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ISO 9613 (1996)

Lateral Diffraction

Three options for lateral diffraction are available for selection:

  • none: No lateral diffraction considered, just the direct path source-receiver is evaluated.
  • only one object: Lateral diffraction is not calculated if more than one object intersects the line connecting the source to the receiver point.
  • some objects: The two shortest possible convex rays around the arrangement of objects are determined and used for the calculation of the lateral diffraction.

if Distance smaller (m)

This setting defines the distance between source and receiver for which lateral diffraction is taken into account. The default value is 1000 m.

Note

The limit of the screening attenuation Dz for single and multiple diffraction accord. to ISO 9613-2, section 7.4, is in CadnaA not applied to the two lateral paths, but to the direct path only. Otherwise, the screening attenuation for single or multiple diffraction resp. would get limited to 20-10 lg (3)=15.2 dB or 25-10 lg (3)=20.2 dB, respectively (see also Barrier Attenuation below).

Lateral Diffraction with multiple Edges or Objects

With CadnaA release 4.3 the strategy when calculating lateral diffraction around objects with more than one diffracting edge or multiple objects has been changed. Now, the parameter e (distance between first and last diffracting edge) is considered when calculating C3 according equation 15 of ISO 9613-2 ISO-9613-1. In previous releases, the constant value C3=1 was used instead.

Note

The change refers to the calculation of the lateral diffraction only. For the direct path, however, the parameter e has always been and is still taken into account when calculating C3.

By a local text block OPT_OLD_CALC and the text SBEUG_e0_X141=1, the former value C3=1 can be set. If this text block does not exist or the value is zero (SBEUG_e0_X141=0), the new strategy is used.

Note

By selecting the option „none“ (i.e. no lateral diffraction, see above) it can be easily checked that the screening effect for the direct path does not change, regardless of whether the text block is set or not.

Level Increase due to Lateral Diffraction

With calculations accord. to ISO 9613-2 the receiver level including an obstacle (e.g a building) and terrain can be higher, than just with terrain (e.g. with the buildings deactivated). When calculating the screening effect by terrain no lateral diffraction is calculated. With an additional obstacle, however, the effect by lateral diffraction by that obstacle is added. This could cause an increase of the local receiver level.

Therefore, in case the screening effect by an obstacle is smaller than without (i.e. with terrain only) the screening effect ignoring the obstacle is considered by CadnaA. This strategy can be deactivated using the local text block OPT_OLD_CALC with the text VERSION_X=115.

Example

Example

Path: Examples\Configuration\ISO 9613

actual default setting for lateral diffraction accord. to ISO: Level with obstacle (e.g. with building) must be lower than without. (no local text block OPT_OLD_CALC with text VERSION_X=115)
file "ISO - lateral diffraction & terrain - 1.cna"
former effect due to lateral diffraction: higher receiver level with building, than without (with local text block OPT_OLD_CALC with text VERSION_X=115)
"file ISO - lateral diffraction & terrain - 2.cna"

Screening & Ground Attenuation

The following options are available to account for the ground attenuation in conjunction with the calculation of the screening effect.

Exclude Ground Attenuation over Barrier

With this setting the ground attenuation is excluded from the total attenuation of screened rays. This is the default setting in CadnaA.

Explanatory Remarks:
ISO 9613-2, equation (12) states (for top edge): $A_{bar}=D_z-A_{gr}>0dB$
In conjunction with equation (4): $A=A_{div}+A_{atm}+A_{gr}+A_{bar}+A_{misc}$
it results that for screened rays no ground attenuation Agr is considered:
$A=A_{div}+A_{atm}+A_{gr}+(D_z-A_{gr})+A_{misc}=A_{div}+A_{atm}+D_z+A_{misc}$
This setting is, therefore, in line with the specifications in ISO 9613-2.

Include Ground Attenuation over Barrier

The ground attenuation is included in the total attenuation of screened rays. The mean height above ground of the ray between source and receiver when passing over the barrier is considered in the calculation.

Explanatory Remarks:
With this option the equation (13) of ISO 9613-2 is applied: $A_{bar}=D_z>0dB$
This equation holds as well for the two lateral paths. For the total attenuation it results according to equation (4):
$A=A_{div}+A_{atm}+A_{gr}+D_z+A_{misc}$
In this case, the ground attenuation Agr is considered also for the diffracted rays.

