Fascinating Facts About Sound Waves

11/01/2007 10:31 PM Eastern

Fascinating Facts About Sound Waves

  • Around 350 B.C., Aristotle observed that a vibrating string was actually striking the air. From this, Aristotle hypothesized that air was needed as a medium through which sound could be conducted. He further reasoned that sound would not be conducted in a vacuum.
  • In the sixth century B.C., Pythagoras of Samos observed a musician plucking a stringed instrument and noticed that the amplitude of a vibration (deduced by the width of the blurred area of motion) related to the perceived loudness of the sound. He also saw that the sound stopped when the vibration stopped and that the shorter strings vibrated more rapidly, which produced a higher-pitched sound.
  • Based on Pythagoras' findings, first-century Roman engineer/architect Marcus Vitruvius Pollio suggested that air not only moved when struck, it vibrated in response to the vibrations of the string. He maintained that it was actually these air vibrations that we heard and perceived as sound.
  • The Roman philosopher Anicius Manlius Severinus Boethius compared the conduction of sound through the air to the waves produced by dropping a pebble into calm water. Today we know that sound waves and water waves represent two distinct types of wave motion (longitudinal and transverse, respectively), but the realization that sound moves as a wave at all was an important step in the study of sound.
  • In 1842, Christian Doppler first identified and quantified the change in pitch that occurs when a source of sound moves toward or away form a stationary observer or vice versa. This is now known as the Doppler Effect.
  • Waves are disturbances in water, air, or another substance, or in a field of force.
  • Sound originates at a point and travels equally strongly in all directions. Sound waves are spherical and three-dimensional.
  • When a sound wave reaches a person's body, it reflects off of the person's head, shoulders, and the curved surface of the outer ear. When reflecting waves bounce more than once or interfere with other waves before reaching the tympanic membrane, it causes changes in the volume or quality. Such occurrences are called head-related transfer functions (HRTFs).
  • Unless you are directly on the axis of a sound source, there is a difference in level between the sound your right ear hears and the sound your left ear hears, called interaural level difference (ILD). The difference between how the sound waves interact with your head and body, and the delay it causes, is called interaural time difference (ITD). The reduction in level is caused by the natural dissipation of the sound wave and by your head absorbing and reflecting a little bit of the sound.
  • Objects that tend to resonate at a single frequency, such as a flute, are often said to produce a pure tone. Other objects, such as a tuba, vibrate and produce more complex waves with overtones that have whole number mathematical relationships between them; these are said to produce a rich sound.
  • Wave interference is the phenomenon that occurs when two waves meet while traveling along the same medium.
  • If two crests that have the same shape meet up with each other while traveling in opposite directions, the medium — air — will take on the shape of a crest with twice the amplitude of the two interfering crests. This is known as constructive interference.
  • If a crest and a trough with the same shape meet up while traveling in opposite directions, the two pulses will cancel each other's effect on the air displacement and the air will assume an equalized position. This is known as destructive interference.
  • Sound is a pressure wave that consists of compressions and rarefactions.
  • The destructive interference of sound waves can also be used to reduce noise, such as in industrial headphones. Such headphones capture sound from the --environment and use computer technology to produce a second sound wave. The combination of the two sound waves within the headset results in destructive interference and, thus, reduces the sound level.


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