# Second sound Second sound is a [[Wave|wave]] made of temperature. In ordinary matter [[Heat_transfer|heat]] spreads by [[Diffusion|diffusion]]: the governing equation is parabolic, a hot spot blurs, and the front creeps out like the square root of time — there is no [[Velocity|speed]] to quote. Below the [[Lambda_point|lambda point]], [[Superfluid_helium-4|liquid helium II]] instead propagates heat. It has a definite velocity, it reflects off walls, it forms standing [[Acoustic_wave|waves]], and you can tune a cavity to it. Nothing is compressed and nothing moves on net; what oscillates is [[Entropy|entropy]]. ## Microsims — three.js <iframe src="https://wikitube-3d-microsims.netlify.app/Second_sound.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" title="Second sound — three.js microsim"></iframe> **`Second_sound` (three.js).** Two tubes of [[Liquid_helium|helium II]] run side by side, one driven by a piston and one by a heater, and the whole point is to watch them behave differently. Turn on `Particles` and the first-sound tube shows orange normal-component and cyan superfluid tracers moving *together*, while the second-sound tube shows them moving in exact opposition — the superfluid travelling less far by the ratio rho_n/rho_s, so the two mass fluxes cancel and the tube is never compressed. Switch `Show` to `Density field` to confirm it: the first-sound tube bulges and pinches, the second-sound tube stays a perfect cylinder. Press `Sweep` to grow the resonance ladder at f_n = n·u/(2L), after which the drive parks itself on the strongest peak. The live HUD quantity is the wave [[Velocity|speed]] *measured* from the running field, u = f·λ, shown against the model value and the tabulated one and flagged LOCKED only when the drive sits on a cavity mode — which is exactly why Peshkov swept the [[Frequency_domain|frequency]] instead of trusting a single tone. Then drag `T` up through 2.1768 K and watch second sound die. ## One wave carries pressure; the other carries temperature Landau's two-fluid [[Fluid_dynamics|hydrodynamics]] gives helium II two independent [[Velocity|velocity]] fields rather than one: a **normal component** of [[Density|density]] rho_n that carries *all* the [[Entropy|entropy]] and *all* the [[Viscosity|viscosity]], and a **[[Superfluidity|superfluid]] component** of density rho_s that carries neither, with rho_s + rho_n = rho. Two fields admit two independent [[Oscillation|oscillation]] modes, and the two modes are distinguished exactly — not loosely, not qualitatively — by the phase relation between the two velocities. **First sound: v_n and v_s oscillate in phase.** The components move together, so the mass flux j = rho_s·v_s + rho_n·v_n oscillates and the total [[Density|density]] oscillates with it. The linearised [[Entropy|entropy]] equation gives ds/dt = 0 identically, so to leading order the temperature does *not* oscillate. This is ordinary pressure [[Sound|sound]] — the same [[Mechanical_wave|mechanical wave]] an [[Ultrasound|ultrasound]] [[Transducer|transducer]] launches into water. **Second sound: v_n and v_s oscillate out of phase**, in the exact ratio rho_s·v_s = −rho_n·v_n, so that j = 0 identically. This is counterflow at constant total [[Density|density]]: nothing moves on net and nothing is compressed. What oscillates is the *split* between the two components — and because only the normal component carries [[Entropy|entropy]], an oscillation of the split is an oscillation of entropy and therefore of temperature. Second sound is not heat riding on a [[Plane_wave|pressure wave]]. It is a wave in which pressure is the quantity that stays still. ## Why there are two modes, and what sets the speed Linearise mass conservation, entropy conservation and the two [[Force|force]] equations about [[Thermodynamic_equilibrium|equilibrium]] and you get a biquadratic [[Wave_equation|dispersion relation]] with two positive roots. In helium II the thermal expansion coefficient is minute, so the specific heats at constant pressure and constant volume are nearly equal and the two roots decouple almost perfectly: one becomes the adiabatic compression [[Velocity|speed]], the other **u₂² = (rho_s / rho_n) · (T · s² / c)** with *s* the [[Entropy|entropy]] per unit mass and *c* the specific heat per unit mass. Each factor does a job. **rho_s/rho_n is the