10 m-Criterion

When a source is higher than 10 m above ground and at least one screening edge is higher than 10 m above ground, accord. to ISO 9613-2, note 15, equation (13) is used to calculate the attenuation by a barrier Abar (with Abar = Dz > 0 dB).

Barrier Attenuation

For the barrier attenuation the following options are available from the list box:

  • Dz without limit
  • Dz with limit (20/25)
  • De,o with limit
  • Dz with limit (20/20)

Note

The limitation refers in any case to the direct path only. The barrier attenuation resulting for the two lateral paths is not limited in CadnaA (see also Lateral Diffraction above).

Dz without limit

With this option activated the barrier attenuation Dz increases with increasing path difference z without any limitation (accord. to equation (14) in ISO 9613-2).

Dz with limit 20/25

According to ISO 9613-2 the barrier attenuation Dz is limited to 20 dB for a single, and to 25 dB for a two or more screening edges. Therefore, this is the default setting.

De,o with limit

The option „De,o with limit“ refers to the former German guidelines VDI 2714 2714-88 and VDI 2720-1 2720-97. This setting is not relevant for ISO-calculations, but has to be kept to ensure downward compatibility with old projects.

Dz with limit 20/20

According the new Austrian edition of ISO 9613-2 (see ÖNORM ISO 9613-2:2008-01) the barrier attenuation Dz of two or more screening edges is for Austria limited to 20 dB, thus deviating from the original document.

No Subtraction of negative Ground Attenuation

With this option activated, a negative ground attenuation (e.g. with reflecting ground) will be ignored when calculating Abar. Consequently, the total attenuation A considers the ground attenuation Agr for the screened ray. So, with reflecting ground, equation (12) of ISO 9613-2 is not applied, but equation (13).

By default, this option is activated as this is recommended by ISO 17534-3 (testcases for ISO 9613-2).

Explanatory Remarks:
Equation (12) in ISO 9613-2 is: $A_{bar}=D_z-A_{gr}>0dB$
For low frequencies and absorbing ground or generally with reflecting ground the ground attenuation Agr becomes negative. In this case, the attenuation due to a barrier Abar would be increased since the ground attenuation Agr would be added to the barrier attenuation Dz. With this option activated, it will be used for the screened ray instead:
$A_{bar}=Dz>0dB$
Thus, when calculation the total attenuation A the ground attenuation Agr is considered according to:
$A=A_{div}+A_{atm}+A_{gr}+D_z+A_{misc}$

No negative path difference

With activated option no barrier attenuation is calculated when the ray from the source to the receiver is above the upper edge of the barrier. By default, this option is deactivated.

The situation with multiple negative path differences in case of multiple screening objects along the path is not treated in ISO 9613-2. In CadnaA, the barrier attenuation Abar is checked individually for each object and the highest value of Abar is used.

Explanatory Remarks:
The barrier attenuation Dz according to equation (14) in ISO 9613-2 produces a distinct screening effect even when the ray passes slightly above the barrier’s top edge. ISO 9613-2, section 7.4, states that the path difference z receives a negative sign in this case.
By this approach, the calculation model according to ISO 9613-2 accounts for the theoretical fact from diffraction theory that the screening effect does not vanish instantly in case the ray passes just a little above the screening edge. If this option is activated, Abar is set to zero at a negative z.

Ground Attenuation

In ISO 9613-2, two methods are mentioned to calculate the ground attenuation Agr:

  1. „general method“: method in frequency bands according to section 7.3.1 with the ground factor G as parameter,

  2. „alternative method“: method for A-weighted levels according to section 7.3.2 not applying the ground factor G.

As in projects the sound emission of sources can either be given in spectral form or as A-weighted levels, several possibilities exist to calculate the ground attenuation. In CadnaA four calculation options are available:

Designation Procedure
none no ground attenuation at all
However, the ground reflection is still accounted for by using equation (11) of ISO 9613-2.
not spectral method 7.3.2 is used in all cases
For spectral sources the same value of Agr results for all octaves resp. 1/3-octaves.
spectral, spectral sources only The method 7.3.1 is used for all spectral sources, in all other cases the method 7.3.2 is used.
spectral, all sources Method 7.3.1 is used for all types of sources, for non-spectral sources the value of Agr of the octave band where the entered frequency is located. When calculating in third-octaves all third-octaves within an octave have the same value Agr.
This is the default setting in CadnaA since this is the „general method of calculation“ according to ISO 9613-2.
WEA interim (-3 dB) A fixed value for Agr of -3 dB is used with A-weighted or for all octaves resp. third octaves with spectral calculations (according to the German LAI interim method for wind turbine noise assessments LAI-2015).
The directivity index D_omega is 0 dB in both cases.
The negative ground attenuation is not subtracted when calculating Abar regardless of the option „No Subtraction of negative Ground Attenuation“ (see above).

