3D-Reflector

The object „3D-Reflector“ is a special object not described by any standard or guideline with respect to its properties and its screening effect.

Inclined, floating 3D reflector in the 3D view

The 3D-reflector models a freely orientated, arbitrarily shaped closed and plane polygon area, where the shortest path difference between source and receiver determines the screening effect. In addition, lateral diffraction and reflections up to the 1st order are taken into account. Distinct edges of 3D-reflector can be excluded from the calculation of lateral diffraction (see below „Exclude Edges“). This prevents for diffracting around the selected edge, for example, at geometrically adjacent obstacles.

The reflector is always considered as a flat surface in the calculation. When polygon points are not in a plane, a plane surface is generated by quadratic averaging.

Normative References

The 3D-reflector as a special object is just compatible with such standards and guidelines, using a diffraction model evaluating the path difference based on path geometry only. Therefore, the 3D-reflector shows no effect with standards and guidelines (i.e. no screening) which are based on a diffraction model using, for example, an additional height Δh or which exclude certain obstacles in the ray path from the diffraction calculation (e.g. „the most efficient barrier“ etc.).

By default, the 3D-reflector is, therefore, just compatible with standards and guidelines whose diffraction models comply with the above conditions and where the screening effect by multiple obstacles in the ray path is evaluated by applying so-called „rubber-band-method“ like e.g.:

ISO 9613-2 (1996) and ISO 9613-2 (2024)
RLS-90
Schall 03 (1990) and Schall 03 (2014)

For more information on guidelines-specific procedures for assessing diffraction and on the effectiveness of the effectiveness of 3D-reflector see Industry Tab, Road Tab, and Railroad Tab.

Configuration of Calculation

The configuration settings on tabs „Industry“, „Road“ and „Railway“ are also relevant when calculating diffraction of the 3D reflector. Please note in special:

For the German procedures „RLS-90“ (road) and „Schall 03“ (railway) the exclusion of the lateral diffraction and the suppression of the ground attenuation is strictly regulated (option „Strictly acc. to ...“).
For industrial sources, the lateral diffraction can be accounted for or not and the screen coefficients may be modified if necessary (see Industry Tab).

Dialog options

Name, Memo-Window, ID, ObjectTree, Master

see Dialog Options Name, ID, INFO, ObjectTree, Master in the manual "Introduction to CadnaA"

Reflection Loss/Absorption Coeff. Alpha

By default, the 3D-reflector does not reflect sound (option „No Reflection“). Selecting one of the options „Reflection Loss“ or „Absorption Coefficient Alpha“ enables to specify via the respective selector symbol either a reflection type (see Chapter 3 - Obstacles, Reflection Properties) or to address an absorption spectrum from the local or the global (by SHIFT-clicking) library of absorptions.

In this case, reflections from the reflector’s surface are taken into account applying the image source method, provided that on tab „Reflection“ (see Reflection Tab) in the configuration of calculation at least the first order of reflection is selected.

Left/Right Side

Regard the definition of „left“ and „right“ reflector side „right-hand-rule“ applies: Suppose the right hand being curved along the outer edge of the polygon, the fingers pointing from the first to last point, the stretched thumb points into normal direction of the „right“ side.

Button „Exclude Edges“

On the dialog Exclude Edges (see below) the reflector‘s edges can be marked not to be taken into account in the calculation of diffraction. Up to 32 edges of the 3D-reflector can be excluded. A 3D-reflector consisting of more than 32 edges, those further edges cannot be excluded.

Dialog Exclude Edges

List „Excluded Edges“

In order to exclude an edge, click once on the identifier in the upper list (e.g. E01) or double-click on the facade in the graphics below. Additionally shortcuts are available to step through the edges (E) and toggle the activation (SPACE).

Graphic Display

An edge having been excluded is displayed in the graphics without a facade point symbol (octagon) attached. A further click on the designation includes the edge again into the calculation of diffraction.

On the lower graphics, the actually selected edge is marked by a thick black line. Alternatively, a double-click onto an edge in the graphics excludes/includes this edge.

Zooming in the Graphics

After a mouse click into the graphics the sketch can be zoomed in/out.

Additional Information

In conjunction with the application of the 3D-reflector, several questions may arise which are answered in the following.

Restriction to specific Standards/Guidelines

The majority of calculation standards or guidelines just allow for barriers standing vertically on the reference plane, with the upper edge being straight and possibly inclined. The 3D-reflector offers a solution for special cases where one or more of the following characteristics have to be considered:

reflection from irregularly limited vertical surfaces,
reflection from surfaces which are neither horizontal, nor vertical,
diffraction at arbitrarily edges, including diffraction upwards.

