For THD+N Level, the measured distortion plus noise (the signal with the stimulus tone removed) is expressed as an rms level.
对于THD+N电平，测量的失真加上噪声（去除刺激音的信号）表示为均方根电平。
For THD+N Ratio, the rms level of the measured distortion plus noise (the signal with the stimulus tone removed) is divided by the rms level of the total signal. The result displayed on a bar meter or as a point on an XY sweep graph. THD+N ratio is most often stated in as a percentage or as a decibel value, where 0 dB represents the total signal.
对于THD+N比，测量的失真加噪声（去除刺激音的信号）的均方根电平除以总信号的均方根电平。在条形图或XY扫描图上显示的结果。THD+N比通常表示为百分比或分贝值，其中0 dB表示总信号。
以上说明截取自AudioPrecision的官网，直白来说，就是THD+N level，是总谐波失真+噪声的均方根电平，THD+N ratio，就是((THD+N level) / 总信号)的均方根值。
举例说明：
上两图是某影音公司三款调音台的指标，其增益均为0dBu，输入信号均为0dBu。以A设备来看，THD+N level值为-85dB，换算成均方根值则为0.00004356Vrms，而输入信号0dBu换算成均方根值则为0.775Vrms，0.00004356Vrms / 0.775Vrms = 0.0000562，与图3a中A设备的THD+N ratio基本吻合，而0.0000562 * 0.775 = 0.000043555Vrms，将其换算为dBu后即为-85dBu.
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