Adi,
I sent the previous message without fully reading your request. So continuing on, what the rating basicly means is that the higher the db per meter rating, the less power you will need to achive the same sound pressure at the same distance. However, power curves are nonlinear. For every 3 db increase in sound pressure you need twice the amp power to drive it. Using an example of comparison of 2 different pairs of speakers. One rated at 87 db and one rated at 90 db. To generate a sound pressure level 90db, you need only half of the power that you would need for the 87 db speakers to achive the same sound pressure level ie 90db.
Clear as mud?? Let me know if you need further clarification.