Non-spectral Ground Attenuation

With non-spectral ground attenuation selected ("alternative method"), the mean height h_m is calculated in CadnaA, starting from release 4.1, based on the 3D distance (distance d in Figure 3, ISO 9613-2 ISO-9613-2-1996). This change, now corresponding strictly with the standard’s specification (d in equation (10) denotes the 3D distance, see Figure 3), causes at receiver points above and below the source the ratio h_m/d to become very small, resulting in Agr = 4.8 dB.

Prior to CadnaA release 4.1 of the horizontal distance (2D distance) was used instead to prevent this effect. This implementation, however, was not strictly in line with the standard’s specification (see illustrations).

Example

Path: Examples\Configuration\ISO 9613

vertical level distribution, Agr from 3D distance,
w/o textblock or with textblock OPT_OLD_CALC with the text ISO_HM_X137=0
(file ISO - alt. proc. - Agr from 3D distance.cna)
vertical level distribution, Agr from 2D distance,
with textblock OPT_OLD_CALC with the text ISO_HM_X137=1
(file ISO - alt. proc. - Agr from 2D distance.cna)

The previously used procedure can be toggled using the local text block OPT_OLD_CALC with the text ISO_HM_X137 = 1.

Mean Height with non-spectral Ground Attenuation

The procedure applied in CadnaA to evaluate the mean height h_m in the non-spectral ground attenuation model („alternative method“ in chapter 7.3.2 of ISO 9613-2 ISO-9613-2-1996) is as follows:

  • The line of sight source-receiver (ray path) is not blocked by obstacles or terrain: CadnaA evaluates the area below the ray path and the 3D-distance source-receiver (as shown in Figure 3, ISO 9613-2). The internal calculation procedure evaluates distances and heights step-by-step for each point of terrain or obstacle/s in the ray path.
  • The line of sight is blocked by obstacles or terrain: This case is not treated in ISO 9613-2 explicitly. As CadnaA applies the "rubber-band-method" to calculate the screening effect by multiple objects, a similar rule is applied here: The area below the diffracted path source-receiver minus the area of obstacles or terrain in path is used to calculate the area F, giving in conjunction with the 3D distance the mean height h_m.

The latter case is illustrated in more detail by a simple example: Assume a hill in line of sight being a symmetrical triangle in cross-section. The area relevant for h_m is the area below the red line (diffracted ray path) minus the green hatched area (hill area).

spectral Ground Attenuation

When calculating the ground absorption in frequency bands („general method“ according to ISO 9613-2) the global ground absorption for areas not covered by the object "Ground Absorption" (see Ground Absorption Areas) is defined via the „Ground Absorption“ tab (see Ground Absorption Tab). A default value of G=1 is used (porous ground).

In contrast to the „alternative method“ the 2D distance is used by the „general method“ to calculate the ground attenuation (dp).

Calculation in 1/3-Octave Bands

With this option is activated, the calculation occurs in third-octave bandwidth. This requires that the emission is also specified in third-octave bandwidth.

For this, sound level spectra (see Sound Level Spectra), spectra of the sound reduction index (see Sound Reduction Spectra) and absorption spectra (see Absorption Spectra) can be entered or imported in third-octave band into the libraries (see Database Import (ODBC & XLSX) and see ASCII-Spectra).

Conversion of Spectra

For point, line or area sources the spectra of which are in octave bandwidth, these will converted to third-octave bandwidth during the calculation automatically. In the opposite case (calculation in third-octaves, emission spectra in third-octaves) also an automatic conversion occurs. The following conversion rules apply:

Conversion from ... Conversion Rule
octaves to 1/3-octaves Each 1/3-octave value within each octave calculates from: L1/3-oct=Loct - 10lg3 (method „Flat (corrected energetically)“, see Spectra,Conversion of Spectra)
1/3-octaves to octaves energetic sum of all 1/3-octave values within an octave (method „energetic sum“, see Spectra,Conversion of Spectra)
A-weighted sum levels to octaves or 1/3-octaves The entered value is used for any octave or 1/3-octave. Thus, with octaves the sum level „Res. PWL“ is larger by 10lg9=9.5 dB, with 1/3-octaves by 10lg27=14.3 dB.

