A mixture of fact with fable. Danny Owens has the most facts:

"For every 3 db increase in sound pressure you need twice the amp power to drive it."

"It is a common fact that a 3db increase in sound pressure requires twice the power. If you can get 100 db with 50 watts, it takes 100 watts to get 103 db."

Sensitivity is how efficent the speaker is at turning the amplifiers power into sound. A speaker rated at +3 db more than another will require only half the power to produce the same output.

paraphrased from www.pioneerelectronics.com

90 db at 1 watt at 1 meter
93 db at 2 watt at 1 meter
96 db at 4 watt at 1 meter
99 db at 8 watt at 1 meter
102 db at 16 watt at 1 meter
105 db at 32 watt at 1 meter
108 db at 64 watt at 1 meter
111 db at 128 watt at 1 meter
114 db at 256 watt at 1 meter
117 db at 512 watt at 1 meter
120 db at 1024 watt at 1 meter

now compair

99 db at 1 watt at 1 meter
102 db at 2 watt at 1 meter
105 db at 4 watt at 1 meter
108 db at 8 watt at 1 meter
111 db at 16 watt at 1 meter
114 db at 32 watt at 1 meter
117 db at 64 watt at 1 meter
120 db at 128 watt at 1 meter
123 db at 256 watt at 1 meter
126 db at 512 watt at 1 meter
129 db at 1024 watt at 1 meter

A speaker with a db rating of 90 db at 1 watt at 1 meter will put out 120db of volume at 1 meter at the cost of 1024 watts.

A speaker with a db rating of 99 db at 1 watt at 1 meter will put out 120db of volume at 1 meter at the cost of 128 watts.

However some of the loudest speakers have what is concidered low sensitivity
for example the Pioneer Premier TS-W5000SPL 12 inch sub that shattered records and won a Deathmatch championship at the 2004 dB Drag Racing World Finals. (car audio) the sub can exceed 170+ dB. and yet the Sensitivity is 88dB.

Danny please correct me if I am wrong.

and no i'm not a Pioneer rep I'm just an audiophile.