Which factor has the least effect on the speed of sound in air?

Which factor has the least effect on the speed of sound in air?

• A. Ambient temperature
• B. Humidity
• C. Air density
• D. Sound frequency

Answer: (D)

The speed of sound in air is guided mainly by the properties of the air itself, especially temperature. When temperature rises, air molecules have more kinetic energy, so pressure waves pass from molecule to molecule faster, increasing the propagation speed. Humidity and air density do affect speed too, but their impact is smaller in typical conditions: humidity lowers density slightly as water vapor replaces some heavier air molecules, which can raise speed a bit, and density influences speed through the basic relationship c ~ sqrt(P/rho), but these effects are relatively modest compared to temperature changes. Frequency, on the other hand, is a property of the sound wave itself, not the medium, and for ordinary audible frequencies air behaves almost non-dispersively—waves of different frequencies travel at nearly the same speed. So the factor that has the least effect on how fast sound travels through air is the frequency.

The speed of sound in air is guided mainly by the properties of the air itself, especially temperature. When temperature rises, air molecules have more kinetic energy, so pressure waves pass from molecule to molecule faster, increasing the propagation speed. Humidity and air density do affect speed too, but their impact is smaller in typical conditions: humidity lowers density slightly as water vapor replaces some heavier air molecules, which can raise speed a bit, and density influences speed through the basic relationship c ~ sqrt(P/rho), but these effects are relatively modest compared to temperature changes. Frequency, on the other hand, is a property of the sound wave itself, not the medium, and for ordinary audible frequencies air behaves almost non-dispersively—waves of different frequencies travel at nearly the same speed. So the factor that has the least effect on how fast sound travels through air is the frequency.