Interpreting artifact levels
Psychoacoustics tells us that loud noise can mask more subtle sound. Soft noise and artifacts are inaudible, until they reach a level where they suddenly become very noticeable. Small differences in artifact levels can cause them to be clearly audible or not at all. Normally, a 1-2 dB difference doesn't have a big impact, but as soon as audio is masked by other audio, changes of this magnitude can have a large impact on our perception of audio quality. This is the case for any codec: not just MicroMPX, but also MP3 for example.
The difference signal of audio that has been through the MP3 codec has pre-ringing, sounds rougher and contains tones that were filtered out in its psychoacoustic stage. MicroMPX does not contain these undesirable characteristics, but its difference signal sounds similar enough to that of MP3 to make comparing their levels useful.
As you can see in the graph below, at 320 kbit/s, MicroMPX adds artifacts that are 6 dB lower than those of MP3. That's the total difference over the entire spectrum, at the frequencies where the artiacts are most noticeable the difference is even bigger - around 12 dB. These artifacts decrease in power by 6 dB, every time we step up MicroMPX's bitrate by 72 kbit/s.
The follwoing graphs show the effect on the waveform (top) and on artifact levels (bottom) of different codecs at different bitrates. The effect of the waveform is the RMS level of the difference between the compressed and the original signal, versus the original signal. The levels in dB's show how high the added noise level is, in absolute values. For MicroMPX these levels are flat across the spectrum, for MPX we have taken the average value.