Gases – Speed of Sound
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About these numbers
Reference values below are typical speeds of sound in selected gases near the stated temperatures at approximately atmospheric pressure. If you enable normalization, each row is rescaled to your target temperature using the ideal-gas square-root law \(c(T) \approx c_0\sqrt{T/T_0}\) (assuming the heat-capacity ratio and composition are unchanged).
- Unit switching: Choose m/s, km/h, mph, or ft/s.
- Sorting: Click any column header to sort (click again to reverse).
- Notes: The “Steam, 6 MPa” row reflects high pressure and is not normalized.
For precise engineering work, consult property libraries for the specific gas across temperature/pressure ranges.
Speed of Sound in Selected Gases
| Gas | Base Temp (°C) | Base Speed (m/s) | Shown Speed (unit-aware) | Notes |
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Understanding the Speed of Sound in Gases
The speed of sound tells you how quickly tiny pressure disturbances travel through a gas. For an ideal gas, the governing relation is \( c = \sqrt{\gamma\,R\,T} \), where \( \gamma = c_p/c_v \) (heat-capacity ratio), \( R \) is the specific gas constant, and \( T \) is absolute temperature in kelvins. This compact formula explains most of the trends you’ll see in the table: sound is faster in lighter gases, in gases with smaller heat capacity (larger \( \gamma \)), and at higher temperatures.
Why helium and hydrogen are “fast,” and SF6 is “slow”
The specific gas constant is \( R = \bar{R}/M \), where \( \bar{R} \) is the universal gas constant and \( M \) is molar mass. Light molecules (helium, hydrogen, neon) have large \( R \), pushing \( c \) upward. Heavy, polyatomic molecules (sulfur hexafluoride, carbon tetrachloride) have smaller \( R \) and usually smaller \( \gamma \) because they store energy in rotational and vibrational modes—both effects reduce sound speed. Monatomic noble gases (He, Ne, Ar, Kr) are simpler: their \( \gamma \approx 5/3 \) is comparatively high, so even at the same temperature, they carry sound faster than many multi-atom gases.
Temperature matters via a square-root law
If composition is fixed, temperature is the dominant variable. Because \( c \propto \sqrt{T} \), going from 0 °C (273.15 K) to 20 °C (293.15 K) increases \( c \) by about \( \sqrt{293.15/273.15} \approx 1.036 \) — roughly a 3.6% bump. That’s why this tool lets you “normalize” a published speed from its base temperature \( T_0 \) to your chosen target \( T_1 \):
\( c(T_1) \approx c(T_0)\,\sqrt{\dfrac{T_1+273.15}{T_0+273.15}} \)
Tip: Normalization is most reliable near room temperature and ordinary pressures, where ideal-gas behavior is a good approximation.
Does pressure change the speed of sound?
At first glance, you might expect denser air to transmit sound faster. For ideal gases, not quite: if the gas composition and temperature are unchanged, pressure cancels out of the formula. That’s why sea-level and high-altitude air at the same temperature have nearly the same \( c \). Pressure only matters indirectly—by influencing phase (e.g., steam at 6 MPa) or composition (e.g., moisture content limits).
Humidity and mixtures
Real air is a mixture. Adding water vapor changes the effective \( R \) and \( \gamma \), nudging \( c \) upward by about 0.5–1% when going from bone-dry to ~50% relative humidity at 20 °C. In specialty gases, trace components can matter too (e.g., CO₂ in “air” for lab work). Our companion tool Air — Speed vs Temperature models this moist-air effect explicitly.
When the simple model breaks down
- Very high pressures / near condensation: Non-ideal effects and changing \( \gamma \) make the square-root scaling less accurate. The “Steam, 6 MPa” row in the table is therefore not normalized.
- High temperatures: Vibrational modes become active; \( \gamma \) declines, so \( c \) grows more slowly than \( \sqrt{T} \) would suggest.
- Reactive or dissociating gases: Composition can shift with temperature, altering both \( R \) and \( \gamma \).
Quick rules of thumb
- Temperature scaling: +10 °C ≈ +1.8–2.0% in many gases near room temperature.
- Molar mass trend: Lower molar mass → higher \( R \) → higher \( c \), all else equal.
- Polyatomic penalty: More internal degrees of freedom → smaller \( \gamma \) → slower \( c \).
From speed to wavelength and Mach
Once you know \( c \), wavelength is \( \lambda = c/f \) (e.g., a 1 kHz tone in air at 20 °C has \( \lambda \approx 0.343 \) m). In aerodynamics and flow acoustics, Mach number is \( \mathrm{Ma} = V/c \). Because \( c \) depends on temperature and gas type, the same vehicle speed may correspond to different Mach numbers in different gases or on different days.
For metrology-grade work across wide ranges, consult full thermophysical property databases that provide temperature-dependent \( c_p \), \( c_v \), and mixture models. For education, quick design checks, and day-to-day acoustics, the ideal-gas picture and the normalization tool here are excellent starting points.