We test the sound quality of the boards with the loopback test of Rightmark Audio Analyzer. Do note: this means we test the quality of both the input and output of the on-board sound cards simultaneously. The 'weakest link' determines the result.
The dynamic range and noise level test are very comparable and show nearly identical results. The dynamic range test measures the different in volume between the loudest and softest observable signal, the noise level test measures the difference between the loudest sound and the noise level.
To put the numbers in perspective: the highest dynamic range c.q. noise level theoretically possible with 16 bit sound is 96 dB. 99.9% of the sound you listen to on your PC (CDs, MP3s, YouTube, games,...) is 16 bit and the graphs proof that the hardware is no bottleneck in almost all cases. We can only achieve the higher scores by testing with 24 bit audio. By the way, we can't imagine someone actually able to distinguish between 91 dB and 100 dB signal/noise ratio.
The ASRock Fatal1ty Z270 Gaming-ITX/ac is not the best board in terms of audio quality, nevertheless its results are much better compared the best you could get a couple of months ago.
The stereo crosstalk tests to which extent sound from one channel (left or right) resounds on the other channel.
Lastly we measured the total harmonic distortion, the average distortion in the frequency domain, using RMAA. The differences seem big, but we're pretty much looking at the difference between excellent and more than excellent.