What Is A Spectrogram
A spectrogram displays signal strength over time at the various frequencies present in a waveform. Spectrograms are two-dimensional graphs with a third variable represented by color. In ObserVIEW, the color scale is red-green-blue, where blue corresponds to low amplitudes, or “loudness,” and red corresponds to high amplitudes.
For vibration testing, spectrograms can be used to analyze the frequency content of a waveform to distinguish different types of vibration. With the data, users can locate strong signals and determine how frequencies change over time.
To generate a spectrogram, a time-domain signal is divided into shorter segments of equal length. Then, the Fast Fourier Transform (FFT) is applied to each segment. The spectrogram is a plot of the spectrum on each segment. The “Frame Count” parameter determines the number of FFTs used to create the spectrogram and, in result, the amount of the overall time signal that is split into independent FFTs.
For instance, it is possible to define a spectrogram covering 10 hours with only 10 FFT frames. However, there would be many gaps between FFT analyses. Conversely, a 1-minute spectrogram can be defined with 1000 FFTs, which would cover all time samples with some overlap between FFT analyses.
In the graphs below, the number of FFTs are reduced from 500 to 50. The result is a jagged spectrogram with many gaps in the data.
How are spectrograms different than other signal processing analyses?
A time-domain analysis can point out a defect in a DUT but does not specify the location or nature of the defect. As a collection of time-frequency analyses, the spectrogram can be used to identify characteristics of nonstationary or nonlinear signals. For this reason, the spectrogram is a helpful tool for analyzing real-world data where there are various frequency components and/or mechanical and electrical noise.
A spectrogram is most helpful for vibration analysis on a changing environment. It illustrates the patterns of energy change which may not be visible in an FFT or PSD. In comparison to an FFT, a spectrogram gives a better look into how the vibration changes over time.
In a spectrogram, there are many indicators of damage and they can be complex. Still, atypical bands can indicate very useful information regarding potential damage.
Using spectrograms in ObserVIEW for analysis
In ObserVIEW, there is a time spectrogram and tachometer spectrogram graph option. For the time spectrogram, the following settings can be adjusted:
- Frame Count: the number of FFTs that make up the spectrogram
- Min Frequency and Max Frequency: limits the frequency range of the spectrogram plot
The FFT spacing and FFT width are properties of the amount of time analyzed with respect to the FFT sample count and number of FFTs. The user cannot define these properties directly.
- FFT Spacing: the distance in time between FFT anchor points (left, center, right); an approximation based on the spectrogram range and frame count
- FFT Width: the width of time data each FFT represents
In addition to these settings, the tach spectrogram includes:
- Min and Max Tach Values: limit the tachometer range of the spectrogram. For example, these values may be changed to focus on a subset of the total RPM range or to purposely exclude errant data.
- Tach Sweep Direction: used to trigger spectrogram readings and can be configured to match the direction of the tach trace sweep. If the tach sweep direction doesn’t match the direction of the tach sweep, the tool will report little to no readings.
There are several graph options that are useful for analysis. These options can be found under Spectrogram Graph Options.
- The mouse cursor displays the X, Y, and Z values of the data under the cursor
- The horizontal cross section displays a graph under the spectrogram with the data for a horizontal slice across the spectrogram. The slice can be moved be dragging the horizontal cross bar up and down on the spectrogram.
- The vertical cross section displays a graph to the right of the spectrogram with the data for a vertical slice down the spectrogram. Again, the slice can be moved by dragging the vertical cross bar left and right across the spectrogram. The vertical cross section can be rotated.
Both the horizontal and vertical cross sections can be used to examine a slice of all data at a specific time or frequency.
In addition to these options, the tach spectrogram includes the option to enable an order cross section graph.
Order Analysis and Spectrograms
Spectrograms can also be used to identify order lines. Orders identify the relationship between the response of a rotational component at a specific amplitude, the RPM, and the frequency of rotation. With order analysis, engineers can identify how the vibration of an individual component contributes to the overall level.
In ObserVIEW, you can automatically generate the top ten orders with the Add Top 10 Orders button. The orders are selected based on the highest peak acceleration level defined within the selected analysis range.
The spectrogram is another tool for device maintenance and detection of error. To learn more, visit our page about the ObserVIEW analysis software.