total harmonic distortion plus noise (THD+N)
Total Harmonic Distortion plus Noise(THD+N) is a characteristic value of audio devices, comparable to the THD. The difference is that with THD+N, the harmonics are measured together with the noise as an RMS value.
In the THD+N measurement, a low- harmonic sinusoidal signal whose harmonic purity is known is fed into the device under test, and the output signal is fed back to the measuring device. Filters are used to filter out the test frequency, and the level of the remaining signal, which includes all harmonics, mains hum, and noise from the device, is measured over the frequency range of interest. For audio, this would be the frequency range between 20 Hz and 20 kHz. This composite signal is measured in RMS and represents the THD+N value.
The THD+N values are given in the specifications, specifying the frequency, frequency range, and level. A similar statement can be made with the signal-to-noise ratio( SNR).