How Does Sound Travel?

Sound travels in waves that are essentially disturbances that move through a medium by causing particles to vibrate back and forth in the direction of the wave’s travel. Imagine a Slinky toy or a coiled spring: when you pull back one end and release it, a wave of compression and expansion of the coils travels along its length. Similarly, sound waves consist of alternating compressions and rarefactions, or regions of high and low pressure, that travel through the medium. Put another way, if you pluck a guitar string, the vibrations of the string disturb the surrounding air particles causing them to move slightly forward and backward. The moving air particles in turn disturb their neighboring air particles, which disturb their neighboring air particles, and so forth. That melodious sound of a guitar string you hear is really air particles being disturbed.

Because sound travels via successive disturbances in a medium, sound cannot travel through a vacuum since there is no medium in a vacuum to carry the disturbances. The speed at which sound travels depends on the medium and its qualities. In dry air at 0 °C (32 °F), sound zips along at about 331.29 meters (1,086.9 feet) per second. But sound travels much faster in water—around 1,439 meters (4,721 feet) per second at 8 °C (46 °F). Because particles in liquids and solids are closer together than in gases, sound waves transmit more efficiently and thus faster.

Sound waves have properties such as frequency and wavelength that affect how we perceive them. Frequency, measured in hertz, determines the pitch of the sound: higher frequencies mean higher pitches and lower frequencies mean lower pitches. Wavelength, the distance between successive compressions or rarefactions, is inversely related to frequency. Thus low frequency sounds have long wavelengths and high frequency sounds have short wavelengths. For this reason, a low sound can travel much further. Certain animals such whales and elephants produce very low sounds in order to communicate over vast distances.

There is also the amplitude of the sound wave, the magnitude of the fluctuation of a wave from equilibrium. The amplitude determines the loudness of the sound. So if you pluck a guitar string harder, it will displace more air and create a louder sound—even more with the help of an amplifier with which sound waves can disturb not just air molecules but also neighbors.