Can someone explain db sensitivity?

The higher the sensitivity, the louder a speaker will play with the same input. This loudness is measured in dB. A difference of 3 dB is twice as loud. So a 90dB speaker will play twice as loud as an 87dB speaker with the same input.

The "what is good" question is debatable. Everthing else being equal, a higher sensitivity is better because the amp doesn't have to work as hard to produce the sound level you want.

However, some will say that in order to achieve this high efficiency, compromises must be made in the speaker design so lower efficiency speakers and more powerful amps are the way to go. Others will argue that low power amps sound better than high power amps so higher efficiency speakers and less powerful amps are the way to go.

Note to Aragain. I beg to differ. I believe you are confusing percieved loudness as measured in dB with amplifier power, which is also measured in dB although they are two different things. The 90dB versus 87dB is the output loudness from the speaker, not the input power to the speaker from the amplifier.

The general rule, as you correctly state, is that to double the loudness from a particular speaker, you need to provide it with 10 times more power. Example: you are listening to an amplifier that is delivering one watt to the speaker. To make it sound twice as loud, it would have to deliver ten watts.

Twice as loud is a change of 3dB in loudness, not in amplifier input power. You need to send ten times (10dB) more power to the 87dB speaker to get the same loudness as the 90dB, not twice as much as you state.

Ok, after doing some research, I now think the 10 times power part of my answer was wrong. I don't know where I got that but I know I've heard it before. Must be the 10 times reserve power mentioned above. Sorry for the confusion.

I also agree with Sean, I just can't type very fast and a post that long would take me about a week.