THD+N
THD+N measures harmonic distortion and noise together relative to the test signal.
The percentage depends on signal level and measurement bandwidth as well as the device. Noise can dominate at low levels, so results need comparable conditions.
THD+N — Headphone Amp
For a headphone amplifier, THD+N describes the loaded headphone output. Gain, load impedance, signal frequency and output level set the operating point.
Compare equivalent conditions and retain any upper-limit notation. A low result into a light load does not establish the same result into a more demanding headphone.
THD+N — Crossover
For an active crossover, THD+N describes a specified signal path or output branch. Gain and filter settings affect the signal reaching that output.
Compare the same branch, settings, test level and bandwidth. A heavily attenuated test signal can change the ratio, so different filter conditions should not be treated as equivalent.
THD+N — DAC
For a DAC, this measurement concerns the analogue line output at a stated digital or analogue reference level. RCA and XLR outputs may have different levels.
Retain the actual THD+N or THD label, bandwidth, frequency and output path. A DAC-chip or headphone-output measurement does not establish the finished line-output result, and intermodulation distortion is a different metric.