Do Acoustic Panels Keep Sound Out (or In)?
Updated
No, acoustic panels don't keep sound out or in. They soak up echo in the room they hang in, and at 0.5 lb/sq ft for a 2 in. fiberglass panel (Owens Corning 703 (opens in a new tab)) they're way too light to change what gets through the wall. Do acoustic panels keep sound out if you hang them on the other side? Nope. The wall is what does the blocking, and it blocks about the same in both directions.
Flipping direction barely moves a wall's rating. In 507 lab tests at the National Research Council of Canada (opens in a new tab), reversing the test direction changed STC by 1 point or less in about 9 of 10 cases. So an insulated 2x4 wall rated STC 34 in USG lab tests (opens in a new tab) keeps your music in about as well as it keeps the neighbor's TV out.
Watch out: Keeping sound in is often the tougher job, because music and home theater bass sit low. A 2 in. panel absorbs only 0.15 of the sound at 125 Hz. And STC is calculated from 125 to 4,000 Hz (Acoustical Society of America (opens in a new tab)), so it tells you nothing about the deepest bass.
Either way, the fix is the same: seal the gaps first, then add mass and decoupling. Resilient channel on one side took USG's insulated wall from STC 34 to 48, the biggest single gain in its tests.
Acoustic panels vs soundproofing explains why absorbing and blocking are two different jobs. Do soundproof panels work gives the verdict problem by problem, and the soundproof panels guide walks through each method. There are more answers in the soundproof panels FAQ.