You gain the Concussion Bomb formula, which doesn't count against your total number of formulas known. If you already have it, gain another formula of your choice. For you, the Bomb is improved in the following ways.
Selectable Save
When you use the Bomb's Explode property, you can choose whether the Bomb forces all targets to make a Strength, Dexterity, or Constitution saving throw (choose each time you use the Bomb).
Silenced
When you use the Bomb's Explode property, you can choose for the area of the explosion to be magically silenced immediately after the explosion until the start of your next turn. Any creature or object entirely inside the silent area has Immunity to Thunder damage, and creatures have the Deafened condition while entirely inside it. Casting a spell that includes a Verbal component is impossible there.