partial fade in

Hello all,

I would like to ramp the volume of an audio file from a given level to
full volume over a specified time interval. For instance, increase the
level linearly from 30% at time t=0 to 100% at time t=10s. In other
words, perform a fade in starting from a level greater than 0.

I can do it by inserting an appropriate amount of silence before the
beginning, performing a fade in, then removing the extra silence. But
this seems unnecessarily complex, especially if the effect is to be
applied at a time other than the beginning. Is there a better way to do
it with SoX ?

Thanks for any help.
Nicolas