Sound is a mechanical wave that travels through matter by making particles vibrate back and forth. The speed of sound tells how quickly a pressure disturbance moves through a material, which matters in music, communication, sonar, medicine, weather, and flight. Unlike light, sound cannot travel through a vacuum because there are no particles to carry the vibration.
Its speed depends strongly on the medium and on conditions such as temperature.
In gases, sound speed increases when temperature rises because warmer molecules move faster and transfer pressure changes more quickly. In liquids and solids, particles are closer together and the material is usually much harder to compress, so sound often travels faster than in gases. A useful comparison is air at room temperature at about 343 m/s, water at about 1480 m/s, and steel at about 5960 m/s.
For fast-moving objects, Mach number compares object speed with local sound speed, so Mach 1 means the object is moving at the speed of sound in that medium.
Key Facts
- Sound is a longitudinal mechanical wave made of compressions and rarefactions in a medium.
- Wave speed formula: v = fλ, where v is speed, f is frequency, and λ is wavelength.
- Speed of sound in dry air near room temperature: v ≈ 343 m/s at 20°C.
- Temperature rule for air: v ≈ 331 m/s + 0.6T, where T is in °C.
- Mach number formula: M = object speed / speed of sound.
- Sound usually travels fastest in solids, slower in liquids, and slowest in gases because particle coupling and stiffness affect energy transfer.
Vocabulary
- Sound wave
- A sound wave is a mechanical disturbance that transfers energy through vibrating particles in a medium.
- Compression
- A compression is a region in a sound wave where particles are crowded closer together and pressure is higher.
- Rarefaction
- A rarefaction is a region in a sound wave where particles are spread farther apart and pressure is lower.
- Medium
- A medium is the material, such as air, water, or steel, through which a mechanical wave travels.
- Mach number
- Mach number is the ratio of an object's speed to the local speed of sound.
Common Mistakes to Avoid
- Confusing sound speed with particle speed is wrong because the wave moves through the medium while individual particles only vibrate around their positions.
- Assuming sound travels at one universal speed is wrong because sound speed changes with medium, temperature, and material properties.
- Thinking louder sound travels much faster is wrong because loudness mainly changes amplitude, not wave speed in ordinary conditions.
- Using 343 m/s for every situation is wrong because that value is only an approximation for sound in air near 20°C.
Practice Questions
- 1 A sound wave in air has a frequency of 500 Hz and travels at 340 m/s. What is its wavelength?
- 2 At 30°C, estimate the speed of sound in air using v ≈ 331 m/s + 0.6T. Then find the Mach number of a plane traveling at 408 m/s.
- 3 Explain why sound travels faster in steel than in air even though steel particles are much heavier than air molecules.