The present invention is to provide an air shaft comprising a hollow cylinder and a plurality of curved members provided on an outer surface of the hollow cylinder and between both ends thereof, wherein the curved members are adapted to move radially relative to the hollow cylinder and are spaced each other with a gap formed between any two adjacent ones of the curved members. A propellant charge is carried by the case and covered by an over-powder member in the absence of a separate projectile. Such disclosed example methods further include replacing the first scale factor with the second scale factor and the first set of mantissas with the second set of mantissas to embed the watermark in the compressed audio stream.