Hold the sensor using a soft resin such as silicone rubber with a hardness of approximately 50. Otherwise the vibration of the sides of the sensor will be transmitted to the case and reflected back to the sensor, re-exciting the sensor and extending reverberation.
Also, avoid using potting agents containing sulfur. If sulfur goes into the inside of the sensor along with water molecules, it will sulfurize the silver electrodes of the piezoelectric element and thereby cause faulty continuity.
(General-purpose rubber materials are not recommendable because they contain vulcanizing agents. Silicone and polyurethane are often used as rubber holding materials.)