Most sonobuoys can operate at several preset depth settings. The shallow setting is used for surface duct propagation, and the deep for sound channel propagation. Sonobuoys have poor values of directivity index, mostly because of their limited size. This is a combination of an active system for transmission at one location with a passive system for reception at another location. The passive system does not suffer from the same reverberation limitations as the standard active system.
Some system use explosive driven projectors with incredible source levels.
The source may be well outside the weapons range of the target. Furthermore, the transmission loss is less than the full two-way loss of normal active. Sometimes the projector is a sonobuoy. Non-Acoustic Detection Visual Submarines near the surface are very vulnerable to visual detection. Anything that protrudes above the surface like a periscope, antenna or mast will leave a significant wake if the submarine is moving at any speed over a few knots.