In the studio we have been taught that microphones with a flat frequency response would be the ideal microphone to capture a voice or an instrument, capturing exactly the sound that resonates from the source. However, when we mixing those perfectly recorded instruments we tend to grab an EQ and cut away certain frequencies and boosting other frequencies all in an attempt to get the instrument to fit the mix better.
Enter the tailored frequency response. These microphones have been designed to produce a certain sound from an instrument without using an EQ, because the microphone already has those EQ characteristics built into the mic.
Let's look at some specific microphones: For many years the AKG D12 was the go to kick drum microphone, but if you listen to that microphone by modern standards, you’ll notice that the midrange is very forward and you would probably want to cut some of the low-mids to make it sound more modern. So when AKG redesigned the D12 they decided to improve the frequency response to suit bass- and kick drums well. The new mic has a boost in the low end and high midrange, probably because they knew engineers were going to do that anyway in the mix. The result is the AKG D112 which for many engineers is the premier kick drum microphone that can cover multitudes of music genres. Other microphones like the Sennheiser e902 have an even bigger scoop in the midrange and without any EQ would almost sound like a kick drum recorded for a metal song.
The most famous of tailored microphones are probably the Shure SM57 & SM58. These mics were designed in the 1960s and are still as relevant today as when it was released. The SM58 is the industry standard live vocal microphone that you will find at almost every live show. These mics are tailored to target the main sound source and reject background noise, meaning that it sound great when a vocalist is singing directly into it and is great at feedback rejection. The SM57 is tailored specifically for instruments and drums and some of the best engineers in the world still consider it the best microphone for capturing snare drums and guitar cabs.
When you look at the frequency response curves of these microphones you would never think that these mics could be used for recording anything, but in practice these microphones do exactly what we need. By simply using the mic for what it was designed to do and utilizing good microphone placement, you can get a professional sounding instrument without spending a small fortune on microphones with a flat frequency response. So it’s a good idea to study the spec sheets of the microphones you have available to check what the manufacturer had in mind when designing the mic. You will find that not all microphones are full range microphones with a flat frequency response, and if you think about it: Do you really always need to have a full range microphone on an instrument?