We apply the Backus-Gilbert approach to normal mode center frequency data, to constrain jumps in P, S, bulk-sound speed and density at the “660” discontinuity in the earth’s mantle (~650-670 km depth). Different 1D models are considered to compute sensitivity kernels. When using model PREM (Dziewonski and Anderson, 1981) as reference, with a “660” at 670 km depth , the best-fitting jumps in density, P and S wave-speeds range from (5.1-8.2)%, (5.3-8.0)\%, (5.0-7.0)%, respectively, so the PREM values lie outside the ranges of acceptable density and P wave-speed jumps. When shifting the depth of “660” to 660 km, the density and S wave-speed jumps increase while the P wave-speed jump decreases. Normal mode data do not support a global transition at 650 km depth. The density jumps are closer to those of pyrolite than PREM while our bulk-sound wave-speed jumps suggest a larger garnet proportion at “660”.