+/- db

Let me see if I can explain this, and if my understanding on this is correct.

it basically means variation in loudness ie deviation from a "flat response"

-Suppose u are hearing a tone the same loudness from say 10Hz to 40Khz and your speaker has a response rated at 40Hz to 20Khz +/- 3db

since the speaker will not have a flat response, it will reproduce this sound with differnce output levels at different freq.
hence a average person will start making out that the loudness is dipping from below 40Hz onwards...
(an average person is just able to make out the difference of 3dB of difference in a sound)

that is why the 10db point is also important as that is the point at which a sound volume will halve

hence two speaker may have the same response at3db from 40 to 20. but one may drop sgharply to a 10db point at 35Hz hence inaudible sound below that..while another may have its 10dB point only at say 28Hz..hence it goes down lower..

Hope I have been clear in teh above !

Clasically, frequency response bandwidth is defined as the frequency extremes at which the outputs drops by 3dB from its value at mid-band or passband,as it gently rolls off (imagine a bow response). Applied to speakers as far as I understand, the +/- dB levels define the cutoff power points that are considered as the limit of usable bandwidth. So if a speaker response is specifiedd as "45Hz to 20KHz, +1dB/-3dB", then it means the output is down by 3dB at 45Hz and 20KHz as compared to midband flat response and also there could be a peak of upto +1dB at some intermediate points - so the power output could vary as much as 4dB within the 45Hz to 20KHz band.