Microphone Frequency Response
The frequency response of a microphone is a graphical representation of how a microphone will respond in the audio spectrum. The x-axis represents the measured frequency of signals, and the y-axis represents the relative response in decibels (dB). It usually spans from 20 to 20,000 Hz.
The flatter the frequency response curve is and more faithfully the microphone will represent reality. Microphones with a non flat curve enhance certain frequencies over others. Therefore, the signal will be distorted and unreal.
Despite it might be an interesting idea to use a microphone that records only a certain band of the audible spectum - in order, for example, to mach the extension of a particular instrument and filter out other instruments that use other bands - it is not wise to use a hardware that has intrinsic limitations. Unwanted frequencies can be easily filtered out with proper software, instead of working with a faulty hardware.
The diagrams above display the frequency response of two famous microphones, the Oktava 319 and the Shure SM58. As it can be see, they are both far from a flat curve. The Shure SM58 in particular is just crap, as it has a 23 dB swing. Considering that 3 dB mean a double volume, 23 dB is almost 8 times the volume. Therefore, certain frequencies will sound 8 times lower than others. Moreover, the frequencies between 3 kHz and 7 kHz will stand over all the others. This may be good for vocals, which are centered around 5 kHz, but the recording will be lacking in harmonics anyway, as it will suppress the higher ones.