lever**: it measures how much inertia-free component is available to counterflow against the entropy-carrying one. It falls to zero at the [[Lambda_point|lambda point]] and diverges as T → 0. **T·s² is the restoring push** — [[Entropy|entropy]] is the wave's amplitude coordinate, and a [[Liquid_helium|liquid]] with more entropy per gram pushes back harder on a given displacement of it. **1/c is the inertia**: heat capacity is the effective mass of a temperature [[Wave|wave]], because a large *c* means a given entropy displacement produces only a small temperature swing. The [[Mathematical_model|formula]] is exact within two-fluid hydrodynamics up to that decoupling approximation, and it is the reason the shape of the u₂ curve is a [[Thermodynamics|thermodynamic]] fact rather than a fitted one — the [[Second_law_of_thermodynamics|entropy]] bookkeeping fixes it, and nothing is tuned. ## The numbers, and the conditions they cannot be quoted without All values below are for [[Helium-4|helium-4]] at **saturated [[Boiling_point|vapour pressure]]**, on the ITS-90 [[Accuracy_and_precision|scale]], from the recommended tabulations of Donnelly and Barenghi. The condition is not decorative — every [[Cryogenics|cryogenic]] number in this cluster shifts with pressure. **First sound** runs u₁ = **238.2 m/s as T → 0**, 234.5 m/s at 1.60 K, 217.5 m/s at T_lambda = 2.1768 K, and 219.4 m/s at 2.20 K in helium I — the ordinary [[Viscosity|viscous]] [[Liquid_helium|liquid]] above the [[Phase_transition|transition]]. The figure "238 m/s" that circulates without qualification is the T → 0 value, and it must carry that condition: at the transition itself first sound is nine per cent slower. **Second sound** is roughly an order of magnitude slower, and unlike u₁ its curve turns twice. It **peaks at 20.37 m/s at 1.65 K**, sits at 20.33 m/s at 1.60 K, and falls away on both sides. Going up: 19.90 m/s at 1.80 K, 16.78 m/s at 2.00 K, 12.69 m/s at 2.10 K, 8.49 m/s at 2.15 K, **2.16 m/s at 2.1760 K** — eight-tenths of a millikelvin below the [[Lambda_point|lambda point]] — and **exactly zero at T_lambda**, because rho_s is. *The commonly repeated form places the ~20 m/s value "around 1.8 K"; the recommended table puts 1.80 K on the falling side and the maximum at 1.65 K.* Going down there is a shallow **minimum of 18.35 m/s at 1.10 K**, and then u₂ climbs steeply: 18.93 m/s at 1.00 K, 21.66 m/s at 0.90 K, 29.28 m/s at 0.80 K, 47.07 m/s at 0.70 K, **83.85 m/s at 0.60 K** and still rising. That climb has a limit, and it is one of the prettiest results in the subject. Below about 0.6 K the excitation gas is pure phonon — a gas of massless [[Vibration|quanta]] with linear dispersion, formally the same object as a [[Photon|photon]] gas — and second sound becomes ordinary sound *in that gas*. For an ideal ultrarelativistic gas in three dimensions the sound speed is the [[Kinetic_theory_of_gases|carrier]] speed divided by √3, so **u₂(T → 0) = u₁(T → 0)/√3 = 238.2 / 1.7320508 = 137.5 m/s.** The [[Estimation_theory|experimental]] confirmation is old and independent. Lane, Fairbank and Fairbank, working at Yale in 1947 with a resonance method, reported the velocity "zero at the λ-point, reaches a maximum of 20.46 m/sec. at 1.7°K and decreases to 19.80 m/sec. at 1.42°K", quoted to ±0.5 per cent — within one per cent of the values recommended half a century later. ## The experimental insight: a heater, not a piston The history is a case study in how a correct equation can sit in front of people while the [[Experimental_system|experiment]] fails. László Tisza proposed the two-fluid picture in 1938 and predicted that [[Entropy|entropy]] or temperature fluctuations should propagate; his [[Velocity|velocity]] formula was wrong, because he extrapolated roton entropy data to [[Thermodynamics|absolute zero]] and so missed the phonon contribution entirely, and because he had no [[Mathematical_model|model]] for rho_n and simply guessed it proportional to *s*. Lev Landau published the correct [[Fluid_dynamics|hydrodynamics]] in 1941, derived both modes, obtained the formula above, and supplied the name **second sound**. Then nothing worked. John Wilks, quoted by Donnelly: "even with the wave propagation equation before them, the first attempts to generate second sound were made using piezoelectric crystals to set up pressure variations. It was only after these had failed . . . that an analysis by [Evgeny] Lifshitz showed that a much more effective method would be by periodic heating." That is the key experimental insight of the whole subject, and the reason is exactly the phase relation above. **A piston pushes mass; a heater pushes entropy.