Note

When entering spectra into the libraries, however, the conversion rule to be applied can be selected individually for each spectrum (see Spectra). To this end, create a copy of the present third-octave or octave spectrum in the respective library and convert the copy afterwards.

Obstacles within Area Source do not Shield

When this option is active, any obstacle (e.g. buildings, foliage, barriers etc.) located within area sources (including parking lots and optimizable sources) provide no screening for any sub-source of that area source.

Example

Area source with 2 buildings inside, option activated: sources with free propagation (with all rays displayed)

With this option activated the result levels are calculated as if the obstacles were not present (i.e. no screening effect).

Area source with 2 buildings inside, option deactivated: sub-sources behind the buildings are screened, sub-sources in front of buildings with free propagation (screened rays in RED)

With this option deactivated lower levels result at the receiver points due to the screening effect of the obstacles inside. Nevertheless, for all other sources (i.e. sources outside of the area source’s border) the obstacles inside cause always screening.

The decision whether an obstacle is located inside of an area source is based on the location of the obstacle’s first polygon point.

Obstacles within Area Sources do not reflect

When this option is active, any reflecting object (e.g. buildings or barriers) located within area sources (including parking lots and optimizable sources) are not considered as reflecting for any sub-source of that area source. In this case, the same levels result at a receiver as if the objects within the area source were not present (as reflectors).

With this option deactivated a higher level result at a receiver due to the reflections of sub-sources located in front of reflecting objects. Nevertheless, for all other sources (i.e. sources outside of the area source’s border) the objects (reflectors) inside cause always reflections.

The decision whether an object is located inside of an area source is based on the location of the first polygon point.

Sources in Building/Cylinder do not shield

By default, a screening effect results for sources inside buildings and cylinders as the outer walls act as barriers.

Note

In this case, prior to the calculation, a message will be displayed (dialog Consistency Check) which enables to proceed with the operation (button „Continue“).

This procedure is due to the fact that buildings and cylinders are treated as open boxes within the calculation of propagation (despite they are displayed as „closed“ objects (i.e. with roof) on the 3D-Special View).

With this option activated, sources located inside of the geometrical borders of buildings or cylinders are not screened as if the obstacle were not present. For sources outside, those obstacles are always screening and - if any - reflecting. This option enables to model e.g. sound radiating chimneys by using point or line sources located inside the cylinder. In this case, the directivity pattern of the chimney - whether entered or selected from the default directivities - will not distorted by the cylinder’s screening edge.

Example

see manual „Introduction to CadnaA“, chapter Directivity

No attenuation for sources within built-up areas

When this option is active, no attenuation regarding built-up areas is considered for any sources or sub-source located within a built-up area. In this case, the same level results at a receiver as if the built-up area is not present.

Barrier Coefficient

The barrier coefficients C1, C2 and C3 according to ISO 9613-2 can be edited. This option enables to adapt the barrier equation to special applications (see ISO-9613-2-1996, section 7.4).

Note

In ISO 9613-2 the coefficient C1 is not expressed explicitly. The first constant („3“) in equation (14) corresponds with C1.

The default values are: C1 = 3 dB; C2 = 20 dB; C3 = 0 dB.

The input C3 = 0 dB causes this value to be calculated according to ISO 9613-2, equations (15), (16) and (17) for single and double barriers automatically.

Additional Information

Calculations for octave 31.5 Hz

By default, calculations according to ISO 9613-2 just cover the frequency range for octaves from 63 to 8000 Hz. Based on user requests, CadnaA offers the octave band at 31.5 Hz in addition.

For predictions in the 31.5 Hz octave band the following rules apply:

  • atmospheric absorption Aatm: The standard ISO 9613-1 ISO-9613-1 provides equations which are used to calculate the air absorption in the 31.5 Hz octave band.
  • frequency dependent ground attenuation Aground: The same equations as for the octave band at 63 Hz are used.
  • barrier attenuation (Dz): The equation (14) as for all other octave bands is used, however, with the wavelength at 31.5 Hz.
  • reflection criterion: The equation (19) as for all other octave bands is used, however, with the wavelength at 31.5 Hz.
  • objects „Foliage“ and „Housing Attenuation“: The same equations as for the octave band at 63 Hz are used.