A null test is a simple check to see if one track is a perfect digital copy of the original. This requires lining up, sample accurate, the original and copy on two separate tracks and reversing the polarity of one track. On playback, with both tracks mixed together and at unity gain, you shouldn’t hear any sound.
This is never going to happen when comparing an original 24bit WAV file with an MP3 version of itself. However, it is very useful at hearing the destruction that is done, and artefacts that are created, when we convert the beautiful original to an MP3.
I performed this test as there were quite a few comments on Twitter about the quality of SoundCloud‘s MP3 conversion to their 128kbps streaming audio. The track used was Julian Cole’s Miasma 2 which you can hear by clicking this link:
I was fortunate to hear the 24bit version of this track which, as you can imagine, sounds stunning in the studio.
If you download this track it will be the original 24bit WAV version so you should have a more accurate listen 🙂