** A piezoelectric [[Transducer|transducer]] drives j ≠ 0 at constant entropy per unit mass — that is the definition of the first-sound mode, and it couples to second sound essentially not at all. A resistive heater drives the [[Heat_transfer|heat]] flux q = rho·s·T·v_n, which is carried entirely by the normal component; mass conservation then forces the [[Superfluidity|superfluid]] to stream the other way at rho_s·v_s = −rho_n·v_n, which is the definition of the second-sound mode. The source has to match the mode. (One practical corollary every such [[Acoustics|experiment]] handles: an ohmic heater driven at [[Angular_frequency|frequency]] *f* dissipates I²R with the [[Electric_current|current]] squared, so its heat output oscillates at 2*f*.) Vasilii Peshkov implemented Lifshitz's suggestion and generated and detected second sound in **1944**, mid-war — heater at one end of a resonator, [[Sensor|thermometer]] at the other, [[Frequency_domain|frequency]] swept until the cavity rang. He reached about 1.2 K by 1948 and saw the velocity flatten; by 1960 he reached about 0.5 K and found the low-temperature rise in very good agreement with Lifshitz's computed curve, settling the Tisza–Landau dispute in Landau's favour. Lane and the Fairbanks confirmed the effect independently in the United States in 1947. ## Above the lambda point there is nothing to oscillate In helium I — the [[Liquid_helium|liquid]] above the [[Lambda_point|lambda point]] — rho_s = 0. There is no second [[Velocity|velocity]] field, no second mode, and u₂ = 0: not small, not damped, absent. Heat reverts to [[Diffusion|diffusing]], and the governing [[Partial_differential_equation|equation]] goes back to being parabolic with the measured [[Thermal_engineering|thermal]] diffusivity of helium I, D = 2.51 × 10⁻⁸ m² s⁻¹ at 2.20 K. The contrast is the article's best single number: across a 2 cm cavity, second sound at 1.65 K arrives in **0.98 ms**, while the diffusive time constant L²/4D is **about 1.1 hours** — a factor of four million. The same two-fluid structure, and so the same second mode, carries over to superfluid [[Helium-3|helium-3]] below its own [[Phase_transition|transition]], roughly a thousand times colder; that the constituent there is a [[Fermion|fermion]] rather than a [[Boson|boson]] changes the pairing and the [[Spin_(physics)|spin]] structure but not the fact that two velocity fields give two [[Wave|waves]]. ## Second sound is not a helium trick The [[Physics|physics]] requires only a gas of excitations whose momentum-conserving collisions dominate the momentum-destroying ones, so that the gas behaves [[Fluid_dynamics|hydrodynamically]] rather than ballistically or diffusively. That is a window, not a threshold: too cold and phonons fly ballistically to the boundary, too warm and umklapp [[Kinetic_theory_of_gases|scattering]] destroys momentum faster than the [[Wave|wave]] can organise it. In a suitable [[Crystal_structure|crystal]] the window opens. It was found in **[[Sodium|sodium]] [[Fluorine|fluoride]] in 1970** by Jackson, Walker and McNelly, at roughly 10–20 K, and in **[[Bismuth|bismuth]] in 1972** by Narayanamurti and Dynes, over 1.2–4.0 K, at a saturated [[Velocity|velocity]] of **(0.78 ± 0.05) × 10⁵ cm/s** — which the authors report as the Debye velocity divided by √3. That is the same factor of √3 that governs [[Helium|helium]] at absolute zero, and it is there for the same reason: in both cases second sound is ordinary sound in a three-dimensional gas of massless [[Vibration|excitations]]. Those results left second sound looking like a liquid-helium-temperature curiosity for four decades. The [[Materials_science|materials]] side has since moved sharply. **Huberman and colleagues observed second sound in [[Carbon|graphite]] above 100 K** (*Science*, 2019), using time-resolved optical [[Heat_transfer|thermal transport]] measurements on 5–20 μm length scales; **Ding and colleagues pushed it over 200 K** in the same material (*Nature Communications*, 2022, with clear signatures at 