A noise canceling microphone is designed to filter out ambient noise from a desired sound or direction. These are particularly useful in noisy environments. The diagram shown here illustrates the pattern of how the microphone receives sound. The microphone faces upward and sound intensity for a particular frequency is plotted for angles radially from 0° to 360°. All noise canceling microphones have at least two ports through which sound enters. The front port is normally oriented toward the desired sound and another port is oriented at a sound that is more distant. Sound that is much closer to the front port than to the rear will make more of a pressure gradient between the front and back of the diaphragm, causing it to move more. The microphone's proximity effect is adjusted so that flat frequency response is achieved for sound sources very close to the front of the microphone. Sounds arriving from other angles are subject to steep midrange and bass roll-off. Noise canceling microphones are found in call centers, helicopters, and race car driver headsets.