"See" sound in realtime on your iPhone by using Spectrogram.

A spectrogram shows you what frequencies you are hearing over time. The color of a pixel in the graph shows how much sound energy is occurring at that particular frequency and time.

• Adjustable sensitivity
• Discover images hidden in music.
• Logarithmic or Linear frequency scale.
• Analyzes frequencies up to 20kHz!
• 2048 FFT window size.
• Frequency scale can be toggled on/off.
• Pause capability.
• Save screen captures to your Camera Roll by pressing the Sleep/Wake & Home buttons together.