200 K and 225 K) using a sub-picosecond transient grating. Beardo and colleagues reported second sound in bulk [[Semiconductor_device|germanium]] from 7 K to room temperature under a rapidly varying temperature field (*Science Advances*, 2021), where how fast the [[Signal|drive]] changes matters as much as how hot the sample is. The honest qualification, which the popular coverage tends to drop: in graphite the wave decays with a characteristic length of order 1–10 μm. This is microscale transient transport in layered [[Solid_mechanics|solids]], not a bulk [[Thermal_engineering|heat pipe]]. ## What it is actually used for **Quench location in superconducting accelerator cavities.** A [[Superconductivity|superconducting]] RF cavity sits in a bath of [[Liquid_helium|helium II]] — the same [[Helium_cryogenics|cryogenic]] engineering that a [[Superconducting_magnet|superconducting magnet]] or an [[Magnetic_resonance_imaging|MRI]] bath rests on. When a surface defect on the [[Niobium|niobium]] goes normal it dumps [[Energy|energy]] at a point, which is to say it is a heater, precisely the right source for second sound. Oscillating [[Porous_medium|superleak]] [[Transducer|transducers]], which sense the fluctuating counterflow [[Velocity|velocity]], register the arrival, and trilateration on the arrival times locates the defect. Conway, Hartill, Padamsee and Smith at Cornell (2008) used eight such [[Sensor|transducers]] and pinned a defect that turned out to be an elliptical pit of 0.12 mm major radius, in agreement with thermometry and optical inspection, from a single cold test. The method is in production use and it is not settled: Plouin and colleagues (2019) report that "most of the experimental measurements on cavities show premature signals" — the wave arrives *earlier* than the known u₂ predicts — and conclude that processes in the niobium itself, not faster helium, dominate the timing. Anyone quoting second-sound trilateration as a solved [[Accuracy_and_precision|metrology]] is ahead of the literature. **Counting vortices.** [[Quantum_mechanics|Quantised]] vortex lines scatter the normal component, so they attenuate second sound; the [[Damping|damping]] of a second-sound resonance is therefore a direct [[Estimation_theory|measure]] of vortex line density, and has been the standard probe of quantum [[Nonlinear_system|turbulence]] in [[Superfluid_helium-4|helium II]] for sixty years (Varga and colleagues, 2019). A [[Wave|wave]] made of temperature turns out to be the best instrument for seeing something that is not made of temperature at all. ## Sources Annotated; one clause each on what the source establishes. Bibliography lines are link-light by house rule (§4). **Thermophysical data (every number in "The numbers" above)** - Donnelly, R. J., and Barenghi, C. F. (1998). "The Observed Properties of Liquid Helium at the Saturated Vapor Pressure." *Journal of Physical and Chemical Reference Data* **27**(6), 1217–1274. doi:[10.1063/1.556028](https://doi.org/10.1063/1.556028) · chapter tables online at [pages.uoregon.edu/rjd](https://pages.uoregon.edu/rjd/vapor1.htm) — Chapter 3 Table 3.3 (first sound: 238.2 m/s at T → 0, 234.5 at 1.60 K, 217.5 at T_lambda, 219.4 at 2.20 K); Chapter 4 Table 4.3 (second sound: 83.85 at 0.60 K, 18.93 at 1.00 K, the 18.35 minimum at 1.10 K, the 20.37 maximum at 1.65 K, 19.90 at 1.80 K, 16.78 at 2.00 K, 8.492 at 2.15 K, 2.164 at 2.1760 K, 0 at T_lambda = 2.1768 K); Chapter 18 Table 18.1 (thermal diffusivity of helium I, 2.510 × 10⁻⁸ m² s⁻¹ at 2.20 K). Read directly from the recommended-value tables, not from a secondary quotation. - The value u₁(0)/√3 = 137.5 m/s and the 2 cm crossing times (0.98 ms against L²/4D ≈ 1.1 h, a ratio near 4 × 10⁶) are this article's own arithmetic on the tabulated values above, not quoted figures. **Theory** - Landau, L. D. (1941). "Theory of the Superfluidity of Helium II." *Physical Review* **60**, 356. doi:[10.1103/PhysRev.60.356](https://doi.org/10.1103/PhysRev.60.356) [also *J. Phys. USSR* **5**, 71 (1941); continued **11**, 91 (1947)] — the two-fluid hydrodynamics, both wave modes, the u₂ formula, and the name. **Paywalled; the DOI resolves and the bibliographic record was checked, but the text was not opened for this pass.** - Tisza, L. (1938). *C. R. Acad. Sci.