Vocal Lab helps you train to sing on key. It listens while you sing and displays the pitch of your voice on a graph, in real time. All you have to do is watch the graph while you sing and try to stay near the lines.
With a little practice, you can greatly improve your intonation. Vocal Lab can help you identify problem areas in your singing, and can also help improve your intonation on various instruments - woodwinds, brass, and strings.
GETTING STARTED
Inspect the Volume Indicator
Talk or play something while watching the volume indicator. Several lighted blue bars (4-7) should appear then vanish again. If no bars appear, too few bars, or the red bar appears often, please read the section entitled, 'Selecting the Microphone'. 
SELECTING THE MICROPHONE
Laidman & Katsura tuning programs work by 'listening' through your Microphone. Because your Mac may support more than one sound input device (Microphone, Line Input, etc.) you may need to choose one.