This graph shows a spectrogram plot over the region from 10Hz to the end of the measurement sweep. The spectrogram is like a waterfall viewed from above, with the level indicated by colour. The scale showing how colour relates to level is displayed to the left of the plot. The vertical axis of the plot shows time, increasing towards the top of the plot. The time starts, by default, the width of the selected window before zero, so that the onset of the response can be seen. The areas where the response is decaying more slowly show up as streaks rising up towards the top of the graph.
The spectrogram plot is generated in the same way as the Spectral Decay plot, shifting the impulse response window to the right by a proportion of the time range to generate each succeeding slice. The window types may be selected via the Spectral Decay entries in the Auswertungseinstellungen. The plot uses logarithmically spaced data at 96 points per octave with 1/48th octave smoothing applied.
To produce the spectrogram plot click the Generate in the bottom left corner of the graph area. The legend panel shows the plot value at the intersection of the vertical and horizontal cursor lines.
An ideal Spectrogram decays very rapidly off the bottom of the scale range. Here is an example of a plot produced from a soundcard loopback measurement.
Match Time Scale to Window and Range adjusts the time axis range so that it starts at the Window width before zero (e.g. -300ms for a 300ms Window setting) and ends at the Time Range (e.g. 1000ms for a 1000ms Time Range) so that the plot shows all the generated data.
Match Top of Scale to Peak adjusts the Scale Top value so that it corresponds to the highest level found in the data.
If Show modal frequencies is selected the theoretical modal frequencies for the room dimensions entered in the Modal Analysis section of the EQ Window are plotted at the bottom of the graph.
3D Enhancement gives the plot a more three-dimensional appearance.
Draw Contours adds contour lines at the dB interval set in the adjacent spinner.
The Colour Scheme for the plot can be changed, the plots above use the "Rainbow" scheme, here is a plot using the "Flame" colour scheme.
This plot uses the "Copper" colour scheme with 3D enhancement active.
Scale Gamma adjusts the way colours are distributed along the scale, gamma values below one emphasis variations at the top of the scale, values above one emphasise variations at the bottom of the scale. A gamma value of 0.5 was used for the copper colour scheme image above.
The Scale Top, Scale Bottom and Scale Range controls adjust how the plot colours correspond to the values in the Spectrogram data. Any values higher than the Scale Top are drawn in the colour at the top of the scale, any values lower than the Scale Bottom are drawn in the colour at the bottom. If the Scale Top setting is changed the Scale Bottom will be adjusted to keep the same Scale Range. If the Scale Bottom is changed the Scale range will be adjusted to keep the same Scale Top. If the Scale Range is changed the Scale Bottom will be adjusted, keeping the same Scale Top.
The Time Range control determines how much spectrogram data will be generated after the time = zero point. The width of the window that is moved along the impulse response to generate the spectrogram is set by the Window control. The corresponding frequency resolution is shown next to the window setting.
The control settings are remembered for the next time REW runs. The Apply Default Settings button restores the controls to their default values.