Frequency response is another factor to consider in selecting a microphone. Each microphone hears sound differently. Microphones will boost or cut certain frequencies across their range, which is what frequency response means. Many microphones aim for a flat response, which means there is little boosting or cutting of frequencies. With a flat response, you get as accurate a representation of the source sound as possible. There are limits to the minimum and maximum frequencies microphones can detect. The lowest frequency that most microphones can hear is 20Hz, and the highest frequency is 20kHz. You will see these numbers in product literature and advertisements (20Hz to 20kHz response). Figure 9-2 shows a frequency response chart for a dynamic microphone—a Shure SM57, a classic microphone that sounds great on guitar amps and snare drums.
Figure 9-2: Microphone frequency response
However, not all microphones go down to 20Hz, and not all go up to 20kHz. So, why is this information important? Consider the boundaries of sound. The lowest note on a piano registers at 27.50Hz, which is about the lowest sound you can really imagine recording. The highest sound on a piano clocks in at around 4kHz. There are, of course, higher frequency sounds. We won't get into this here, but all sounds spread into many frequencies, and there are always higher overtones present. Microphones need to pick up these higher frequencies for our ears to accept the sound as “natural.”
The loudness of any sound is measured by the level of sound pressure present, measured in decibels (dB). An airport runway is about 120dB and a normal conversation is about 60dB. Loud amplifiers, screaming singers, and certain drums can put out extremely high sound-pressure levels. If the microphone you choose can't handle the sound-pressure level, you can damage or distort the microphone over time, rendering the signal unusable.