Sound is a vibration that travels through a medium as a wave. It needs a medium to travel. It cannot travel through a vacuum. That is why space is silent. Sound waves are longitudinal, meaning the particles of the medium vibrate in the same direction the wave travels. They create regions of compression and rarefaction. The speed of sound depends on the medium. It travels at about 343 meters per second in air at room temperature. It travels faster in water, about 1,480 meters per second, and faster still in steel, about 5,960 meters per second.

Sound has several properties. Frequency determines pitch. High frequency means high pitch. Low frequency means low pitch. Amplitude determines loudness. Larger amplitude means louder sound. The human ear can detect frequencies from about 20 Hz to 20,000 Hz. Below that is infrasound. Above that is ultrasound. Sound can be reflected, refracted, diffracted, and absorbed. Echoes are reflected sound. Sonar uses reflected sound to map the ocean. Ultrasound is used in medical imaging. Sound is also used in music, speech, and communication. It is one of the most important ways we interact with the world.

Sound facts

• Definition. Vibration traveling through a medium.
• Medium. Needs one. Cannot travel through a vacuum.
• Speed. 343 m/s in air at room temperature.
• Frequency. Determines pitch.
• Amplitude. Determines loudness.

Sound is not the same as light. Sound needs a medium. Light does not.