A flow-through device for use on-line in a conduit and capable of determining real-time characteristics of particles dispersed in a medium is disclosed. The device uses a sensor and a flow-through cell. The sensor has a stationary transducer positioned in a wall of the flow-through conduit to emit sound waves into the medium and a reflector or a second stationary transducer positioned opposite the transducer. The sensor provides a plurality of different acoustic path lengths through the medium between the transducer and reflector, thereby providing a plurality of different acoustic path lengths in the sensor and no moving parts for the transducer and reflectors. A method for determining the characteristics of particles dispersed in a medium in real time for an industrial process using the flow-through device also is disclosed. Further, a method of controlling particle size distribution in real time in a process using the flow-through device is disclosed.