Application "Sound Meter HQ Pro" uses built-in microphone to calculate sound level (noise level) of surrounding. Result of the measurements is displayed in decibels on the screen of you phone.

DISCLAIMER:
Don't treat this utility app as a professional device to measure decibels. This tool uses built-in microphone, which was designed to capture human's voice. Such microphone is not able to capture sounds above ~90 - 100 dB (Maximum value differs depending on kind of microphone you have). In addition, some devices are equipped with AGC (Automatic Gain Control). This system can also disturb correct sound pressure level measurement.