Spectra
Definition
A spectrum is a series of numbers, each of these figures indicates the value of the sound level, the sound insulation or sound absorption in an octave or a third-octave band. Entering a value of 0 (zero) and „no entry“ (or space) have a different meaning. The zero stands for a sound level of 0 dB, causing two sources having 0 dB in the same frequency band adding up to 3 dB (10lg2). A space or an empty octave or third-octave band input box, however, indicate that this frequency band is not valid.
When spectra are assigned to industrial sources (see Common Input Data), the calculation and display of results at receivers occurs in each spectral band. A missing value in a octave or third-octave band of a single source included in the calculation will cause that this band will not be displayed on the dialog Receiver.
Display in octaves or 1/3 octaves
In all library tables, spectra objects are displayed in 1/3-octave bandwidth. For a display in 1/1-octave bandwidth, the configuration files supplied in the installation directory can be loaded via the buttons "Edit" and then "Open". The installation directory is typically located at "C:/Program Files/Datakustik/CadnaA/". The configuration files are named "lib_spec_xxx_oct.tbl".
Weighting of Noise Spectra
A frequency spectrum for levels is unambiguously only when the respective weighting - without (i.e. linear), A-, B-, C- or D-weighting - is known. The concept applied in CadnaA avoids the risk of mixing different weighted frequency spectra and still provides the ability to assess spectral shape of a spectrum.
When entering spectra in octave or third-octave bandwidth in CadnaA the following rules apply:
- Linear (unweighted) spectra or spectra with A, B, C or D weighting can be entered. The correctness of the data entered and the settings remain to be the user‘s responsibility.
- If - accidentally - a spectrum has been entered with a wrong setting of weighting, this can be corrected by inverting the weighting and, if any, by assigning a different weighting.
- For display on the screen and export/print any of these weighting may be used. The calculation is not tied to the weighting, set when entering a noise spectrum.
- Toggling the weighting on the dialog Spectrum does not alter the frequency spectrum entered. Therefore, select the weighting prior to entering the frequency spectrum.
Examples
dialog Spectrum: with setting „Spectrum, linear“ and 0 dB at all octaves
dialog Spectrum: switching to „Spectrum, A-weighted“ shows the A-weighting at all octaves.
The spectrum has not been altered by this - as can been when switching back to „linear“.
Conversion of Spectra
In the dialogs Spectrum in the libraries „Sound Level“ (see Sound Level Spectra), „Sound Reduction Indices“ (see Sound Reduction Spectra) and „Absorptions“ (see Absorption Spectra) via the „Convert“ button octave spectra can get converted into third-octave spectra and vice versa.
By clicking OK, the selected method is applied and the dialog is closed:
1/1 Octaves to 1/3 Octaves
- When having an octave spectrum presently, it is converted into a third-octave spectrum using the method set by „1/1 Octaves to 1/3 Octaves“. The option „1/3-Octave Band Spectrum“ on the Spectrum dialog is set.The following methods are available:
| Method | Procedure |
| Flat | The octave value is used for all third-octaves within each octave. |
| Flat (corrected energetically) | Each third-octave value within an octave calculates from: L1/3oct=Loct - 10lg3 |
| only middle 1/3 octave band | The value of the middle third-octave is used as octave value. |
1/3 Octaves to 1/1 Octaves
- When having an third-octave spectrum presently, it is converted into an octave spectrum using the method set by „1/3 Octaves to 1/1 Octaves“. The option „1/3-Octave Band Spectrum“ on the Spectrum dialog is deactivated.The following methods are available:
| Method | Procedure |
| middle 1/3 octave band | value of the middle 1/3-octave within each octave |
| left 1/3 octave band | value of the lower 1/3-octave within each octave |
| right 1/3 octave band | value of the upper 1/3-octave within each octave |
| median 1/3 octave band | middle value after sorting the three 1/3-octave values in each octave using an increasing order |
| maximum 1/3 octave band | maximum 1/3-octave value within each octave |
| minimum 1/3 octave band | minimum 1/3-octave value within each octave |
| arithmethic sum | arithmetic sum of all 1/3-octave values within an octave |
| energetic sum | energetic sum of all 1/3-octave values within an octave |
| negative-energetic sum | negative-energetic sum of all 1/3-octave values within an octave (for sound reductions) |
| arithmethic mean | arithmetic mean of all 1/3-octaves within an octave (for absorptions) |
| energetic mean | energetic mean of all 1/3-octaves within an octave |
| negative-energetic mean | negative-energetic mean of all 1/3-octave values within an octave (for sound reductions) |
| reciprocal mean | arithmetic mean of the reciprocal values of all 1/3-octaves within an octave (for absorptions) |
Option „Keep Total Level“
When enabled, the total level of the converted spectrum remains the same as the input spectrum. It can be selected which weighting as criterion for the total level shall be used (Linear, A, B, C, or D).