The dB ratings tell you the relative sensitivity of a speaker, generally measured with 1 watt of input and at a distance of 1 meter. It is generally measured @ 1000hz.
1 Hz is one cycle per second. 20Hz is the lowest frequency that most people can hear. 1 kHz is 1 kilohertz or 1000 Hz. 20 kHz is generally the highest frequency that people can hear. Generally a component set will have a frequency range listed and generally it is BS. The speaker might reproduce the low frequency listed, but usually only at low levels. If you turn it up and try to play that frequency, you will usually cook the speaker. If you have a sub, the low frequency extension of most speakers is enough. In fact, you usually want to cross the mids over in their linear range to keep from asking too much of them. If you are stuck using 5.25s, you might want to run a dedicated midbass to fill in the gap between the sub freqs and the midrange if you are using a large sub.
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