Has this ever happened to you? You see a flash of lightning on the horizon, but several seconds pass before you hear the rumble of thunder. The reason? The speed of light is much faster than the speed of sound.
What Is the Speed of Sound?
The speed of sound is the distance that sound waves travel in a given amount of time. You’ll often see the speed of sound given as 343 meters per second. But that’s just the speed of sound under a certain set of conditions, specifically, through dry air at 20 °C. The speed of sound may be very different through other matter or at other temperatures.
Speed of Sound in Different Media
Sound waves are mechanical waves, and mechanical waves can only travel through matter. The matter through which the waves travel is called the medium (plural, media). The Table below gives the speed of sound in several different media. Generally, sound waves travel most quickly through solids, followed by liquids, and then by gases. Particles of matter are closest together in solids and farthest apart in gases. When particles are closer together, they can more quickly pass the energy of vibrations to nearby particles. You can explore the speed of sound in different media at this URL:
| Medium (20 °C) | Speed of Sound Waves (m/s) |
|---|---|
| Dry Air | 343 |
| Water | 1437 |
| Wood | 3850 |
| Glass | 4540 |
| Aluminum | 6320 |
Q: The table gives the speed of sound in dry air. Do you think that sound travels more or less quickly through air that contains water vapor? (Hint: Compare the speed of sound in water and air in the table.)
A: Sound travels at a higher speed through water than air, so it travels more quickly through air that contains water vapor than it does through dry air.
Temperature and Speed of Sound
The speed of sound also depends on the temperature of the medium. For a given medium, sound has a slower speed at lower temperatures. You can compare the speed of sound in dry air at different temperatures in the following Table below. At a lower temperature, particles of the medium are moving more slowly, so it takes them longer to transfer the energy of the sound waves.
| Temperature of Air | Speed of Sound Waves (m/s) |
|---|---|
| 0 °C | 331 |
| 20 °C | 343 |
| 100 °C | 386 |
Q: What do you think the speed of sound might be in dry air at a temperature of -20 °C?
A: For each 1 degree Celsius that temperature decreases, the speed of sound decreases by 0.6 m/s. So sound travels through dry, -20 °C air at a speed of 319 m/s.
Summary
- The speed of sound is the distance that sound waves travel in a given amount of time. The speed of sound in dry air at 20 °C is 343 meters per second.
- Generally, sound waves travel most quickly through solids, followed by liquids, and then by gases.
- For a given medium, sound waves travel more slowly at lower temperatures.
Vocabulary
- speed of sound: Speed at which sound waves travel, which is 343 m/s in dry air at 20 °C.
Practice
At the following URL, read about the speed of sound in different materials. Be sure to play the animation. Then answer the questions below.http://www.ndt-ed.org/EducationResources/HighSchool/Sound/speedinmaterials.htm
- Describe what you hear when you play the animation. Explain your observations.
- Name two properties of materials that affect the speed of sound waves. How do they affect the speed of sound?
- Explain why sound waves moves more quickly through warmer air than cooler air.
Review
- What is the speed of sound in dry air at 20 °C?
- Describe variation in the speed of sound through various media.
- Explain how temperature affects the speed of sound.
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