SoundMeter turns your iOS device into a handheld sound level meter (SLM).
The built-in and headset microphones are suitable for certain, basic sound level measurements, but high precision external microphones may also be used with appropriate interface hardware. Even though default sensitivity values are included for the built-in and headset microphones, for best results, SoundMeter should be calibrated before use. Sound levels may be off by a few dB when using the default microphone sensitivity.

For iPhone/iPad/iPod touch audio I/O compatibility and frequency response measurements, visit http://blog.FaberAcoustical.com.
NOTE: SoundMeter has not been shown to meet ANSI or IEC standards for sound level meters.

FEATURES
– Measure time-weighted and equivalent sound levels.
– Employ Flat, A, or C frequency weighting.
– Time-weighted measurement options include Fast, Slow, and Impulse weightings.
– Keep track of peak and maximum sound levels.
– Save high-resolution sound level display images to PDF files or to the device’s Camera Roll photo album. Descriptive text may be added to the image before it is saved.
– SoundMeter also offers data sharing via photo and email.
– Calibration controls enable automatic calibration, relative to a calibrated sound level meter, or manual entry of the microphone sensitivity.
– SoundMeter supports rotating an iPhone upside down, so the built-in microphone sits on top of the device when measuring sound levels.
– With the iPhone’s built-in microphone, SoundMeter can measure peak sound levels of up to approximately 105 dB. With the iPhone’s included headset microphone, SoundMeter can measure peak sound levels of up to approximately 100 dB. (These limits are due to iPhone hardware, not the SoundMeter software.)
Results may vary between iPhones and headset microphones. Faber Acoustical, LLC Web SiteSoundMeter Support What’s New in Version 2.0.3 Version 2.0.3 – An issue which sometimes prevented audio output from working properly has been corrected.
– Users are prompted once to reset default microphone and headset input sensitivities.

http://itunes.apple.com/us/app/soundmeter/id287615105