The wave thus generated is harmonic because it is produced by a simple harmonic oscillation, and longitudinal because the gas particles oscillate from left to right (along the x-axis) parallel to the direction of wave propagation. In the figure is represented the state of the gas for a fixed instant of time. These zones will go back and forth from high pressure to low pressure. Initially the gas is at equilibrium at pressure p 0.Īs the piston moves in and out of the cylinder undergoing simple harmonic motion of amplitude s m as shown in red in the figure, the gas particles will undergo simple harmonic motion as well, and therefore there will be inside the cylinder certain zones where pressure will be higher than p 0, called compressions (in dark blue) and others where pressure will be lower than p 0, called rarefactions (in light blue). A cylinder of infinite length with a movable piston at one end contains a gas. ![]() In order to see how a sound wave is produced and how it is described mathematically, we will consider the situation represented in the figure below. In this page we will focus on the study of one-dimensional sound waves. Sound waves can be one-dimensional, two-dimensional and three-dimensional. And the essential characteristic of a longitudinal wave that distinguishes it from other types of waves is that the particles of the medium move in a direction parallel to the direction of energy transport.A sound wave is disturbance consisting of a succession of compressions and rarefactions traveling through a material medium. Regardless of the source of the sound wave - whether it is a vibrating string or the vibrating tines of a tuning fork - sound waves traveling through air are longitudinal waves. The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air. Since air molecules (the particles of the medium) are moving in a direction that is parallel to the direction that the wave moves, the sound wave is referred to as a longitudinal wave. Other surrounding particles begin to move rightward and leftward, thus sending a wave to the right. These back and forth vibrations are imparted to adjacent neighbors by particle-to-particle interaction. The molecules move rightward as the string moves rightward and then leftward as the string moves leftward. The back and forth vibration of the string causes individual air molecules (or a layer of air molecules) in the region immediately to the right of the string to continually vibrate back and forth horizontally. The lower pressure to the right of the string causes air molecules in that region immediately to the right of the string to expand into a large region of space. As the vibrating string moves in the reverse direction (leftward), it lowers the pressure of the air immediately to its right, thus causing air molecules to move back leftward. This causes the air molecules to the right of the string to be compressed into a small region of space. As the vibrating string moves in the forward direction, it begins to push upon surrounding air molecules, moving them to the right towards their nearest neighbor. A vibrating string can create longitudinal waves as depicted in the animation below. Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves. ![]() In such a case, each individual coil of the medium is set into vibrational motion in directions parallel to the direction that the energy is transported. ![]() A longitudinal wave can be created in a slinky if the slinky is stretched out in a horizontal direction and the first coils of the slinky are vibrated horizontally. Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction that is parallel to the direction of energy transport. For a sound wave traveling through air, the vibrations of the particles are best described as longitudinal. The vibrations of the object set particles in the surrounding medium in vibrational motion, thus transporting energy through the medium. In the first part of Lesson 1, it was mentioned that sound is a mechanical wave that is created by a vibrating object.
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