I have also found another formula in the net which is an aproximation for vey small bubbles, but gives radical deviation for bigger quantities. In this formula the raduis of the bubbles raised to the power of -1. While in Landau's formula it in -3/2 (i.e. the resonance frequency is reversly proportional to square root of the 3-rd power of the raduis).

If I'ld be very gratefull if someone gave me the exact formula.