While WhisperRoom warns on their site that there's no such thing as a soundproof enclosure, they'll help you get pretty close. The sheds are made out of heavy materials rather than lightweight foam and the like, with single or double walls of wood and steel. The reason WhisperRoom forgoes foam as a primary sound wall is that while foam will help tailor the interior sound of a chamber, it won't stop the passage of sound into the surrounding areas. In some cases, too much foam might even deaden the sound quality of a room interior, according to the site.
WhisperRooms come in an impressive 19 sizes with two different levels of sound protection–basically a single wall to contain spillover, or a double-walled sound shed. The cost can run pretty high, though. For the smallest 3.5ftx2.5ft, one-wall unit you're looking at nearly $3000. If you're serious about sound and need room to operate, you can grab one of the doubles: 8.5ftx15.5ft for about $24,000. But what you're paying for, according to WhisperRoom, is a space that has a quality sound interior as well as some of the best soundproofing one can manage.