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A spectral map is a three-dimensional display of sound or vibration spectra as a function of time or speed. The spectra can be frequency or order spectra. A spectral map provides an overview of the frequency or order content of a signal related to time or speed. A spectral map can help you locate strong sound or vibration components, identify the components changing with the rotational speed, and identify the fixed components within a certain frequency range.

The following front panel shows the spectral map, in colormap display, of the vibration signal acquired from a gearbox casing in a run-up and run-down test.

The strong order components on this spectral map change with time. The vibrations in the frequency range from 1.8 kHz to 3.0 kHz are stronger than vibrations in other frequency ranges. This range is the resonance range of the gearbox. In general, a spectral map helps you obtain overview information such as how signal components change and the location of significant frequencies or order components.

Because a spectral map provides overall signal information, you perform a spectral map calculation as the first step in order analysis applications. You can locate the signal components of interest from the view of time, speed, frequency, or order. After you locate the components of interest, you can perform further analysis with other functions such as the order power spectrum, order waveform, or order magnitude and phase.

You can use the Spectral Map Express VI to generate a spectral map and display the spectral map as a colormap or waterfall graph for offline or online analysis.

Colormap

A colormap displays spectral map data in a customized intensity graph. The colormap uses different colors on the plot to represent the signal power distribution.

When displaying a signal with a colormap, you can select any one of eight plot types to view different information related to time, speed, frequency, and order. The following front panel shows vibration results from a run-up test in Frequency-Time and RPM-Order displays.

You typically use a colormap for offline analysis. By comparing the display results with different formats and units, you can get a complete knowledge of the signal, such as how the amplitude of the signal relates to time, speed, frequency, and order. You then can determine further analysis steps.

Waterfall Graph

Use a waterfall graph to observe frequency or order spectrum changes versus time. A waterfall graph consists of a series of spectra acquired at consecutive times. The x-axis displays frequency or order. The y-axis displays the amplitude or power. The z-axis displays the time. The following front panel shows an example of a waterfall graph for a run-up test.

A waterfall graph shows how vibration changes with time and indicates which components are related to rotational speed. Use a waterfall graph for both online and offline analysis.

You can change the display perspective of a waterfall graph to obtain a cascade plot view. A cascade plot displays frequency or order changes versus rotational speed. A cascade plot also consists of a series of spectra acquired at consecutive increasing or decreasing speeds. The x-axis displays frequency or order. The y-axis displays the amplitude or power, and the z-axis displays the speed. The following front panel shows an example of a cascade plot.

You can use a cascade plot for both online and offline analysis. You primarily use cascade plots to show results for tests such as run-up and run-down tests. The components that move across the plot as the speed changes are the order components. You can use a cascade plot to recognize machine resonances that occur at fixed frequencies.