TrueRTA is a software based audio analyzer for testing and evaluating audio systems using just a PC with basic sound capability. The instruments found in TrueRTA include a low distortion signal generator, a digital level meter, a crest factor meter, a dual trace oscilloscope and a high-resolution real time audio spectrum analyzer. By creating these test instruments in software and employing the signal input and output capability of your PC's sound card we are able to offer a level of performance that could only be replaced by many thousands of dollars worth of traditional test instruments.
TrueRTA turns your PC into a powerful audio spectrum analyzer normally seen only in better audio research and design laboratories.

more info available at:
http://www.trueaudio.com/rta_abt1.htm