Products VACS - Screen-Shots SPL-Impedance-Plot

Importing SPL and impedance curves into an isotropic grid graph

This example demonstrates the import of sound pressure, distortion products and impedance files, and the formatting into a graph with two ordinate-axis and an isotropic grid. Also sampling and smoothing is demonstrated.

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Select the four files in the file-dialog or drag from File-Manager. Because these files are text-files the Import Control Dialog opens.

These files are measurement files from the Klippel Distortion Analyzer. The content can be viewed after clicking the button View file.... The first column are frequencies and the second are SPL or impedance-amplitude in ohm.

The estimator of the import engine has interpreted the impedance file as sound pressure, so we have to correct for this. When done we sort the files so that the impedance is at the bottom of the list by clicking on the table header Impedance.

After clicking Apply all data-sets are streamed to a single graph.

On import of multiple and different curves VACS checks if the current data-set fits to the graph of already imported curves. If not then the import engine tries to assign the current curve to the right hand ordinate. This is the reason why we ordered the file-list in a way so that the impedance curve is at the bottom. In this way it is assigned directly to the right ordinate. However, this is just a trick. If we do not sort the list we would end up with multiple graphs, and use drag and drop to collect the curves sub-sequentially.

The next step is to adjust the ranging and the layout of the graph so that we get a proper SPL-Impedance-Plot. Open the Layout-Dialog of the graph (menu Graph > Layout...) on page Grid. There select Sizing = To window, which makes that the graphs' size follows the size of the containing window.

Second go to page zR-Axis and select Detached lower half, which makes the right hand ordinate half the size of the graphing area and independent.

Finally we have to adjust the ranges and switch into iso-grid mode. For this open the Range-Dialog (menu Graph > Range...) first on page zL-Axis. There select Iso-Grid = Max anchored and z-Range/x-Decades = 40.

Iso-Grid keeps the grid aspect-ratio to the value given in z-Range/x-Decades. The value 40 tells VACS to render 40dB per decade. Because the range 20...20kHz covers three decades our plot will be such that 120dB appears as high as three decades are wide on the screen.

We can apply the Iso-Grid-Mode also to the right hand ordinate. In order to do so select page zR-Axis in the Range-Dialog.

First select Bode type = Amplitude and Axis type = Log const ratio. Why? This axis type maps the ordinate values logarithmically but arranges grid-lines with a constant spacing, which is equivalent to a constant ratio on log-mappings. The ratio can be specified in the field Step ratio. Here, we may enter 2, which yields a binary distribution.

Then select Iso-Grid = Max anchored, Max = 1000 and enter, for example, z-Decades/x-Decades = 1. The latter means that the decade of the ordinate is rendered as wide as the decade of the x-axis.

Close the Range-Dialog and play with the size of the window. You will notice that VACS does not distort "the graphing-paper". You may wonder why the right hand ordinate goes to "1024" instead of "1000" as specified in the Max-field of the Range Dialog. This is so, because there we also have selected Rounding mode = Headroom, which causes VACS to round to the next "nice" number, which on a binary grid is 1024.

In the next step we want to down-sample and smooth the sound pressure data. Before applying the processing, we hide the impedance curve because we do not want to smooth this curve. Then simply issue Processing > Smoothing response.

With Number of sampling points = 400 the down-sampling rate is significant as can be seen in table of the dialog.

For re-sampling the impedance curve let us do something more exotic. The impedance curve exhibits 27292 points on a linear grid, as can be seen when pressing F3 on the selected impedance curve. For down-sampling we would like to have a distribution, which is dense in the proximity of the resonance pole at 50Hz and coarse at HF. Exactly this can be achieved with the ArSinh-sampling.

As a preparation hide all SPL curves in the original page, but the impedance curve. This time issue Processing > Resampling general..., select page ArSinh, enter Number of points = 400, Shift = 50 and Stretch = 100. The latter means that we want the linear part of the ArSinh densely distributed and centered at 50Hz.

The following graph shows a comparison of the different sampling models.

For purpose of demonstration the sampling points are reduced to 50 points. The blue curve is sampling on a logarithmic grid, the purple curve is linearly sampled and the black curve shows the ArSinh sampling. The latter obviously yields a concentration of points around 50Hz and exhibits log-behavior elsewhere.

Back to our SPL-Impedance plot. After having down-sampled the impedance curve as well we copy the sampled impedance curve into graph of the sampled SPL curves. For convenience you can drag the curve and drop it on the target page-tab. The final result could look like this:

This screen-shot does not show the "working" graph as we had before but the Export as Picture dialog (menu File). This dialog is the stepping stone for exporting the graph as a bitmap or Metafile, either to the clipboard or to a file.

This example showed the step-by-step formatting of the graph from the default to an individual layout. It is also possible to "store" any layout with the help of Styles.

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