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]
(acoustics)
A transverse sound wave which propagates in smectic liquid crystals, and whose behavior resembles mathematically that of second sound in superfluid helium.
(cryogenics)
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 is made by the closing of atrioventricular valves and is heard loudly in the apex; the second heart sound is made by the closing of the aortic valve and pulmonary valve and is heard loudly in the base of the heart.
Consult a specialist for an abnormal second heart sound or click and/or an abnormal ECG or chest x-ray finding.
More than 90% of patients have an accentuated pulmonic component of the second heart sound.
The first heart sound can be separated into two portions, while the second heart sound is single.
She hears a wide physiological splitting of the second heart sound, with a loud aortic component, but no third heart sound.
On auscultation first heart sound was sharp and loud, and pulmonary component of second heart sound also loud at pulmonary region.
The amplitude ratio of the first to second heart sound is reduced in left ventricular systolic dysfunction.
2, 7 8] A fixed split of second heart sound is the auscultatory hallmark of ASD.
During the exam, feel for a hyperdynamic precordium, and check for an increased and sometimes palpable P2-a single, loud second heart sound.
A cardiovascular exam showed normal precordial activity; a n normal first and second heart sound, and a soft systolic murmur at the lower left sternal border that decreased in intensity when she stood up.
The pediatrician heard a continuous, machinery-like murmur at the left upper sternal border and fixed split second heart sound.
First heart sound was normal with aortic component of second heart sound loud whereas the third and fourth sounds were not audible.