how to achieve a reduction of noise of -60 dB in as little thickness as possible of insulation material?
Think about what you are hoping for. A 10dB reduction means reducing the sound power by a factor of 10. Dropping the sound power by 60dB therefore means reducing it by a factor of 1 million, or stopping 99.9999% 

To put this in context, this is equivalent to building a wall that blocks 100% of the sound, (let's say the wall is 9' tall by 10' wide), and then leaving a 1/8" diameter hole in it.

See also the wikipedia article on sound transmission class, which suggests that typical interior wall construction gives STC 33, and that STC 45 is sufficient for loud speech to be inaudible. STC 45 can be achieved with double layer of 1/2" drywall on each side of the partition, over wood studs with batt insulation in the wall.