Skip to content

Entering a Sound Radiating Facade

Object Snap

A vertical area source is attached to the outside of a building’s facade by use of the command Object Snap.

  • Enter a building with 20 m of edge length using the mouse and set the building’s height to 10 m relative (dialog Building|Geometry).
  • Select the command Object Snap from the Options menu.
  • On the dialog Object Snap a snap radius can either be entered in meters (i.e. scale dependent) or in pixels (i.e. not scale dependent). Enter a snap radius of 10 pixels.

If the building’s facade is located inside this snap radius upon mouse click, CadnaA catches the point at the defined distance of 0.05 m off the facade. This procedure prevents sources to be entered inside a building’s envelope.

Dialog Object Snap, snap radius 10 pixels

Entering a vertical area source

  • Select the vertical area source (symbol: ) from the toolbox and click consecutively at two points outside the building’s facade, close to the corners. Finalize the input by clicking the right mouse button.

  • By zooming in with the magnifying glass (+) the vertical area source. outside the building polygon can be recognized.

  • Select the auxiliary polygon symbol () and measure the distance between the facade and the vertical area source by clicking with the mouse on the building’s border and dragging the auxiliary polygon to the projection line of the vertical area source.

Distance building - vertical area source = 0.05 m

  • Finalize this procedure by pressing the right mouse button.
  • Now, click with the right mouse button on the vertical area source and select from the context menu the command Set Length. Enter a length of 19 m.

  • Open the dialog of the vertical area source.
  • Click on button „Geometry“ and enter a height of 10 m for the upper edge of the vertical area source. Close the dialog Geometry.

Editing the vertical area source

  • Specify on the dialog Vertical Area Source a z-extent 10 m. The z-extent is the vertical dimension from the upper edge downwards.

Entering the z-extent

Directivity index K0

With vertical area sources the directivity index K0 is set to 3 dB automatically („K0 without ground“). This default setting takes the reflection at the building into account causing the radiated sound power to be increased apparently by 3 dB - ignoring the ground reflection. This is equivalent to the statement that the source radiated into half-space, instead of into full space. Here, the ground as a reflector is not considered since the ground reflection is taken into account automatically by CadnaA. The following cases apply:

  • source at a height above ground: K0 = 0 dB

  • source at a height above ground in front of a wall: K0 = 3 dB

  • source at a height above ground in a corner: K0 = 6 dB

Making use of the directivity index K0 (DΩ in ISO 9613-2) requires that no calculation of reflections at the building near to the source occurs. Therefore, on menu Calculation|Configuration, tab Reflection, a value of 0.1 m is entered for "Min. Distance Source-Reflector". For sources located closer to a reflector than this distance no calculation of reflections is carried out.

3D-Special View

  • Press the key combination CTRL+3 the enable the 3D-Special view.
  • Move the camera location using the arrow keys (forward/backward) and the mouse (left mouse button pressed), to catch a view of the vertical area source.

  • Double-click in 3D-Special view onto the area source.
  • Activate on the lower left hand side of the dialog the option „Trans.Loss“. The dialog switches from PWL/PWL“ to the interior level.

Entering interior level and transmission loss

  • Enter an interior level of 90 dB(A) and a transmission loss of 25 dB.

After entering of interior level and transmission lossthe resulting sound power level PWL and PWL“ are displayed.

The A-weighted sound power radiated by the area source calculates from:

\(L_{wA}=L_i-R_w+10\lg(S/S0)-4dB=90-25+22.8-4=83.8dB(A)\)

\(L_{wA}"=L_i-R_w-4dB=90-25-4=61.0dB(A)\)

Note

For the calculation in frequency bands the radiated sound power per octave band width results from: \(L_{w,okt}=L_i-R+10\lg(S/S0)-6dB\) or \(Lw_{w,okt}" = L_i - R - 6 dB\)

For the modeling of sound radiation from buildings or from building parts, in principal, the point, the line, and the area source are also suitable as well as the vertical area source. Whether to model the radiation from the one or the other object type depends on the available input data, the situation (e.g. distance and partial-screening), and on precision requirements.