spectmorph (analyze audio samples and combine them)

SpectMorph is a free software project which allows to analyze samples
of musical instruments, and to combine them (morphing). It can be used
to construct hybrid sounds, for instance a sound between a trumpet
and a flute; or smooth transitions, for instance a sound that starts
as a trumpet and then gradually changes to a flute. In its current
version, SpectMorph ships with many ready-to-use instruments which can
be combined using morphing.