* **207**, 1035 and 1186 — the two-fluid picture and the first prediction of propagating temperature waves, with the velocity formula that turned out wrong. **[UNVERIFIED]**: cited as it appears in Donnelly's reference list; not independently opened. - Schmitt, A. (2014). *Introduction to Superfluidity.* [arXiv:1404.1284](https://arxiv.org/abs/1404.1284) — the modern derivation; the second-sound speed as eq. (2.80b), u₂ = √(s²Tρ_s / ρ c_V ρ_n), and the low-temperature limit stated explicitly as eq. (2.82), "u₂(T → 0) = c/√3 = u₁(T → 0)/√3". The load-bearing source for the √3. - Maris, H. J. (1974). "Hydrodynamics of superfluid helium below 0.6 K. III. Propagation of temperature waves." *Physical Review A* **9**(3), 1412. doi:[10.1103/PhysRevA.9.1412](https://doi.org/10.1103/PhysRevA.9.1412) — the phonon-regime dispersion relation, confirming "the usual second-sound mode with velocity c₀/√3" at low frequency and its rise toward c₀ at high frequency. - Michigan State University PHY451 superfluidity guide, [web.pa.msu.edu](https://web.pa.msu.edu/courses/2016spring/PHY451/Experiments/superfluidity/guide_msu_superfluidity_second_sound.pdf) — the counterflow-at-constant-density statement and the same u₂ formula in per-unit-volume form; a teaching source, used only for corroboration. **History** - Donnelly, R. J. (2009). "The two-fluid theory and second sound in liquid helium." *Physics Today* **62**(10), 34–39. [full text](https://www.physics.umd.edu/courses/Phys404/Anlage_Spring10/Donnelly%20Two%20Fluid%20LHe.pdf) — the authoritative short history and the source of the load-bearing quotation, itself quoting Wilks (1967): "the first attempts to generate second sound were made using piezoelectric crystals to set up pressure variations. It was only after these had failed . . . that an analysis by [Evgeny] Lifshitz showed that a much more effective method would be by periodic heating." Also: "The first successful attempt to generate and detect second sound was in 1944—during the war—by Vasilii Peshkov, who implemented Lifshitz's suggested approach"; Peshkov reaching "about 1.2 K in 1948" and "about 0.5 K" in 1960 "in very good agreement with Lifshitz . . . and settle the dispute once and for all"; the naming of second sound by Landau; and Tisza's two errors (extrapolating entropy to absolute zero, "thus missing the phonon contribution", and guessing rho_n proportional to the entropy). - Peshkov, V. P. (1944). "Second sound in helium II." *J. Phys. USSR* **8**, 381–389 — the first successful generation and detection. **Citation dispute worth recording:** this is the form given by most of the literature, but the AAPPS review below cites the 1944 result as *C. R. Acad. Sci. l'URSS* **45**, 365–366, and Donnelly's own reference list points instead to *J. Phys. (Moscow)* **10**, 389 (1946), *J. Expt. Theor. Phys. (USSR)* **18**, 951 (1948) and *Sov. Phys. JETP* **11**, 580 (1960). The 1944 *date* is verified in Donnelly's text; the exact 1944 volume and pagination are not. **[UNVERIFIED]** — the 1960 JETP paper is openly hosted at jetp.ras.ru but that host is outside this session's egress allowlist and was not opened. - Lane, C. T., Fairbank, H. A., and Fairbank, W. M. (1947). "Second Sound in Liquid Helium II." *Physical Review* **71**(9), 600. doi:[10.1103/PhysRev.71.600](https://doi.org/10.1103/PhysRev.71.600) — the independent US confirmation; abstract verbatim: "the velocity is found to be zero at the λ-point, reaches a maximum of 20.46 m/sec. at 1.7°K and decreases to 19.80 m/sec. at 1.42°K. The accuracy of the measurements is at least ±0.5 percent." - Hu, H., Yao, X.-C., and Liu, X.-J. (2022). "Second sound with ultracold atoms: a brief review." *AAPPS Bulletin* **32**, 26. doi:[10.1007/s43673-022-00055-2](https://doi.org/10.1007/s43673-022-00055-2) — the out-of-phase definition verbatim ("the out-of-phase motion leads to fluctuations in relative density and hence in entropy") and the c₂ = c₁/√3 saturation; also the alternative Peshkov citation noted above. **Second sound in solids** - Jackson, H. E., Walker, C. T., and McNelly, T. F. (1970). "Second Sound in NaF." *Physical Review Letters* **25**(1), 26. doi:[10.1103/PhysRevLett.25.26](https://doi.org/10.1103/PhysRevLett.25.26) — heat-pulse propagation in a purer crystal to higher temperatures, where "the second-sound velocity fails to level off at the theoretically predicted limiting