Rarefaction in the context of "Neon lighting"

Click consonants, or clicks, are speech sounds that occur as consonants in many languages of Southern Africa and in three languages of East Africa. Examples familiar to English-speakers are the tut-tut (British spelling) or tsk! tsk! (American spelling) used to express disapproval or pity (IPA [ǀ]), the tchick! used to spur on a horse (IPA [ǁ]), and the clip-clop! sound children make with their tongue to imitate a horse trotting (IPA [ǃ]). However, these paralinguistic sounds in English are not full click consonants, as they only involve the front of the tongue, without the release of the back of the tongue that is required for clicks to combine with vowels and form syllables.

Anatomically, clicks are obstruents articulated with two closures (points of contact) in the mouth, one forward and one at the back. The enclosed pocket of air is rarefied by a sucking action of the tongue (in technical terminology, clicks have a lingual ingressive airstream mechanism). The forward closure is then released, producing what may be the loudest consonants in the language, although in some languages such as Hadza and Sandawe, clicks can be more subtle and may even be mistaken for ejectives.
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In physics, magnetosonic waves, also known as magnetoacoustic waves, are low-frequency compressive waves driven by mutual interaction between an electrically conducting fluid and a magnetic field. They are associated with compression and rarefaction of both the fluid and the magnetic field, as well as with an effective tension that acts to straighten bent magnetic field lines. The properties of magnetosonic waves are highly dependent on the angle between the wavevector and the equilibrium magnetic field and on the relative importance of fluid and magnetic processes in the medium. They only propagate with frequencies much smaller than the ion cyclotron or ion plasma frequencies of the medium, and they are nondispersive at small amplitudes.

There are two types of magnetosonic waves, fast magnetosonic waves and slow magnetosonic waves, which—together with Alfvén waves—are the normal modes of ideal magnetohydro­dynamics. The fast and slow modes are distinguished by magnetic and gas pressure oscillations that are either in-phase or anti-phase, respectively. This results in the phase velocity of any given fast mode always being greater than or equal to that of any slow mode in the same medium, among other differences.

Longitudinal waves are waves which oscillate in the direction which is parallel to the direction in which the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal waves are also called compressional or compression waves, because they produce compression and rarefaction when travelling through a medium, and pressure waves, because they produce increases and decreases in pressure. A wave along the length of a stretched Slinky toy, where the distance between coils increases and decreases, is a good visualization. Real-world examples include sound waves (vibrations in pressure, a particle of displacement, and particle velocity propagated in an elastic medium) and seismic P waves (created by earthquakes and explosions).

The other main type of wave is the transverse wave, in which the displacements of the medium are at right angles to the direction of propagation. Transverse waves, for instance, describe some bulk sound waves in solid materials (but not in fluids); these are also called "shear waves" to differentiate them from the (longitudinal) pressure waves that these materials also support.