Audio Calculators Online

[Variac]

A list of useful programs online for audio related calculations:

Theses are not necessarily the best, please add others..

Air Core Inductors:
http://www.pronine.ca/multind.htm
http://www.diyaudioandvideo.com/Calculator/Inductor/

Passive Crossovers:
http://www.the12volt.com/info/diagrams.asp#pas

First Order Butterworth:
http://www.diyaudioandvideo.com/Tables/XOver/1stOrder.aspx

Second Order Butterworth:
http://www.diyaudioandvideo.com/Tables/XOver/2ndOrder.aspx

Third Order Butterworth:
http://www.diyaudioandvideo.com/Tables/XOver/3rdOrder.aspx

Series Notch:
http://www.diyaudioandvideo.com/Calculator/SeriesNotch/

Parallel Notch:
http://www.diyaudioandvideo.com/Calculator/ParallelNotch/

L-Pad
http://www.diyaudioandvideo.com/Calculator/LPad/

2 way:
http://www.diyaudioandvideo.com/Calculator/XOver/

3 way:
http://www.diyaudioandvideo.com/FAQ/XOver/

Speaker Box:
http://www.diyaudioandvideo.com/Calculator/Box/

I find these two most useful, for xover part values (remembering to use the actual value of driver impedance at the crossover frequency) :
http://ccs.exl.info/calc_cr.html#second
for Lpads:
http://www.sengpielaudio.com/calculator-Lpad.htm