Text in subscript

For my text-in-mathmode needs in LaTeX, I use \mbox. However, it does not seem to behave well in subscripts. How can I write text that adjusts its height?

$3 \mbox{dB} = f_{ 3 \mbox{dB} }$

compiles to

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see the FAQ: tex.ac.uk/cgi-bin/texfaq2html?label=mathstext –  Philipp Mar 7 '11 at 1:35
@Philipp: Thanks, I hadn't read that. –  Tim N Mar 7 '11 at 1:43

I do not quite agree, say you want to specify the radius of a circular lake, then $R_{\textup{lake}}$ would be a better choice. Why try setting it in an italic context. So for named indices I'd use \textup, one could use \mathrm or \textrm, but then what if one is using a sans serif context, so in these cases \textup (i.e. non italic text font) is better. But for general comments in math, \text is the right choice –  daleif Mar 6 '11 at 23:13