I do all sorts of audio work and I have to constantly properly determine an audio's true quality. As such, I have two recommendations over that site, which I've tried before and aren't a fan of:
#1- Spek (FREE - http://spek.cc/): A small, cross platform, and simple spectrum analyser.
- Supports all popular lossy and lossless audio file formats thanks to the FFmpeg libraries.
- Ultra-fast signal processing, uses multiple threads to further speed up the analysis.
- Shows the codec name and the audio signal parameters.
- Allows to save the spectrogram as an image file.
- Drag-and-drop support; associates with common audio file formats.
- Auto-fitting time, frequency and spectral density rulers.
- Adjustable spectral density range.
- Translated into 19 languages.
#2- Adobe Audition (PAID): If you already have Audition, there's a built in option by going to View -> Show Spectral Frequency Display. This is the one I live by.