iOscilloscope (App Store link) is a new $.99 app that turns your iPhone into a virtual oscilloscope that can measure sound waves in real time.

iOscilloscope lets you do do several different things:

• FFT allows you to view the sound waves in Fast Fourier transform mode.
• Mute, mutes the sound so that there is no feedback.
• Sonogram allows you to view the sound waves in a sonogram form.

Note that iOscilloscope will not function when headphones are plugged into your iPhone.