[PD] convert reson filter bandwidth to decay time

[Libero Mureddu]

Hi,
I have  a simple patch with a [click~] object connected to a [reson~]
filter. I'd like to know how it is possible to convert the bandwith
parameter of the filter to decay lenght (in milliseconds).
In SuperCollider there is a particular version of the "resonz" filter
called "ringz", here is the description:
Ringz.ar(in, freq, decaytime, mul, add)

This is the same as Resonz, except that instead of a resonance
parameter, the bandwidth is
specified in a 60dB ring decay time.

My knowledge of filters is too little to be able to convert the
between the two parameters by myself, any help appreciated!

thanks,

libero