Table of Contents
What Is the Speed of Sound?
The speed of sound is the rate at which pressure waves propagate through a medium. In dry air at 20°C (68°F), sound travels at approximately 343 m/s (1,235 km/h or 767 mph). Unlike light, sound requires a medium to travel and cannot propagate through a vacuum. The speed depends primarily on temperature and the properties of the medium.
Sound travels faster in warmer air because the air molecules have more kinetic energy and vibrate more quickly, transmitting pressure waves faster. At sea level and 0°C, the speed is about 331.3 m/s, increasing by roughly 0.6 m/s for each degree Celsius rise in temperature.
Formula
Where T is the temperature in degrees Celsius. A simplified approximation often used is v = 331.3 + 0.606 × T. Humidity has a small positive effect because water vapor (M=18) is lighter than the nitrogen (M=28) and oxygen (M=32) it displaces.
Speed at Various Temperatures
| Temperature (°C) | Speed (m/s) | Speed (mph) |
|---|---|---|
| -40 | 306.2 | 685 |
| -20 | 319.1 | 714 |
| 0 | 331.3 | 741 |
| 15 | 340.3 | 761 |
| 20 | 343.2 | 768 |
| 30 | 349.0 | 781 |
| 40 | 354.7 | 793 |
Factors Affecting Speed
- Temperature: Higher temperature increases molecular speed, increasing sound speed.
- Humidity: Moist air is slightly less dense, so sound travels about 0.1-0.6% faster.
- Altitude: At high altitude, lower temperature decreases the speed of sound.
- Medium: Sound is much faster in water (~1,480 m/s) and steel (~5,960 m/s).
Frequently Asked Questions
How do I estimate lightning distance?
Count the seconds between seeing lightning and hearing thunder. Divide by approximately 3 to get the distance in kilometers, or divide by 5 for miles. This works because light is nearly instantaneous while sound takes about 3 seconds per kilometer.
What is Mach number?
Mach number is the ratio of an object's speed to the local speed of sound. Mach 1 equals the speed of sound. At Mach 1, a sonic boom is created. Fighter jets typically fly at Mach 1.5-2.5, while the SR-71 Blackbird reached Mach 3.3.
Why does sound travel faster in water than air?
Although water is denser than air, it is also much less compressible. The speed of sound depends on the ratio of bulk modulus (stiffness) to density. Water's very high bulk modulus more than compensates for its higher density, resulting in sound traveling about 4.3 times faster in water.