A sonar receiver functioning by transforming underwater sound signals (pressure waves) into electrical signals. In side scan sonar, the functions of the hydrophone and projector are performed by one transducer (active sonar). Often a hydrophone is a passive device doing no transmitting on its own. Hydrophones are commonly used to receive seismic echoes from explosive devices or other low frequency acoustic signals. In these applications, hydrophones are placed in “streamers” or long flexible tubes and towed behind survey ships for deep sub bottom imaging.