Where exactly do you get the frequency response. It is from the rating on the speaker or from the amp. I would like to get down to around 20 so can I just change the speaker or the amp or both?
All audio components are measured for frequency response, the flatter the better. Electronics can do ruler-flat reponse while transducers are trickier to get flat.
As for reaching a low of 20 Hz, you need a superior speaker; the most ambitious (and expensive) subwoofers start rolling off at the 30 Hz point.
Yes, I am talking about subwoofers. So I can just swap out the speaker and use the same amp and get down to 20 Hz as long as that is what the speaker is rated at.
No,it's not that simple. If the cabinet is not tuned to go that low, and doesn't match well with the parameters of the new driver, it may or may not get down to 20Hz, and will give you unpredictable results - you'll get sound, but it may not sound "right".
The amp has to be capable of handling the new driver as well - if you drop a heavy duty driver into an old sub with a low powered amp that was intended to drive a lesser driver, you may fry it or the new driver, or both.
Frequency response is not built into the sub or driver. A frequency is a specific sound wave moving through the air. The driver in a speaker moves air to produce a sound wave. Depending on how big or small the air movement produces different frequencies. (oversimplified I know) A 1" tweeter could move in and out slow enough for a 20hz wave, but it would not be able to move enough air to be audible. Since bass has the biggest waves it takes the most to produce. It needs the combination of the driver, the box, and any ports to move enough air for 20hz to be audiable. So you can't just change 1 thing and magically have 20hz. Whatever you do has to have the abality to move enough air to produce the note you want at the volume you want.