What is speaker’s Sensitivity ?

It is defined as the loudness in dB measured at 1 meter from the speaker when one watt is being applied to the speaker. Let’s say the spec reads 90 dB.  A scale would look something like this:

90 dB = 1 watt

93 dB = 2 watts

96 dB = 4 watts

99 dB = 16 watts

102 dB = 32 watts

105 dB = 64 watts

108 dB = 128 watts

111 dB = 256 watts

For each 3 dB gain (the amount that your ears can hear as a change), you have to double the wattage.  In our example, a 100 watt amplifer would drive your 90 dB sensitive speaker to a maximum output of 105 dB.  Anything above that would probably be distortion.