A modification of the cairo-spectrum audacious plugin for bass frequencies. It's not very good code, and uses a lot of magic numbers that were just randomly tweaked until it looked reasonable.
Works with audacious 3.4.3 and 3.5, probably needs changes for other versions.
Install it by putting the .so in ~/.local/share/audacious/Plugins/ for 3.4.3. For 3.5, audacious doesn't look in this folder anymore for whatever reason, so you'll have to put it in /usr/lib//audacious/Visualization/ instead.
Example gif (playing Pegboard Nerds - Self Destruct):
Inspired by the old Monstercat video format and uses example FFTW3 code from http://www.fftw.org/pruned.html.
Original cairo-spectrum license:
/*
* Copyright (c) 2011 William Pitcock <nenolod@dereferenced.org>.
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
* INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
* (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
* IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/