value." - Narayanamurti, V., and Dynes, R. C. (1972). "Observation of Second Sound in Bismuth." *Physical Review Letters* **28**(22), 1461. doi:[10.1103/PhysRevLett.28.1461](https://doi.org/10.1103/PhysRevLett.28.1461) — "in the temperature range of 1.2 to 4.0 K"; the saturated velocity "(0.78±0.05)×10⁵ cm/sec (1/3√3 times the Debye velocity)"; and the ballistic → second-sound → diffusive crossover in one material. The parenthetical renders in APS plain text as "1/3√3" and means (1/3)·√3, i.e. 1/√3 — read the other way it would be a factor of three too small, and it does not reproduce the quoted velocity. - Huberman, S., Duncan, R. A., Chen, K., Song, B., Chiloyan, V., Ding, Z., Maznev, A. A., Chen, G., and Nelson, K. A. (2019). "Observation of second sound in graphite at temperatures above 100 K." *Science* **364**(6438), 375–379. doi:[10.1126/science.aav3548](https://doi.org/10.1126/science.aav3548) · [arXiv:1901.09160](https://arxiv.org/abs/1901.09160) — abstract verbatim on the qualification this article insists on: wavelike transport "on 5-20 μm length scales", with ab initio calculations predicting "wavelike phonon hydrodynamics on ~ 1-μm length scale up to almost room temperature." - Ding, Z., Chen, K., Song, B., Shin, J., Maznev, A. A., Nelson, K. A., and Chen, G. (2022). "Observation of second sound in graphite over 200 K." *Nature Communications* **13**, 285. doi:[10.1038/s41467-021-27907-z](https://doi.org/10.1038/s41467-021-27907-z) — the higher-temperature result, by sub-picosecond transient grating, with clear signatures at 200 K and 225 K. Published 12 January 2022; frequently miscited to 2021 because of the online-first date. - Beardo, A., López-Suárez, M., Pérez, L. A., Sendra, L., Alonso, M. I., Melis, C., Bafaluy, J., Camacho, J., Colombo, L., Rurali, R., Alvarez, F. X., and Reparaz, J. S. (2021). "Observation of second sound in a rapidly varying temperature field in Ge." *Science Advances* **7**(27), eabg4677. doi:[10.1126/sciadv.abg4677](https://doi.org/10.1126/sciadv.abg4677) · [arXiv:2007.05487](https://arxiv.org/abs/2007.05487) — bulk natural germanium from 7 K to 300 K under frequency-domain optical pump-probe, the point being that the *rate of change* of the driving field, not only the temperature, opens the hydrodynamic window. **Applications** - Conway, Z. A., Hartill, D. L., Padamsee, H. S., and Smith, E. N. (2008). "Oscillating Superleak Transducers for Quench Detection in Superconducting ILC Cavities Cooled with He-II." *Proceedings of LINAC08*, Victoria BC. [classe.cornell.edu](https://www.classe.cornell.edu/rsrc/Home/Research/SRF/2009/Oscillating_Superleak_Transducers_for_Quench_Detection_in_Superconducting_ILC_Cavities_Cooled_with_He-II.pdf) — OSTs sensing "the fluctuating counterflow velocity", an eight-transducer array at about 17.2 cm from the beam axis, and the located defect confirmed as a pit of 0.12 mm major and 0.06 mm minor radius by thermometry and optical inspection. - Plouin, J., Baudouy, B., Four, A., Charrier, J. P., Maurice, L., Novo, J., Peters, B. J., and Liao, K. (2019). "Experimental study of second sound quench detection for superconducting cavities." *Physical Review Accelerators and Beams* **22**, 083202. doi:[10.1103/PhysRevAccelBeams.22.083202](https://doi.org/10.1103/PhysRevAccelBeams.22.083202) — verbatim: "most of the experimental measurements on cavities show premature signals, i.e., the second sound signals arrive earlier on the OSTs than expected", and the conclusion that "processes in niobium play a prominent role in the second sound detection for superconducting cavities". The open problem, stated by the people who use the technique. - Varga, E., Jackson, M. J., Schmoranzer, D., and Skrbek, L. (2019). "The Use of Second Sound in Investigations of Quantum Turbulence in He II." *Journal of Low Temperature Physics* **197**, 130–148. doi:[10.1007/s10909-019-02208-4](https://doi.org/10.1007/s10909-019-02208-4) — second sound as both generator and detector of quantised vorticity. **Abstract verified; full text paywalled and not opened, so the attenuation mechanism is stated here from standard theory rather than quoted.** ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Second_sound) : [Wikitube](https://en.wikitube.io/wiki/Second_sound) ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Helium]], [[PORTAL_Helium-3]].