This solution represents a „stand-alone“ solution, since the calculation of the reflection and the diffraction at the 3D-reflector occurs without considering the (potential) interactions with other screening/diffracting objects. In order to calculate the screening effect, only those standards and guidelines are suitable which are just using the geometrical path length difference, not using an additional height (e.g. in order to correct for a parabolic ray path across the top edge). In addition, no standards or guidelines are appropriate, based on the so-called „most effective“ barrier or similar criteria. These methods are not suitable in the case of 3D-reflector since the edge determining the final screening effect can be oriented freely in space where above corrections do not apply.

Restriction to plane barriers

The equations used by standards to calculate the screening effect just refer to plane barriers. Thus, it requires to enforce a plane barrier by the software if necessary. This is done in the first calculation step (see Software-internal procedures below).

Restriction to 1st order reflections

Since the 3D-reflector is freely orientable in space, reflecting rays from the 3D-reflector‘s surface (for 1st order of reflection) may point to the ground.

Further, since all algorithms used for prediction of the sound propagation outdoors do not use image source to consider ground reflection (but based on source/receiver height, as well as their distance), this ground reflection could not be included into the image source method without violating the basic principles of sound propagation models. Therefore, reflections higher than the 1st order cannot not be allowed.

Note, furthermore, that reflections - as with all other types of reflective objects - are included in the calculation only when the respective ray paths can be constructed geometrically (with angle of incidence = angle of reflection). In case a reflection path cannot be constructed, the reflection from this object can, nevertheless, be relevant acoustically, but cannot be included in the calculation. This illustrates that the construction of reflecting paths using rays just serves as a reasonable approximation.

Software-internal procedures

As part of the calculation the following software-internal procedural steps are carried out:

The input of a 3D-reflector occurs using a polygon having any shape and with an individual height at each polygon point. In this step it is checked whether the entered polygon creates a plane surface (see „user defined 3d polygon“ in the figure below). If this is not the case, the new z-coordinates are determined causing all polygon points to be in-plane and the sum of the squared height differences dH are a minimum (see „plane 3D polygon used for calculations“). The entered object coordinates, however, are kept.

The image source method is applied to calculate of reflections the reflection point, if any. The reflection (specified by the absorption coefficient or the reflection loss) is taking into account only when the reflection point lays inside of the polygon (causing a separate reflected ray).

In the calculation of diffraction it is assumed that three diffracted rays contribute to the receiver level: across the direct path and two lateral paths. For this purpose, the following strategy is applied:

If source and receiver are located at different sides of the 3D-reflector (i.e. the line between source and receiver intersects with the 3D-reflector), the intersection point S inside the polygon is calculated (see figure below).

The ray path having the smallest path length difference and thus provides the highest contribution is evaluated. This is achieved by calculating the distances of all polygon edges from the intersection point. The shortest distance determines the edge with the largest contribution. Using the length (Q-S1*-P) - (Q-P) the attenuation for this path calculated (see figure below).

The calculation of lateral screening is carried out as follows: The two points S2* and S3* that are used for the calculation of lateral diffraction are found by calculating the intersection of the straight line through S (laying in the reflector‘s plane) having a right angle towards S-S1* (see figure below).

The contributions of these two lateral diffraction rays and of the diffracted direct path results the final screening effect. In the figure below all three diffracted ray paths are shown in blue.

Joints to other obstacles

Joints between normative objects (e.g. between buildings and barriers) are recognized by CadnaA automatically, thus requiring no intervention by the user.

With the 3D-reflector, however, interactions with other screening objects are not seen and not considered. For example, is not checked and not automatically detected whether a 3D-reflector is situated close to any other obstacle (e.g. a neighboring building). In these situations, the diffraction around the edge in question is not prevented for automatically (see also Screening effect further in this chapter).

However, the user may exclude one or more edges of 3D-reflector from the calculation of diffraction.

Note

Please note again that the calculation of reflections and of the screening effect of the object „3D-reflector“ is not treated in any standard or guideline. The solution provided in CadnaA may be understood as a plausible extension of e.g. ISO 9613-2, RLS-90 and Schall03 (1990) or Schall03 (2014). Even with these standards or guidelines it cannot be assured that always reasonable screening effects result in arbitrary object combinations.

Screening effect

The following section describes what screening effect results using a single 3D-reflector or in combination with other screening objects.

Single 3D-reflector in ray path

With just a single 3D-reflector in the ray path, the strategy as described in the Software-internal procedures is applied (see above).

Several 3D-reflectors in ray path

With several 3D-reflectors in the ray path, the screening effect by each 3D- reflector is calculated individually, using the strategy as described in Software-internal procedures. Afterwards the final screening effect equals to the highest screening of all 3D reflectors in the path. This value is used to calculate the final barrier attenuation A_bar..

