air molecules.
passages: This gives the 9% difference, and would be a typical ratio for speeds of sound at room temperature in helium vs. deuterium, each with a molecular weight of 4. Sound travels faster in helium than deuterium because adiabatic compression heats helium more since the helium molecules can store heat energy from compression only in translation, but not rotation. Thus helium molecules (monatomic molecules) travel faster in a sound wave and transmit sound faster. (Sound travels at about 70% of the mean molecular speed in gases; the figure is 75% in monatomic gases and 68% in diatomic gases).
   In low molecular weight gases such as helium, sound propagates faster as compared to heavier gases such as xenon. For monatomic gases, the speed of sound is about 75% of the mean speed that the atoms move in that gas.
   In fluids, only the medium's compressibility and density are the important factors, since fluids do not transmit shear stresses. In heterogeneous fluids, such as a liquid filled with gas bubbles, the density of the liquid and the compressibility of the gas affect the speed of sound in an additive manner, as demonstrated in the hot chocolate effect.
   The speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract. The fundamental frequency (sometimes called pitch) does not change, since this is produced by direct vibration of the vocal folds, which is unchanged. However, the higher resonant frequencies cause a change in timbre, resulting in a reedy, duck-like vocal quality. The opposite effect, lowering resonant frequencies, can be obtained by inhaling a dense gas such as sulfur hexafluoride or xenon.
   A graph comparing results of the two equations is at right, using the slightly different value of for the speed of sound at .
  
  Section::::Details.
  
  Section::::Details.:Speed of sound in ideal gases and air.
  
  For an ideal gas, "K" (the bulk modulus in equations above, equivalent to C, the coefficient of stiffness in solids) is given by
  
  thus, from the Newton–Laplace equation above, the speed of sound in an ideal gas is given by
  
  where
   In gases, adiabatic compressibility is directly related to pressure through the heat capacity ratio (adiabatic index), while pressure and density are inversely related to the temperature and molecular weight, thus making only the completely independent properties of "temperature and molecular structure" important (heat capacity ratio may be determined by temperature and molecular structure, but simple molecular weight is not sufficient to determine it).
   The molecular composition of the gas contributes both as the mass (M) of the molecules, and their heat capacities, and so both have an influence on speed of sound. In general, at the same molecular mass, monatomic gases have slightly higher speed of sound (over 9% higher) because they have a higher "γ" (...) than diatomics do (). Thus, at the same molecular mass, the speed of sound of a monatomic gas goes up by a factor of
   In the rest frame of the atoms with velocity, formula_8, in the atomic beam, the frequency of the laser beam is shifted by formula_9. In the presence of a magnetic field formula_10, the atomic transition is Zeeman shifted by an amount formula_11 (where formula_12 is the magnetic moment of the transition). Thus, the effective detuning of the laser from the zero-field resonant frequency of the atoms is
  
  The atoms for which formula_14 will experience the largest acceleration, namely
  
  where formula_16 and formula_17.
   Because of its low molecular weight, helium enters and leaves tissues more rapidly than nitrogen as the pressure is increased or reduced (this is called on-gassing and off-gassing). Because of its lower solubility, helium does not load tissues as heavily as nitrogen, but at the same time the tissues can not support as high an amount of helium when super-saturated. In effect, helium is a faster gas to saturate and desaturate, which is a distinct advantage in saturation diving, but less so in bounce diving, where the increased rate of off-gassing is largely counterbalanced by the equivalently increased rate of on-gassing.
   Since the "Hindenburg" disaster in 1937, helium has replaced hydrogen as a lifting gas in blimps and balloons due to its lightness and incombustibility, despite an 8.6% decrease in buoyancy.
   BULLET::::- The diffusion issue shared with Hydrogen (though, as Helium's molecular radius is smaller, it diffuses through more materials than Hydrogen).
  
  BULLET::::- Helium is expensive.
   Helium II is a superfluid, a quantum mechanical state (see: macroscopic quantum phenomena) of matter with strange properties. For example, when it flows through capillaries as thin as 10 to 10 m it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current theory explains this using the "two-fluid model" for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a ground state, which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.
   Helium II also exhibits a creeping effect. When a surface extends past the level of helium II, the helium II moves along the surface, against the force of gravity. Helium II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 nm-thick film regardless of surface material. This film is called a Rollin film and is named after the man who first characterized this trait, Bernard V. Rollin. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the van der Waals force. These waves are known as "third sound".
   Here it is the case that "v" = "fλ".
  
