Third octave analysis

[pesko]

Hi,

Anyone that can explain exactly how the Third/Octave analysis module works?

Does the data used for 1/3oct analysis have to pass through a data window (hanning + overlap), before the actual 1/3oct analysis?

I am trying to use it for some noise analysis but I don't understand how the dB evaluation works. In my opinion it should work like the one in the FFT module where it is possible to get dB out of the module. It seems that the T/O module simply looks for the highest level and sets that to the dB level you input in the reference value field.

If you convert the 1/3 octave output levels to dB's and the calculate the sum of dB's you can't get the same value as if you calculate the sound level from the raw signal.

I am running the system at 40kHz and a block size of 20k, using Dasylab V12.0, generating a 1/3oct spectra for every block and then using a block average module to get the mean value of 20 spectra's (10sec), The normal sound level is a RMS value over 10sec that is scaled to dB.

Another issue is that if I am using a y/t display for the 1/3 oct values it is not possible to use the cursors, is that a program bug?

Rgds.

Pesko

[CJ_Butler__DASYLab_Pro]

It really would help if you would mention the version.

I say this because the help was updated for V12, and is very good.

The V11 Help says this:

Control Y/t Chart

The Third-/Octave Module modifies the settings of the connected Y/t Chart module to simplify display handling. The module automatically switches to the bar display mode. Other settings are optional if the checkbox "Control active" is set:

• Autoscaling in linear display

• Y-max: Setting for the maximum y-value in the Y/t Chart. If the linear display is activate, the setting is only available if the auto-scale function is disabled.

• Y-min: Setting for the minimum y-value in the Y/t Chart. Can only be accessed when dB-evaluation active.

The module shows a check box for db Evaluation and you can enter the reference (V11). The output data is a special block of data, and it's flagged, so that the Y/t Chart limits the features that you can use.

Have you looked at the nth Harmonic module?