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Barrier with transmission

All calculation standards for the sound propagation outdoors treat the screening at obstacles (e.g. at barriers) not accounting for sound transmission through the obstacle. With this approach it is implicitly assumed that the screening effect (possibly including lateral diffraction) is the dominating effect when calculating the sound propagation.

Validity of diffraction equations

In order to ensure that this requirement is met, several calculation standards specify a lower limit of the surface mass or a minimum (weighted) sound reduction index (transmission loss or STC) of the screening obstacle. For example, ISO 9613-2, section 7.4, states ISO-9613-2-2024:

„An object shall be taken into account as a screening obstacle (...) if it meets the following requirements:

  • the surface density is at least 10 kg/m²;
  • the object has a closed surface without large cracks or gaps ...

Each object that fulfills these requirements shall be represented by a barrier with vertical edges. ...“

Considering a finite sound transmission

Considering the transmitted energy through the barrier requires that the weighted sound reduction index Rw (or STC) or the sound reduction spectrum (transmission loss spectrum) of the barrier is available.

Proceed as follows to model a barrier with a finite transmission:

  • Activate the object snap on dialog Options|Object Snap with a „Distance Points-Facade“ of 0.05 m (default value).
  • Place a receiver in the middle of the barrier on the sound source side.
  • When the barrier has reflective properties, make sure that in the configurations, „Reflection“ tab, a „Min. Distance Receiver-Reflector“ of 1.00 m and a „Min. Distance Source - Reflector“ of 0.10 m is specified (see Reflection Tab).

Note

In conjunction with the setting for the ObjectSnap, these standard configuration settings ensure that the level calculated at the receivers does not include any reflection component (thus, being the free field level) and that the sound power radiated by the vertical area source does not include any reflection component from explicit rays (i.e. the reflection is considered by the directivity index K0=3 dB on the source dialog, see Common Input Data, K0 without Ground).

  • Calculate the receiver level, either as an A-weighted single value or as a spectrum.
  • Enter on the far side of the barrier (as seen from the source) a vertical area source (see Industrial Sources). Apply the command Parallel Object from the barrier‘s context menu (entering a distance of 0.05 m, a height difference of 0 m, and a Z-extend of 8 m corresponding to the barrier‘s height). The directivity index „K0 w/o ground“ of 3 dB is maintained as the source radiates into half sphere (i.e. not considering the ground reflection here).
  • The emission of the vertical area source results from the receiver level on the source side and from the sound reduction index (or STC) of the barrier:
    • when the receiver level on the source-side is an A-weighted level: SPLi = SPLsource side + 4 dB
    • when the receiver level on the source-side is a spectrum:SPLi (f) = SPLsource side (f) + 6 dB (in each frequency band).

Note

This numerical approach for the calculation of the interior levels compensates for the procedure used in the VDI guideline 2571 2571-76 (see Common Input Data, Transmission Loss).

  • Activate the option „TransLoss“ on the dialog vert. Area Source and enter into the box „Interior Level“ the value SPLi as determined or establish a reference to a sound level spectrum (after having entered spectrum via the menu Tables|Libraries (local)|Sound Levels).
  • Enter the weighted sound reduction index or establish a reference to a transmission loss spectrum (after having entered spectrum via the menu Tables|Libraries (local)|Sound Reduction Indeces).
  • Restart the calculation.

Now, the resulting levels contain the transmitted energy fraction.

Note

Note that the procedure described above is an approximation making use of some assumptions. For example, this approach assumes a uniform sound distribution on the source-side of the barrier. Moreover, it is no taken into account that the sound reduction (transmission loss) may depend on the angle of the inciding sound.

Example 1: Procedure with A-weighted levels

The example file shows the following arrangement of objects (here for variant 2 „with Transmission“)

... with the following input data for the vertical area source when calculating using weighted data (A-weighted interior level and sound reduction index, Rw (STC) = 10 dB):

Example

Path: Examples\Obstacles\Barrier with Transmission\example 1 - barrier transmission using A-weighted level.cna

Toggle variants „without transmission“ and „with transmission“ in order to assess the influence of the transmission in this example.

Note

In this example, the non-spectral ground attenuation according to ISO 9613-2, section 7.3.2, is used ISO-9613-2-1996.

Example 2: Procedure using spectra

The sample file shows the arrangement of objects (again for variant 2 „with Transmission“)

... with the following input data for the vertical area source for invoice with spectral data (spectral indoor level and spectral transmission loss):

Example

Path: Examples\Obstacles\Barrier with Transmission\example 2 - barrier transmission using spectrum.cna

Toggle variants „without transmission“ and „with transmission“ in order to assess the influence of the transmission in this example.

Note

In this example, the spectral ground attenuation according to ISO 9613-2, section 7.3.1, is used ISO-9613-1.