Abstract: A method of and an apparatus for testing the internal pressure of a sealed container carried on a conveyor belt by striking the container with an electromagnetic pulse thereby causing its wall to vibrate, and to check the internal pressure of the sealed container based upon the frequency of the vibratory sound produced by the sealed container. The vibratory sound is converted to an electrical signal by an electroacoustic transducer and is returned to a container striking device in a feedback path thereby to strike repetitively the sealed container with the same frequency of the vibratory sound. The repetitive striking produces continuously without damping a vibratory sound at a frequency commensurate with the natural frequency of vibration of the container. The testing of the internal pressure of the sealed container can be carried out in a stable manner with increased accuracy. A reject system is responsive to the vibratory frequency to permit passage of or to remove the container from the conveyor belt.