Dark Sphere of Silence

Casting this spell creates a bubble of inky blackness in a 20 ft sphere where no sounds can enter to exit. The sphere can not pass through walls or other physical obstacles. Inside the sphere sound works normally, as does sight, it is only the outer edge of the spell is dark and stops all sound. The spell is canceled if a medium sized creature or item passes through the walls of the spell, or after 1 minutes duration, or when the spell caster wishes it to end.