Good Materials for Curtains

I asked a similiar question back in Jan./03....this was Richard from Rives Audio reply

01-11-03: Rives
Hunter vertical blinds makes a Honey comb. It works quite well for some absoption and diffusion. For absorption heavy natural fiber of two different types, have a liner of heavey loose woven material like denium or wool and a lighter fabric on the outside. You need a gather of about 50% when the drapes are closed. This will work quite well. Short of replacing the inserts in the glass there is little you can do about the bass leakage.
Rives (Threads | Answers)

I put up some velvet curtains on the front and side walls and they seem to have eliminated enough of the early reflections to increase focus, depth and imaging quite noticeably.

Get the curtains that look best in your room and then get the AudioPrism vibration control dots to tune-in your windows. Velvet curtains??? Yeh, baby!!

If you live in a city, I would use curtains that shield against RFI, something with a metal braid woven inside them.

If I may, I know the curtains won't soundproof. but why the companies claim that they're making soundproof curtains https://www.geekwrapped.com/guides/best-soundproof-curtains see reviews on them at the first time. 

It might knock down a bit of the upper frequencies but the mids and low end will go right through it.