When a frequency response range is given and identified as "-3dB" it means that the high and low values shown are the points where speaker output has dipped 3dB from the baseline output. Think of the response as being nearly flat across most of the speaker's range (from perhaps 100Hz to close to 20,000Hz), but with slopes dropping off at the left of right. Where those slopes pass the point 3dB below that baseline is where these upper and lower limits are. It's a way of consistently comparing apples to apples for frequency response between different brands of speakers (it's also useful for determining crossover points, of course). Your LSi15's, for example, list an "overall" frequency response of 22Hz to 27,000Hz, but the -3dB response range is 30Hz to 26,000Hz. This means that they may be capable of producing a 22Hz tone, but it will not be at or very close to the same "loudness" as tones from higher frequencies - that slope will have fallen off several dB more.