Second Sound

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Second sound

A type of wave propagated in the superfluid phase of liquid helium (helium II) and in certain other substances under special conditions. The name is misleading since second sound is not in any sense a sound wave, but a temperature or entropy wave. In ordinary or first sound, pressure and density variations propagate with very small accompanying variations in temperature; in second sound, temperature variations propagate with no appreciable variation in density or pressure. See Liquid helium, Superfluidity

The two-fluid model of helium II provides further insight into the nature of second sound. In this model the liquid can be described as consisting of superfluid and normal components of densities ρs and ρn, respectively, such that the total density ρ = ρs + ρn. The superfluid component is frictionless and devoid of entropy; the normal component has a normal viscosity and contains the entropy and thermal energy of the system. In a temperature or second-sound wave, the normal and superfluid flows are oppositely directed so that ρs V s + ρn V n = 0, where V s and V n are the superfluid and normal flow velocities. Thus a variation in relative densities of the two components, and hence a temperature fluctuation, propagates with no change in total density or pressure. In a first-sound wave, the two components move in phase, that is, V nV s.

Theoretical predictions that second sound should exist in certain solid dielectric crystals under suitable conditions have been confirmed experimentally for solid helium single crystals at temperatures between 0.4 and 1.0 K (-459.0 and -457.9°F). See Dielectric materials

Another quite different class of materials can exhibit second sound. In smectic A liquid crystals, when the wave vector is oblique with respect to the layers of these ordered structures, a modulation of the interlayer spacing can propagate at nearly constant density.

Second Sound


weakly attenuated temperature waves that are propagated in superfluid liquid helium (He II) along with ordinary sound waves. Near a temperature of absolute zero the velocity v2 of second sound and the velocity v of ordinary sound (compression waves) are associated by the ratio Second Sound At the point of phase transition from He II to He I (the A-point), v2 vanishes. Second sound is radiated from a heater with a variable temperature and is detected by a sensitive thermometer.

second sound

[′sek·ənd ′sau̇nd]
A transverse sound wave which propagates in smectic liquid crystals, and whose behavior resembles mathematically that of second sound in superfluid helium.
A type of wave propagated in the superfluid phase of liquid helium (helium II), in which temperature and entropy variations propagate with no appreciable variation in density or pressure.
References in periodicals archive ?
The first heart sound was soft, the second heart sound was widely split with an accentuated pulmonary component, and the left ventricular third heart sound was heard over the apex.
Auscultation revealed a first and second heart sound with no added sounds or murmurs and normal breath sounds.
A loud, single second heart sound was audible and no murmur was present.
Precordial examination demonstrated a parasternal heave and a loud second heart sound.
More than 90% of patients have an accentuated pulmonic component of the second heart sound.
She hears a wide physiological splitting of the second heart sound, with a loud aortic component, but no third heart sound.
A 4+/6+ continuous murmur peaked at the second heart sound and was best heard in the second left intercostal space.
Pertinent physical findings were a regular pulse at 96 beats/ min; a blood pressure of 116/60 mm Hg; normal neck veins; brisk, full, symmetrical arterial pulses in the arms and legs with no radial-femoral delay; a loud and palpable second heart sound along the upper and mid left sternal border; and a small apical impulse just outside the left mid-clavicular line.