Abstract In the vortex liquid state the Josephson plasma frequency ω p is strongly affected by fluctuations in the vortex pancakes. We have measured in detail the variation of ω p with field B and tilt angle θ in Bi 2Sr 2CaCu 2O 8. We compare the data with Koshelev's expression relating ω p to the correlation function of the local phase S 1. The agreement is quite close for θ > 1°, provided f varies with B with a weak exponent ( f measures the number of pancakes within a correkated area). A scattering picture is introduced to clarify how the plasma resonance is equivalent to a Fourier transform of | S 1| 2. From the fit, we fin that, at 50 K and 1 T, f ≈ 1.4 pancakes, J c0 equals 4160 A/cm 2 and 〈cos φ n, n+1 〉 = 0.028. Deviations observed when B is aligned with the planes are discussed.