Transmission Loss scenario (Simcenter Nastran FEM acoustics and Acoustics BEM)

Duct acoustics

In duct acoustics, you calculate a transmission loss (TL) curve from acoustic pressure results to assess the sound absorbing capacity of your model, such as a duct system or muffler. To calculate the TL curve, using the three-point method, you must select three microphone points: two points near the inlet and one point near the outlet. The software calculates the TL curve from the SORT2 acoustic pressure results at these three points.

You can request the appropriate acoustic pressure results for the TL curve on the Acoustic Pressure page of the Acoustic Output Requests or Vibro-Acoustic Output Requests dialog box when you select the Enable PRESSURE Request check box and choose SORT2 from the Sorting list.

Muffler example	Example TL curve
Muffler example showing:(1) inlet(2) muffler(3) outlet(4) first microphone at inlet(5) second microphone at inlet(6) third microphone at outlet

Theory

You define three microphones: two at the inlet and one at the outlet. It is assumed that the outlet duct is sufficiently long to eliminate acoustic reflections back to the inlet, or that an AML (Automatically Matched Layer) is at the end of the outlet duct. If this assumption is not true, you may have to use a different method to calculate the TL.

The incident pressure is calculated from

where:

= Incident pressure

= Pressure at microphone 1

= Pressure at microphone 2

= Wave number

= Distance between microphone 1 and 2

The transmission loss (TL) is then calculated from

where:

= Transmission Loss

= Incident pressure

= Transmitted pressure (pressure at microphone 3)

= Inlet area

= Outlet area

Note:

In any model, if the computed transmission loss is negative, you must ensure that the inlet nodes and outlet node are selected in the correct order, and that the model is correctly set up.

For example, a model may also require an AML boundary condition at the inlet and outlet to absorb acoustic reflected waves.

Parameters

The following table lists the parameters available for defining Transmission Loss post-processing scenarios. They appear in the Parameters node of the Scenario Based Data-Visualization Navigator. For example, you can create what-if scenarios for the Transmission Loss scenario by manipulating speed of sound and microphone distance. You can also use these parameters as a diagnostic tool.

Parameter	Description
Override Speed of Sound	Overrides the speed of sound extracted from the solution, so you can see how the results change with different speeds. This allows you to create what-if scenarios.
Speed of Sound	Available when the Override Speed of Sound check box is selected.Sets a value and a unit for the speed of sound.
Surface Area Ratio Inlet/Outlet	Sets the surface ratio between the outlet and inlet. This ratio is used in the TL equation:TL = 20{log_{10}}\frac{{p_i}}{{p_o}} + 10{log_{10}}\frac{{S_i}}{{S_o}}where:pi** is the inlet pressure extracted from the pressures of the two selected microphone points at the inlet.po is the outlet pressure at the selected microphone point at the outlet.S**i is the inlet surface area.S**o is the outlet surface area.
Override Microphone Distance	Overrides the microphone distance that is the distance between the two selected microphone points at the inlet, so you can see how the results change with different distances. This allows you to create what-if scenarios.
Microphone Distance	Available when the Override Microphone Distance check box is selected.Sets the distance and corresponding unit.

Look up more details

Acoustic Output Requests and Vibro-Acoustic Output Requests dialog boxes (Simcenter Nastran)

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Transmission Loss scenario (Simcenter Nastran FEM acoustics and Acoustics BEM), Simcenter 3D 2021.1 Series

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