Hi LAD,
If ever you need a high precision A-weighting filter (as used for
sound level metering), you can find one in the usual place:
<http://users.skynet.be/solaris/linuxaudio>
The tarball contains a C++ class implementing the filter (easily
converted to C if you want that), and both a JACK in-process client
and a LADPSA plugin using it.
--
FA