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ISO 9613 (1996) and ISO 9613 (2024)

Temperature / rel. Humidity

The air attenuation is calculated based on the temperature and the relative humidity according to ISO 9613-1. Intermediate frequency values result from linear interpolation. The temperature specified serves also as the ambient temperature when calculating the exhaust stack’s directivity.

Note

Regarding the calculation of the air attenuation for octave 31.5 Hz see Additional Information at the end of chapter ISO 9613 (1996).

Explanatory Remarks:
CadnaA calculates the air attenuation according to equations (3), (4) and (5) of section 6.2 ISO 9613-1ISO-9613-1. This also holds for intermediate input values and for input values outside the range of list boxes.
By default, the above-mentioned equations just apply to the air attenuation of pure tones. Section 7 of ISO 9613-1 provides information on the accuracy of the attenuation coefficients for certain ranges of input values (for final accuracies of +/- 10%, +/- 20%, +/- 50%). Section 8 of ISO 9613-1 provides information on the accuracy of the air attenuation when transferring the pure-tone method to band-level attenuation (for third-octaves or octaves). Refer to this information - if required - when using input data outside the range of tabulated attenuation coefficients of ISO 9613-1:

  • frequency from 50 Hz to 10 kHz,
  • temperature from -20 ° C to +50 ° C,
  • relative humidity from 10% to 100%, and
  • air pressure of 101,325 kPa (one atmosphere).

User-defined Air Attenuation

For calculations according to ISO 9613-2 a user-defined air attenuation spectrum can be referenced. Enter a spectrum in the local library "Absorptions" with the ID "SYS_ATM_ABSORB". The values have to be specified in dB/km. Missing octave band data will be considered as 0 dB/km.

Wind Speed for Directivity (m/s)

The entered wind speed will be used in the calculation of the directivity of chimneys (see Sound Radiation from Chimneys or Stacks).

Meteorology

For calculations according to ISO 9613 the following options are available:

none

No correction is applied. The resulting equivalent continuous sound pressure level is calculated applying equation (3) for the meteorological situation as described in ISO 9613-2 (downwind).

Cmet, C0 constant

This option calculates the long-term average A-weighted sound pressure level according to ISO 9613-2. This requires to specify the value of C0 for the periods Day/Evening/Night where the meteorological correction Cmet is calculated from according to ISO 9613-2 (2024), equations (31) and (32) resp. (equals equations (21) and (22) for SO 9613-2 (1996))

The factor C0 may be estimated from an elementary analysis of the local meteorological statistics or may be established by the local authorities.

Cmet, C0 from wind statistics

With this option two methods for calculation of C0 from wind statistics are available on the subsequent dialog Wind Statistics:

- method LfU Bayern: The approach used by the LfU-Bayern (Bavarian Environmental Agency) to calculate C<sub>0</sub> is:

\(C_0=-10\lg(\frac{T_m}{100}10^{-\frac{K_m}{10}}+\frac{T_q}{100}10^{-\frac{K_q}{10}}+\frac{T_g}{100}10^{-\frac{K_g}{10}})\)

with

  • Tm: percentage of annual downwind situations and calmness (inversions) in %
  • Km: level difference to the downwind situation in dB
  • Tq: percentage of annual cross-wind situations in %
  • Kq: level difference of cross-wind situations to the downwind situation in dB
  • Tg: percentage of annual upwind situations in %
  • Kg: level difference of upwind situations to the downwind situation in dB

The percentages T for downwind, crosswind, and upwind result from the wind frequencies in the following sectors:

  • down: +/-45° in direction of propagation (=90° sector) and calmness,
  • cross: 45° to 135° and 225° to 315° in direction of propagation,
  • up: +-45° opposite to the direction of propagation (=90° sector).

North direction

With this option a deviating north direction can be defined. The following options are available:

  • Get from first North Arrow Symbol (default): If no (rotated) north arrow has been entered in the model, the north direction is defined upwards.
  • Angle: Input of the north direction in degrees in counterclockwise direction.

Example for method „LfU Bayern“

see Correction based on Wind Statistics in the manual „Introduction to CadnaA“.

The approach used by the LUA NRW (Environmental Agency of North-Rhine-Westphalia) to calculate C0 is:

\(\Delta L_Z(\epsilon)=5-5*\cos(\epsilon-45° \sin(\epsilon))\)

with ε: wind direction referring to downwind

and \(c=h(\delta)*10\lg 10^{0.1*\Delta L(\epsilon)}\)

with the frequency h(δ) in wind sector α1 to α2:

\(h(\delta)=h(\alpha 1)+[h(\alpha 2)-h(\alpha 1)]*\frac{\delta-\alpha 1}{\alpha2-\alpha 2}\)

with δ: angle 0-360 degrees The value C0 results by summation from:

\(C_0 = 10 \lg(\frac{\sum c}{100})\:in\:dB\)

This means that at large distances and on a long-term annual average basis the attenuation with cross winds is 1.5 dB and with upwind 10 dB. The frequency of calmness is evenly distributed among the 12 wind sectors. The values of C0 for the locations investigated by LUA NRW range from 1 dB to 3.9 dB.

VBUI

relevant for Germany only!

According to the „Vorläufige Berechnungsmethode für den Umgebungslärm durch Industrie und Gewerbe (VBUI)“ VBUSch-2006 the following values for C0 are to used when calculating Cmet:

C0,Day = 2 dB; C0,Evening = 1 dB; C0,Night = 0 dB

ISO 9613-2 (2024) Annex C

Annex C of ISO 9613-2 (2024) desribes the meteorological correction due to the dependency of C0 from the angular wind distribution. This methos is not available in CadnaA.