The goal of many audio devices is to have a flat frequency response, meaning the signal going into the device is the same across the entire frequency spectrum as the signal coming out. Technically, any circuit will change the sound a tiny bit simply by passing the signal, though we mostly want our audio devices to change the sound only when we intentionally change the settings.
In addition to this post I made a video to help explain the concept, which includes a segment from the Essential Live Sound Training:
Some devices inherently change the sound more than others. If we actually measure this change we can graph it across the frequency spectrum. This is the frequency response of the device and is especially important when considering the transducers in our audio system: microphones and speakers.
Since the process of turning acoustic energy into electrical signal (or the other way around) is pretty inefficient, our transducers aren’t perfect machines. We need tiny fragile circuits in microphones, and a ton of current driving magnets in the speakers. It’s very difficult to get a flat response from them.
But that’s actually just fine, especially with microphones. A unique frequency response gives each one its own sonic characteristics and having a variety of colors to work with is an important tool for audio engineers.
For example, if you’re micing a bass drum, you probably want something that has a good low-end response. Looking at the graph for a common large diaphragm dynamic mic, you can actually see that it’ll work well for instruments with low frequencies.
Or, if you have a microphone that sounds really open and airy, the frequency response graph would probably have a bump in the high frequencies. This would lend itself to specific uses like softer vocals or acoustic instruments. Many condenser mics have this kind of high frequency bump.
With directional mics (anything except an omni polar pattern) the frequency response actually changes depending on how far away you are. When you get really close to the diaphragm, the low frequencies get more pronounced. So, if you're right up on the mic, your voice is going to sound boomier than if you're even a few inches away.
This is called proximity effect. It’s an important consideration for singers, and good ones will use mic technique to keep the right distance. But it’s also just something to be aware of as you’re deciding where to place the mics on any instrument.
If you’re curious about any specific microphone, you can see the frequency response on the manufacturer’s website. Even though there are other factors involved in how the microphone sounds in real life, this is a great way to get a sense of the overall sonic characteristic.