[css3-values][css3-speech] dB & calc()

Hi,
In CSS2015 [1], there is a link to css3-values and css3-speech 
In css3-speech, voice-volume defines a new unit, dB [2], and this 
to the following questions:

Why isn’t dB in css3-values ?
Why calc() can’t apply to dB (as currently written in css3-values)
Is 0 an acceptable value for 0dB like for other units ?
Thanks,

[1] https://www.w3.org/TR/css-2015/#css
[2] https://www.w3.org/TR/css3-speech/#mixing-props-voice-volume

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Baroula que barouleras, au tiéu toujou t'entourneras.

        ~~Yves