When referring to curtains and drapes, the word "soundproof" is actually a bit of a misnomer. Drapes cannot be used to soundproof a room, venue or doorway but they can certainly help. True soundproofing involves very expensive design, building materials and construction techniques. A soundproof studio needs to be acoustically isolated from the outside world. A famous example of a non-isolated studio is shown right at the end of David Bowie's "Life on Mars" when the studio telephone can be heard ringing.
The most common problems that occur are-
1. Amplified music that can be heard outside a venue (usually it is the bass that is the problem)
2. External sound getting in to a studio (road noise, voices, telephones)
3. Sound being transmitted through a doorway between a factory and office space (noisy machinery).
Drapes can be designed to include multiple layers (each with different properties) to act as a barrier to the sound being transmitted. These are always most effective in the mid- and higher range of frequencies.
For a more heavy duty solution to soundproofing problems, please refer to our "Acoustic Walls" section on the Acoustic Panels page.