Originally posted by: YOyoYOhowsDAjello
I'm sure you could look online and find a better explination of what I can give you, but a +/- 3dB rating gives you a range where the speaker or sub produces an even volume. There can be fluctuations within the range, but from the rated range (like 50hz to 20khz), the speaker is supposed to stay within 3dB louder or softer than a reference point.
It's easier if you look at some graphs.
Ascend 170s are known for having a very flat response over a lot of the audio spectrum, but they really need a sub to complement them.
You can see from that top graph that the line is very flat (horizontal) from about 90hz to almost 20khz (it's a logarithmic scale).
The vertical axis is basically how loud it's going to be for a given frequency (the horixontal axis)
Ascend rates these at 69hz to 20khz +/- 3dB. This +/- 3dB rating is a guide for how high or low something can get before you discount that range. You can see that below 70 and especially below 40hz, there is significantly less output than for the +/- 3dB range.
A 3dB change in volume is not too noticable, a 5dB change would be readily noticable, and a 10dB change would seem like something is twice as loud or half as loud.
So having this +/- 3dB range is a standard way of measuring a frequency range a speaker is able to produce at a consistent volume.
Here are some speakers that state a frequency response of 20hz to 20khz
must be awesome
So yeah, they might be producing some sort of output at all those values, but it's not going to be at an even level, especially the low end.
That was just an extreme example of the kind of arbitrary ratings that are out there. A -3dB point will give you a better idea of where the bass will start to roll off for a given speaker than looking at a full frequency range measurement without a +/- 3dB rating.
What kind of power supply are you using? 
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