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An introduction to phononic crystals (2)

Acoustic and elastic waves

Before going further into our subject, let us introduce some general ideas on acoustic and elastic waves. Such waves are actually part of our everyday experiment and of our closer environment. Sonic waves propagate in the atmosphere. They convey human sound and inform us on what is around. Acoustic waves are used in various areas, such as ultrasound imaging of the human body, detection and location of of underwater objects (the sonar), or sismology. The quartz crystals in our wristwatches take advantage of particular resonances of the acoustic waves that propagate within them. Mobile phones and televisions include electronic filters exploiting high frequency acoustic waves in exotic synthetic crystals, such as lithium tantalate (a material with the chemical formula LiTaO3). All acoustic waves are at the microscopic level composed of progressive vibrations of the atoms of the propagation medium. In a solid for instance, atoms are constrained to remain on average around their equilibrium position, and waves propagate by causing the motion of a succession of crystalline planes. In this case, the terminology refers to elastic waves rather than acoustic waves. The signal velocities involved are of the order of a few kilometers per second, as a general rule. They can reach up to 20000 m/s in diamond, for instance. In the case of sonic waves, in air, or of acoustic waves, in water, the atoms composing the fluid are not compelled to stay at a specific mean position in space, but the wave still manifests itself as a collective motion being communicated from atom to atom in a given direction. The wave velocities involved are of the order of 340 m/s in air and 1480 m/s in water, under standard conditions.

The figure presented below gives a few examples of applications and phenomena based on sonic, acoustic and elastic waves at different operating frequencies. The frequency, measured in Hertz, is a direct measurement of the number of oscillations per unit of time, i.e. per second.

Frequency domains for acoustic waves

Frequency domains for acoustic waves