Sound is produced by vibrations and reaches our ears by traveling through substances such as air. Let us also pay attention to how sound differs from light.
Basic Knowledge
When an object vibrates, its vibrations change the density of the surrounding air, producing alternating compressions and rarefactions, and travel as sound waves. Sound is a longitudinal wave (compression wave) and travels through substances such as air, water, and metal. It does not travel through a vacuum (a major difference from light).
Approximate speeds at which sound travels:
・In air (15℃): approximately 340 m/s
・In water: approximately 1500 m/s
・In iron (solid): approximately 5000 m/s
Sound travels fastest through solids, followed by liquids and then gases. Compared with the speed of light, it is extremely slow, which is why during a thunderstorm we see the lightning before hearing the thunder.
Vibration (the back-and-forth motion of an object. The source of sound)
Sound source (sounding body) (an object producing sound. A drum・vocal cords・speaker)
Longitudinal wave (compression wave) (a wave in which the direction of vibration of the medium is the same as the direction of travel)
Speed of sound (the speed at which sound travels. Approximately 340 m/s in air)
Medium (a substance that transmits sound. Air・water・metal, and so on)
Echo (reverberation) (the phenomenon in which reflected sound is heard again)
Further Exploration (Background and Significance)
If the time from seeing lightning to hearing thunder is t seconds, the distance to the lightning can be estimated as approximately L = 340 × t (m). This is a familiar example that helps us experience the slowness of sound.
The speed of sound also changes with temperature. When the air temperature is T℃, the speed of sound in air is approximately v = 331.5 + 0.6T (m/s). Sound travels faster at higher temperatures.
The property of sound being reflected after striking an object appears as an echo and is used in many technologies, including fish finders (finding fish by reflecting ultrasound), sonar (searching for submarines), and ultrasound examinations (medical echography).
- Sound is a longitudinal wave in which vibrations transmit compressions and rarefactions through air
- Sound does not travel through a vacuum (light does)
- Speed of sound in air (15℃): approximately 340 m/s
- Sound travels faster in solids > liquids > gases
- The higher the temperature, the greater the speed of sound
- Distance to lightning ≒ 340 × (the seconds from seeing the flash to hearing the sound)
- Applications: sonar・fish finders・ultrasound echography
Points to Note (Easily Confused)
① Light travels through a vacuum, but sound does not travel through a vacuum. ② Sound is a longitudinal wave, unlike surface water waves and vibrations in strings, which are transverse waves. ③ The speed of sound is approximately one-millionth of the speed of light. This is why there is a time lag between lightning and thunder. ④ The speed of sound is determined by the type and temperature of the medium, not by the loudness or pitch of the sound.
Practice
- What is the approximate speed of sound traveling through air at 15℃?
- In which of solids, liquids, and gases does sound travel fastest?
- Thunder was heard 3 seconds after the lightning flashed. What is the approximate distance to the lightning in meters?