Simcenter Testing Solutions Time Domain Transfer Path Analysis: Listening to Paths!

Direct YouTube link: https://youtu.be/YLXgS7u96Tc


Time Domain Transfer Path Analysis: Listening to Paths!

After creating a Transfer Path Analysis (TPA) model that characterizes the noise and vibration of a structure, it is possible to listen to it!

Why listen to a TPA model? Listening interactively to different paths can help gain additional insight versus traditional assessments of sound. For example:

• 2D Graph versus Sound: Just because a potential modification shows a large drop in decibel level in a two dimensional graph (for example, amplitude vs rpm), it does not mean that it is better sounding.  Listening to the sound before and after modification helps assess the changes with more insight.
• Transient sounds: Transient sounds, for example injector tick, are not well represented in a frequency domain model.  Having the time history contribution allows metrics like time varying loudness to be used to assess the sound.  Time varying loudness cannot not be calculated from a frequency based curve.

Creating time histories for listening can be done in Simcenter Testlab Transfer Path Analysis using the Time Domain TPA add-in and TPA Synthesis add-in.  

This article explains the steps for listening to a Transfer Path model:
1.    Background
2.    Getting Started
3.    Creating Time Histories
4.    Navigator Attributes
5.    Listening to Path Contributions

1. Background

To create time histories from a Transfer Path model, a frequency domain model is used in conjunction with the measured time histories.  The measured input time histories are applied to the model in the frequency domain to calculate output time histories of the various path contributions (Figure 1).
 

• Frequency Response Functions (FRFs):  FRF functions characterize the TPA structure.  Force is the input (or denominator) of the FRF function, and the output (numerator) is usually either acceleration or sound pressure.
• Operating Data:  Orders, spectrums, or other functions taken from the operating condition of interest.  These might be directly measured forces or indirect measurements (acceleration or otherwise) used to derive the force.

• Acquired Simultaneously:  All channels must have been acquired at the same time to have consistent phasing in the time domain.  They cannot be acquired in batches and patched together.
• Consistent Sampling Rate: All channels used in the time domain model must have the same sampling rate.  Having channels in the same recording with different sampling rates will not work with the Simcenter Testlab software.

The frequency range of the time history will either be limited by the frequency range of the FRF measurements or the operating data measurements, whichever is lower.  Generally speaking, it is easier to measure to a higher frequency on the operating data than the FRF measurement.  Some further guidelines:

• The FRFs are often acquired using an impact hammer, and it is difficult to measure to a high frequency range depending on the hammer tip used.  See the knowledge article "Selecting a Modal Impact hammer tip".
• If the TPA model is composed of orders, it might be prudent to include additional orders (higher and lower) so that the frequency content of the model covers a suitable range for sound playback.

The steps for doing a time domain transfer path listening in Simcenter Testlab are covered next.

2. Getting Started

In Simcenter Testlab Transfer Path Analysis, select the input time history and add it to the Input Basket as shown in Figure 2.
 

1. Read Input Basket: Click on the read input basket button.  The throughput file should appear just below the button, and the red column of PointId information (lower left) should turn green with the letter X in each box.
2. Cell Selection: Click on any of the cells with the green background to view the time histories. Use the arrow key to move up and down the cells to view the time data.

1. Select the desired frequency range of the path contribution time histories (use the default if nothing specific is desired)
2. Press the “Calculate” button.

The results can be viewed in the Navigator worksheet.

4. Navigator Attributes

The path contribution time histories will be found in a new analysis that ends with the extension “TTPA” (assuming the defaults are used).

When viewing the time data, there is a mixture of the originally acquired data and the newly calculated contribution time histories in the same file.

To tell the difference, some attribute columns can be added in the Navigator (Figure 7) to help differentiate the time histories. Right click at the top of the columns and choose “Select Columns” to add the attributes.
 

1. Set the Origin to Testlab, and the Type to Block.
2. Enter “tpa” in the Quick Find area.
3. Choose the attributes “TPA Path” and “TPA Result Type” and use the arrow to put them in the selected field.

Press OK when finished.

Possible attribute values include (Figure 9):

1. Total Contribution: The calculated total time history at the target location (Point ID field).  This can be used for comparison to the original measured time history at the target location.
2. Partial Contribution: Many of the Point ID fields or target location names are the same.  Partial Contribution indicates a path contribution from a location (TPA Path) to the target location. For example, the contribution of a left engine mount (TPA Path) to the driver ear (Point ID).
3. Load-MI: To calculate the contribution of a path, the force time history must be calculated. This is indicated by the LOAD-MI label.

With the time histories calculated, they can be listened to interactively.

5. Listening to Path Contributions

With the “TPA Synthesis” add-in set to ON (Figure 10):

1. Click on the Definition subsheet located at the top of the screen.
2. If desired, select a smaller time segment for listening.  Use the Double X cursor and press the “Apply segment selection” button.

With the time segment of interest selected, switch to the Replay subsheet, and do the following (Figure 11):

1. Choose the Replay subsheet at the top of screen
2. Pick the Transfer Path model for listening.  The top row is the originally measured data, and the model is contained in the rows below.  Any models created with “TPA Component Editing” will also be listed. While replay is in session, click on the different models to hear the differences.
3. Press the Replay button to start listening.
4. Use the check boxes on the right side to turn paths on and off while listening. Paths cannot be turned on and off on the original measured data, they can only be turned on and off on Transfer Path models.

Instead of turning on and off individual paths, if the groupset feature of Simcenter Testlab was used, entire sets of paths can be turned on and off at the same time. 

For example, the entire airborne contribution of a motor could be compared to the entire structureborne contribution of the motor in this manner.  See the knowledge article "Simcenter Testlab Transfer Path Analysis: Groupsets".

Visualize a live spectrum of any path by right clicking on the path and selecting “Add Eavesdropping” (Figure 12).
 
In the same manner, it is also possible to add a filter on the path.  Filter types include Band-Pass, Band-Stop, High-Pass, Low-Pass, or Notch.

Questions? Email peter.schaldenbrand@siemens.com or contact Siemens Support Center.

Related Links

• Index of Simcenter Testing Knowledge Articles
• Introduction to Transfer Path Analysis
• Simcenter Testlab Transfer Path Analysis: Component Editing
• Simcenter Testlab Transfer Path Analysis: Groupsets
• Blocked Forces versus Contact Forces in Transfer Path Analysis (TPA)
• FRF Based Substructuring
• Obtaining Invariant Loads: Practical Examples
• What is Frequency Response Function (FRF)?
• Dynamic Stiffness, Compliance, Mobility, and more...
• Transfer Path Analysis Data Naming
• Simcenter Testlab: Matrix-Heatmap Display
• What is the Acoustic Quantity called Q?
• Simcenter Testlab Virtual Prototype Assembly
• Transfer Path Analysis Seminar
• Virtual Prototype Assembly (VPA) Seminar
• Transfer Path Analysis YouTube Playlist