well-developed tool in ultracold gases used to study density fluctuations (so far, mainly in the collisionless region [20, 38, 39]), is directly proportional to Im χ$_{nn}$ ( q, ω ). In contrast, the low- frequency limit of the dynamic structure factor S ( q, ω ) = − Im χ$_{nn}$ ( q, ω ) /πn [1 − exp( − βω )] measured in Brillouin light scattering, and also the amplitude of density pulses that can be generated in dilute gases [10], are proportional to Im χ$_{nn}$ ( q, ω ) /ω . This extra factor of 1 /ω leads to a large enhancement of the weight of low frequency second sound in S ( q, ω ) compared to first sound. The relative spectral weights of first and second sound in χ$_{nn}$ ( q, ω ) are very dependent on how strongly temperature fluctuations couple to density fluctuations. For S ( q, ω ), this coupling is conveniently parametrized by the value of the dimensionless Landau–Placzek ratio ϵ$_{LP}$ = γ − 1, where γ , defined in (16), is the ratio of the specific heats (per units mass) at constant pressure and volume. At finite temperatures, the relative weight of first and second sound ( W$_{2}$/W$_{1}$ ) is equal to ϵ$_{LP}$ in both unitary Fermi gases and superfluid $^{4}$He. The key difference between superfluid $^{4}$He and a unitary Fermi superfluid lies in the value of ϵ$_{LP}$ . Apart from a small region very close to the transition temperature T$_{c}$ , one has ϵ$_{LP}$ ≪ 1 in superfluid $^{4}$He [16, 17]. In contrast, for T ≳ 0 . 7 T$_{c}$ , one finds that ϵ$_{LP}$ is of order unity in Fermi superfluids (see figures 3 and 4) and hence the second sound resonance has appreciable weight in S ( q, ω ). Density pulse experiments as suggested in [10] seem to be a promising way of detecting both first and second sound pulses in Fermi gases, although more work is needed to clarify the nature of first and second sound in cylindrical geometries where such pulses would be generated. (See, for instance, recent work on this problem in [40].) The experimental results reported in [22], where a blue-detuned laser was used to generate a density pulse in a Fermi gas close to unitarity, revealed no clear signs of a second sound pulse. This experiment was carried out at very low temperatures, however, where second sound will not have appreciable weight in S ( q, ω ) (see figures 3 and 4). Alternately, even though the weight of second sound is comparatively small in Bragg scattering, Bragg scattering has the advantage that it can be used to probe in a very precise way the properties of uniform superfluid Fermi gases [37], including the velocity of second sound. In section V, we compare the two-fluid hydrodynamics in a uniform, dilute, weakly in- teracting Bose condensed gas (figure 6) with a strongly interacting Fermi superfluid gas. In such Bose superfluids, the thermal expansion (and hence, ϵ$_{LP}$ ) is much larger, with the 24