NoiseLevel

App Storeの説明

NoiseLevel is a fun way to measure the sounds in your surroundings. Measure the sound level of the music you listen to, your kids playing or your office environment.

NoiseLevel uses your iPhone's built-in microphone to first pick up sound and then displays it in dB (= decibels) on a status bar at the bottom of NoiseLevel's main screen.

Sounds are also displayed visually as a balloon whose size and color change according to the level of the sound measured. See the balloon burst at high sound levels - you can also make it burst by creating a sharp, high sound, for an example by clapping your hands. Have fun experimenting!

For best results, turn the iPhone so the microphone is pointing directly at the sound source. The screen flips accordingly.

Disclaimer:
NoiseLevel is fairly accurate but it should NOT be considered a professional sound level meter. The built-in microphone of the iPhone can only measure values between 35 to 110 dB and values may vary slightly between iPhones.