This application measures ambient noise in decibel using the built in microphone of the iPhone and iPhone 3G / 3GS and iPhone4. Apart from a modern and vintage look it also features a reference view showing typical noise producers in relation to the currently measured noise. It can record up to 500 samples, display them graphically and send them via eMail for further analysis.
Disclaimer: Even though the dBMeter Pro Software has been calibrated using professional grade equipment, do no use dBMeter Pro to carry out professional measurements. The author can not be held liable for any damage which might arise by using dBMeter Pro. 
 
http://itunes.apple.com/nl/app/dbmeter-pro/id296264086