Divers use underwater breathing systems for recreational, scientific, commercial, and military purposes. There are three types of self-contained underwater breathing apparatus (SCUBA): “traditional” steel or aluminum alloy cylinder of compressed air, semi-enclosed circuit re-breathers (SECRs), and fully closed circuit re-breather systems (CCRs). The traditional SCUBA systems use a two-stage “demand regulator” to reduce the air pressure in the cylinder to the external pressure for the diver to inhale. With these systems, when the diver exhales, bubbles are released into the surrounding water. Semi and fully closed rebreather systems produce few or no bubbles.Traditional SCUBA systems generate low frequency sounds associated with the gas bubbles they release. SCUBA bubbles produce low energy noise in the frequency range of approximately 100 to 400 Hz. Periodic bursts of sound (1.3-5 kHz) also occur as a diver inhales and air flows through the regulator.