How to plot the decibel level of the sound in the atmosphere using iPhone's inbuilt mic, without recording the sound?

I want to plot the decibel level of the sound in the atmosphere that hearing through the iPhone's inbuilt mic. I don't want to record the sound to plot. I want a sound l开发者_开发技巧evel indicator, with a graph that frequently indicates the decibel level with respect to the sound. How can I do this?

http://www.politepix.com/2010/06/18/decibel-metering-from-an-iphone-audio-unit/