The Distortion graph shows the measurement, its harmonic distortion components up to the tenth harmonic and Total Harmonic Distortion (THD). The plots are derived from analysis of the impulse response. Impulse responses measured using logarithmic sweeps separate distortion from the linear part of the system response, the distortion components appear at negative times, behind the main impulse. Analysing the frequency content of these components allows plots of distortion harmonics to be generated.
The harmonic plots can only be generated for frequencies within the bandwidth of the measurement. For example, if a measurement is made to 20 kHz, the second harmonic plot can only be generated to 10 kHz, as the 2nd harmonic of 10k Hz is 20 kHz. Similarly the third harmonic plot can only be generated to 6.67 kHz (20/3). When opening measurements made before the Distortion graph was added to REW fewer harmonics may be available.
Total Harmonic Distortion is generated from the available harmonics up to the tenth. At higher frequencies the THD plot will incorporate fewer harmonics, according to which are available.
The plots of the Fundamental (the linear part of the measurement) and the distortion harmonics do not include mic/meter or soundcard calibration corrections. This is to avoid the effect of the corrections generating a misleading view of distortion levels. For example, mic/meter and soundcard calibration corrections boost the lowest frequencies of measurements to counter the roll-off of the mic/meter and soundcard interfaces, but adding those corrections to a distortion plot would make distortion appear to rise at low frequencies, hence their omission.
The fundamental and harmonic plots are smoothed to 1/24 octave. This cannot be adjusted. The distortion data can be exported to a text file using File -> Export -> Distortion data as text.
The control panel for the graph has these controls:
The Distortion Figures control selects the units that are used for the harmonic distortion levels displayed on the graph legend. The choices are dB SPL, which shows the actual sound pressure level of each harmonic; dB Relative, which shows how many dB the harmonic is below the fundamental at the cursor frequency; and Percent, which shows the harmonic level as a percentage of the fundamental.
The Highest Harmonic control allows the higher harmonics to be hidden if they are not of interest. For example, if Highest Harmonic were set to 3 only the second and third harmonic traces would appear on the graph and in the graph legend.
Here is a distortion plot generated from a loopback measurement of a soundcard, produced at a high sweep level (-4dB FS, which resulted in 2 dB of headroom at the gain settings used). The readings in the legend are with the cursor at 1 kHz. Note that this plot has a very large SPL range, 140 dB, to make the harmonics visible on the plot.
The THD trace has been omitted, as it overlays the 2nd harmonic trace (in red) which is the dominant component, 0.07%. The 3rd harmonic (in orange) is much lower at around 0.01%, whilst the higher harmonics are largely within the noise floor.
This is the impulse response for that measurement, the distortion peaks are to the left of the main peak. The first peak to the left is the 2nd harmonic, the next is the third harmonic and so on.
The next plot is from a room measurement. The 2nd (red), 3rd (orange) and 4th (yellow) harmonic traces are shown, along with the THD (black). Higher harmonics were within the noise floor. The measurement shows a sharp rise in 3rd harmonic distortion at 94 Hz, and a dramatic rise in all distortion components from about 2 kHz upwards. Further measurements at differing signal levels established that this distortion was being introduce by the SPL meter used for the measurement.
This is the impulse response for the in-room measurement, the distortion peaks are clearly visible to the left of the main peak.