  Section::::Experimental methods.:High-precision measurements in air.
   However, helium is costly and its flow rate, and thus consumption, is higher. To improve acceleration capability, nitrogen gas is heated up to about 900 °C. As a result, deposition efficiency and tensile strength of deposits increase.
  
  Section::::Cold spraying.:Warm spraying.
   BULLET::::- Helium is used in helium–neon lasers and in some thyratrons rated for high currents and high voltages. Helium provides about as short deionization time as hydrogen, but can withstand lower voltage, so it is used much less often.
   The speed of sound in an ideal gas depends only on its temperature and composition. The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior.
   Helium speech unscramblers are a partial technical solution. They improve intelligibility of transmitted speech to surface personnel.
  
  Section::::Video communications.
   For instance, sound will travel 1.59 times faster in nickel than in bronze, due to the greater stiffness of nickel at about the same density. Similarly, sound travels about 1.41 times faster in light hydrogen (protium) gas than in heavy hydrogen (deuterium) gas, since deuterium has similar properties but twice the density. At the same time, "compression-type" sound will travel faster in solids than in liquids, and faster in liquids than in gases, because the solids are more difficult to compress than liquids, while liquids in turn are more difficult to compress than gases.
   BULLET::::- Any increase in gas fraction of nitrogen in the decompression gas should be limited to 1/5 of the decrease in gas fraction of helium.
  
  This rule has been found to successfully avoid ICD on hundreds of deep trimix dives.
  
  Section::::Ultrasonic bubble detection in decompression research.
   The speed of sound in helium is about three times that in air, and in hydrogen 3.8 times that in air. The speed of sound also increases with the temperature of the fluid (but is independent of the pressure), so the heat formed by the compression of the working fluid serves to increase the maximum possible speed. Spring piston airguns increase the temperature of the air in the chamber by adiabatic heating; this raises the local speed of sound enough to overcome frictional and other efficiency losses and propel the projectile at more than the speed of sound in the ambient conditions.
   In the Earth's atmosphere, the chief factor affecting the speed of sound is the temperature. For a given ideal gas with constant heat capacity and composition, the speed of sound is dependent "solely" upon temperature; see Details below. In such an ideal case, the effects of decreased density and decreased pressure of altitude cancel each other out, save for the residual effect of temperature.
   Section::::Non-gaseous media.
  
  Section::::Non-gaseous media.:Speed of sound in solids.
  
  Section::::Non-gaseous media.:Speed of sound in solids.:Three-dimensional solids.
  
  In a solid, there is a non-zero stiffness both for volumetric deformations and shear deformations. Hence, it is possible to generate sound waves with different velocities dependent
   The thermal conductivity of nitrogen and oxygen are very similar, and that of helium very different so that the ratio of oxygen and nitrogen in the mix is relatively unimportant, and need not be compensated. This allows a direct reading of helium fraction from these instruments. However, for greater accuracy and compensation to oxygen cross-sensitivity, some instruments include an oxygen cell, and in these cases can generally give a full helium and oxygen analysis of the mixture simultaneously.
  
  Section::::Speed of sound principle.
   Speeds of sound of the elements
  
  The speed of sound in any chemical element in the fluid phase has one temperature-dependent value. In the solid phase, different types of sound wave may be propagated, each with its own speed: among these types of wave are longitudinal (as in fluids), transversal, and (along a surface or plate) extensional.
  
  Section::::Notes.
  
  BULLET::::- Ref. CRC: Values are "at room temperature" unless noted, and "for normal atmospheric pressure" ("at 1 atm" for gases).
  
presuppositions: Because helium is a less dense gas than air, sound should travel slower within it. 
corrections: Helium is lighter than both nitrogen and oxygen which makes up the majority of air, due to this, sound waves will cause helium to move faster than they would inside of air molecules. 
raw_presuppositions: Because helium is a less dense gas than air, sound should travel slower within it.  Sound should travel slower in helium.
raw_corrections: Helium is lighter than both nitrogen and oxygen which makes up the majority of air, due to this, sound waves will cause helium to move faster than they would inside of air molecules.  Sound travels faster in helium because it is lighter.