Distortion measurements (THD, THD+N and IMD) are traditionally expressed in terms of percent. But what do 1%, 0.1%, 0.01% or 0.001% mean in terms of loudness or audibility?
If you are like most people you just know that 0.001% is the best of the three numbers listed above. If you are a well-trained geek you will recognize that each added leading zero represents a 20 dB improvement. 0.01% is 20 dB lower than 0.1% and this is 20 dB lower than 1%. The well-trained geek will convert % to dB in order to give meaning to these numbers.
When distortion reaches 1%, it is only 40 dB lower than the music (-40 dB), and it should be easily audible. When distortion reaches 0.1%, it is 60 dB lower than the music (-60 dB). When distortion reaches 0.01%, it is 80 dB lower than the music (-80 dB). Obviously the -40 dB, -60 dB and -80 dB figures are easier to understand than 1%, 0.1% and 0.01%.