Adi,

Most speakers are rated in db per watt per meter. In other words if a speaker is delevered 1 watt or power it will generate sound pressure that is (in your example) 90 db at 1 meter. Some speakers are rated at db=watts/2.83volts. If you do the math for 8 ohms, power = voltage squared divided by impedence. (2.83^2)/8 = 1. So you see that the watts per meter rating is the same as the watts per 2.83 volts at 8 ohms. Of course the higher the sensitivity, the more effecient the speaker.
Danny