Here are some great resources for building a soundproofed room:
Software:
REW is free software for room acoustic measurement, loudspeaker measurement and audio device measurement.
Free software to design QRD diffuser
Online Calculators:
Easy explained scientific fundamentals for building a soundproofed room and suitable room acoustics.
Tools to understand, explain or improve your room acoustics.
Common absorber Calculators
Schroeder diffuser calculations
The Calculator will automatically calculate cells’ width and depth, and also the bounds of effective frequencies diapason.
Useful Links:
Jochen Schulz - Sound Enginering
Tutorial Videos:
Understanding room modes / standing waves
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I will expand the list with more information in the future.