As a general design rule, the interference tube of a line microphone must be lengthened to narrow the acceptance angle and increase the working distance. While shorter line microphones may not provide as great a working distance as their longer counterparts, their wider acceptance angle is preferred for some applications, because aiming does not need to be precise. If you are in a very noisy environment, and can point the microphone away from the noise, a comparison will show a better ratio of wanted to unwanted sound with the cardioid than with the omni. As you get within about two inches, you'll notice a rising bass response in most cardioid microphones. This is known as proximity effect, a characteristic that is not shared with the omni microphone used for comparison.
Proximity effect can either be a blessing or a curse, depending on how it is used.
A singer can get a deep, earthy sound by singing very close, then change to a more penetrating sound by singing louder while moving the microphone away. This kind of creative use takes some practice, but is very effective. Figure 8 Influence of Proximity Effect on Directional Microphone Response Proximity effect can be used effectively to cut feedback in a sound reinforcement situation. If the performer works very close to the mic, and doesn't need the extra bass, an equalizer can be used to turn down the channel's bass response. This makes the microphone less sensitive to feedback at low frequencies, since it is now less sensitive to any low-frequency signal arriving from more than a foot away.