[CF-metadata] Decibel units in CF standard names

From: Martin Juckes - UKRI STFC <martin.juckes>
Date: Tue, 30 Oct 2018 13:42:13 +0000

Hello All,


The CF standard names have several variables using decibels ("dB") as units, and one using "decibels of Z (dBZ)":

sound_intensity_level_in_air 1e-12 W m-2 10 log10(I/I0)
sound_intensity_level_in_water 6.7e-19 W m-2 10 log10(I/I0)
sound_pressure_level_in_air 2e-5 Pa 20 log_10(p/p0)
sound_pressure_level_in_water 1e-6 Pa 20 log_10(p/p0)
equivalent_reflectivity_factor 1 mm6 m-3 10 log_10(Z/Z0)


Each has a different reference level, and two use an additional factor two in the definition of the decibel level.


There are a few issues here, the main one is that "dB" is not a valid Udunits string. There is a secondary point that the details of the definitions are not easily available to software reading the files.


Where Udunits does support decibels, it is units such as dBZ, dBW for which a specific reference value is defined. As all these variables have different reference values, that would require 4 new units.


The reference values are currently specified within the standard name description. It maattry make sense to add explicit attributes so that this information can be made more accessible to users and software reading the file. E.g. "decibel_reference_level" with the name of a scalar variable holding the reference value (specifying standard name, units and value) and "decibel_scale_factor" set to "10" or "20". With these modifications it would be possible to compute the power/intensity etc from the decibel parameter.


regards,

Martin
Received on Tue Oct 30 2018 - 07:42:13 GMT