See the AUR
See ebuilds here
Install them as follows:
x86_64: tar -xvf ddb_spectrogram_x86_64.tar.gz -C ~/.local/lib/deadbeef
i686: tar -xvf ddb_spectrogram_i686.tar.gz -C ~/.local/lib/deadbeef
First install DeaDBeeF (>=0.6) and fftw3
make
./userinstall.sh