In order to understand this approach, the following hints are useful to consider: The strategy (based on the shortest path difference of the direct path) would, being applied consistently for multiple 3D-reflectors in path, cause a more or less meandering path of the diffracted ray around all obstacles. Since every 3D-reflector may have an arbitrary shape and freely orientated, the diffracted ray would not travel always across the upper edges of obstacles, but could travel once across the top edge, then across the bottom edge or around the left or right edge. It will be appreciated that this approach would lead to an unrealistically large path difference and thus an heavily over-estimated screening effects. In the end, this effect must be controlled and suppressed by the algorithm used.

3D-reflector and other obstacles in ray path

When there are one or more 3D-reflectors and other „conventional“ obstacles in path (e.g. buildings, barriers, embankments etc.), first the combined screening effect A_bar,conv of these obstacles using the „rubber- band-method“ is determined. Subsequently, for each 3D-reflector the screening effect A_bar,3D,n is determined individually (see above). The final screening effect for this ray path equals to the highest A_{bar, conv} or A_{bar, 3D,n} and is used to calculate the final barrier attenuation A_bar.

Application

On the following, some situations are described, which may be modeled using either the 3D-reflector or another type of obstacle object or a combination of obstacles. Some of the situations shown, can neither be modeled using the available types of obstacles or the solution provides just an approximation.

surface providing screening upwards with a source/s below or with a lateral obstacles in addition (keyword: „gas station roof“)

screening surface in free space	screening surface in free spacewith lateral screening vertical surface
CORRECT: 3D-reflector used for modeling a elevated screening surface above the ground, no other screening objects attached to its edges The sound power of the source/s below needs to be increased to consider roof-to-ground multiple reflections.	WRONG: modeled using several 3D-reflectors (for wall and roof) or vertical barrier (wall) and 3D-reflector (roof) Explanation: The ray path is not determined by common path difference since the joint wall/roof is not „seen“. CORRECT: modeling by using a barrier with a cantilever The sound power of the source/s below needs to be increased to consider roof-to-ground multiple reflections.

cantilevered canopies at buildings („delivery/supply ramp“)


WRONG: modeled the canopy by a 3D-reflector attached to a building Explanation: The ray path is not determined by common path difference since the joint 3d-reflector - building is not „seen“. The screening effect in the ray path is determined by that obstacle only providing the highest screening effect.	CORRECT: modeled the canopy by a cantilevered barrier attached to the building Explanation: At the cantilevered barrier the common ray path, from the vertical and the horizontal part of the barrier, is considered for all path directions. The sound power of the source/s below needs to be increased to consider roof-to-ground multiple reflections.

semi-open or partially open enclosures etc. (source inside)

WRONG: modeled using several 3D-reflectors (for wall and roof) or vertical barrier (wall) and 3D-reflector (roof) or barrier/s with cantilever
Explanation: The ray path is not determined by common path difference since the joint wall/roof is not „seen“.
Even when just using barriers with cantilever the interior level is not correct since it does not include roof-to-ground multiple reflections.

CORRECT: modeled using a building and a vertical area source modeling the opening
sound power of the area source does not include the possibly increased emission due to internal reflections,
A simplified approach would be to use a point source instead of a vertical area source attached to the building‘s facade.

buildings on top of each other or with changing cross-section with height or with inclined facades towards the ground

WRONG: modeled by a more or less complex combination of 3D-reflectors (each being triangles) arranged in free space.
Explanation: With multiple 3D-reflectors in the ray path, the screening effect is not based the common path length difference. In addition, the joints between adjacent 3D-reflectors are not „seen“.
The easiest approach would be to replace the complex combination of 3D-reflectors just by a single (possibly floating) barrier, perpendicular to the ray‘s direction.

CORRECT: using the object „Building“ to model the exterior shape. Note that the object „Building“ always raises from the ground (i.e. not floating buildings possible/allowed).
This approach represents an approximate solution based on the available normative obstacles. Alternatively, an arrangement of barriers or floating barriers (see Floating Barrier) may be used.

see also: see Building, Button "Geometry"

comparing the screening effect caused by a 3D-reflector and a common vertical barrier

Since the screening effect by the 3D-reflector results from the shortest path length difference around all edges (plus lateral diffraction) the path across the top edge does not necessarily cause the shortest path length difference.
A vertical 3D-reflector standing on the ground will, therefore, not cause the same resulting level for each source-receiver combination compared to a barrier with the same dimensions and location.

With the object „Barrier“ the screening effect always results from the ray path across the barrier‘s top edge (plus lateral diffraction).