Frequency Response   

a measure of the range (spectrum) of frequencies which a given system will be able to process.  in humans our frequency response is normally 20 - 20,000 Hz with differing sensitivity depending on where in the spectrum the signal is. (see equal loudness contours) in electronic systems the frequency response will also include a graph of intensity over time to indicate how the system deals with different frequency ranges.  the ideal system would be indicated by a flat line; most drop off at the extremes, indicating the system's lower sensitivity to very low or high frequency signals.  

the intensity is usually measured in dB.  thus a sound system may be said to have a frequency response of .30 - 21KHz +/- 1dB.  this would indicate a fairly "flat" response with little colouration of the sound by the system.

The limits of most common playback systems are such that even if one had excellent hearing, the extremes of the frequency spectrum (20-50 Hz and 14 KHz - 20 KHz would likely be lost. This example starts somewhere below 100 Hz and drops out somewhere around 16 KHz. partial sweep of the audio spectrum