The 1% Rule: A 1% opening will allow up to 50% of sound to pass through a wall, door, sound barrier, etc.
This rule comes into play most often when it comes to doors. Whether a regular door or a soundproof door, small openings almost always exist at the bottom, top and sides.
It is because of the 1% Rule that these openings around doors effectively cancel out any acoustic functionality.
For example, a 1/8″ opening around all four sides of an acoustic door(STC rating of 56) can significantly reduce the door’s STC rating (to as low as 21), which means the door is close to useless acoustically.