led audio level

hi !
i need to build a level mic indicator, like old led vu-meter (very simple : 3 green - one yellow - one red)
about graphics i have no problem ...
but ... are two days i'm turn around the problem : seems i did not try anything (libraries - java) about real-time output level in db about audio output (mic or speacker)

please give me a little help ... is there a b4a library or java tip for that ?

many thanks

peppe