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 motherboards with the ALC1220 score significantly higher in this test than the models equipped with the ALC892. It's worth noting that the dynamic range is better on the ASUS Strix Z270G Gaming, whereas the ASRock Z270M Extreme 4 does better in the noise level test.
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.