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Acoustic Transparency (%)

The object „Building“ may have a characteristic called „acoustic transparency“ in %. The default value is 0% which means that the building is acoustically non-transparent.

Note

This option has been implemented based on user request.

Dialog Building, option „acoustic Transparency“

This feature enables to model more or less open structures, e.g., an arrangement of pipes, tanks and other technical equipment penetrated by sound. This is modeled using the object „Building“ with a transparency value addressed.

Restriction to VDI resp. ISO propagation model

With an acoustic transparency larger than zero it will be considered in the calculation of the screening effect that the object is (partially) transparent. This, however, applies with the calculation procedures VDI 2714/2720 and ISO 9613-2 only.

For other calculation procedures and source types Industry, Road, Railway and Aircraft Noise, the option „acoustic transparency“ has no effect unless the propagation model is based on VDI 2714/2720 or ISO 9613-2. Presently (as of 2017), the acoustic transparency has an effect - besides the industrial standards mentioned above - for HJ 2.4 (Industry), Schall03 (2014) and ONR 305011 (both Railway) since they are based on the ISO propagation model.

Note

Use acoustic transparency with VDI 2714/2720 and ISO 9613-2 only.

Calculation Strategy

The following calculation strategy is applied:

  • At first it is checked which objects cause diffraction applying the „ribbon band method“.
  • An acoustic transparency of X% gives a transmission factor of X/100. If the sound energy at the receiver is reduced by dE according to the screening calculation, then with a collective transparency of X% the energy (X/100)*dE is added to the sound energy calculated with transparency zero.
  • The total degree of transmission for many buildings in path is the product of all single transmission factors.
  • A building having no transparency or a barrier (i.e transparency is zero) in path will cause a total transparency of zero which means that all buildings are considered non-transparent.
  • The same rule applies to terrain, i.e. a screening terrain contour (explicit or triangulated) in path will also cause a total transparency of zero.

At the receiver point two sound contributions are added energetically:

  • The first contribution is the diffracted sound over and if necessary around the arrangement and is calculated as for non-transparent objects.
  • The second contribution is calculated as direct sound energy, not taking into account the shielding objects, but multiplying this energy value by the resulting transmission factor defined above. With direct sound contribution Edir and the resulting transmission factor τ, this second contribution is Edir * τ.

Note

This procedure was implemented due to user demands. It is not described in any standard or guideline. The depth of an object with transparency in ray direction has no impact on the calculation result. The value of the acoustic transparency is considered to be the same for all directions.

Examples

Example

Path: Obstacles\Transparency\Example 1 - without lateral diffraction.cna

The example shows a point source with PWL=100 dB(A), a building and a receiver. There are three variants defined in the file:

  • variant 1: non-transparent building
  • variant 2: transparent building (acoustic transparency 10%)
  • variant 3: without building

The calculations for all three variants without lateral diffraction (option Lateral Diffraction „none“ on tab „Industry“, see e.g. ISO 9613 (1996)).

Example 1 - without lateral diffraction, variant 2: building transparent

In the three following variants result level is calculated:

  • variant 1: SPL=40.2 dB(A)
  • variant 2: SPL=55.0 dB(A)
  • variant 3: SPL=64.9 dB(A)

Example

Path: Obstacles\Transparency\Calculation Example 1 - without lateral diffraction.xlsx

From the referenced MS-Excel file the calculation procedure can be studied leading to these results applying the strategy described above.

Example

Path: Obstacles\Transparency\Example 2 - with lateral diffraction.cna

In this example, the calculations are carried out for all three variants including lateral diffraction (option Lateral Diffraction „some objects“ on tab „Industry“, see e.g. ISO 9613 (1996).

Example 2 - with lateral diffraction, variant 2: building transparent

For the three following variants the following receiver level are calculated (in variant 2 again with an acoustic transparency of 10%):

  • variant 1: SPL=45.0 dB(A)
  • variant 2: SPL=55.3 dB(A)
  • variant 3: SPL=64.9 dB(A)

The level difference between the variants 1 without/with lateral diffraction is 4.8 dB (10 log 3) in this example, since object geometry causes that the lateral path length difference is equal to the path length difference of the direct path across the house. In this case, for the non-transparent building the resulting level with lateral diffraction is 4.8 dB larger than the level without lateral diffraction.

Example

Path: Obstacles\Transparency\Calculation Example 2 - with lateral diffraction.xlsx

From the referenced MS-Excel file the calculation procedure can be studied leading to these results applying the strategy described above.

Note

The above statements apply to grid and facade points analogously.

Sources inside of transparent buildings

In real situations, sources may be located within a partially transparent plant or plant sub-structure. For example, valves within bundles of pipes represent point sources which should - in this case - not be shielded by the surrounding partially transparent building.

The option „Obstacles within Area Sources do not shield“ (on tab „Industry“, see ISO 9613 (1996)) enables to enter point, line and area sources inside the buildings perimeter. When enabled, the building causes not shielding to sources inside. For sources outside of the buildings perimeter, however, the screening effect is calculated using the specified configuration.

Examples

option „Obstacles within Area Sources do not shield“ active: source inside the (semi-transparent) building is not screened.
option „Obstacles within Area Sources do not shield“ inactive: source inside the building is screened. In this case, the level is also determined by a semi-transparent building due to the transmitted energy.
NOTE: With this setting of the configuration option the consistency check in CadnaA indicates that there is a source inside of a building.

Transparency at Reflections

An acoustically transparent building causes the same amount of reflections as does a non-transparent building. This means that the transparency effect applies to the diffracted energy, but not to the reflected one.

Example

Building with no transparency

Building with transparency 80%: Level in front of building (source side) remains the same compared to no transparency