Free Flooded Rings (FFR Transducers)

free flooded ring transducer (SX116-00)The open body design of free flooded ring transducers provides an inherent pressure balancing mechanism. This delivers one of the design’s principle advantages: an unlimited depth rating.  Unlike most other common transducers, free flooded ring performance remains virtually unchanged regardless of hydrostatic pressure.

Free flooded rings are efficient transducers. They perform well in both transmitting and receiving.  They offer a low Q so they can broadcast across a wide band.  Additionally, any number of free flooded rings can be assembled into an array to boost output and/or increase directivity.

And because of their simple construction, free flooded rings are reliable, requiring little maintenance and offering a long service life. They are also rugged, making them less prone to damage from impact or shock that they may experience in function, during deployment or in transportation.

Free Flooded Ring Beam Pattern

Free flooded rings have a beam that is toroidal in the horizontal plane.  It is sometimes described as omnidirectional in the horizontal plane, although this is just an alternate label for the same beam pattern.

The transducers also radiate along their primary axis, out the open ends of the tube. The width of this axial beam can be controlled by changing the length of the tube.  However, this is likely of little importance for most applications given that the amplitude of the axial radiation is much lower than that emitted radially.

Free Flooded Ring Resonance Modes

The free flooded ring is simply an encapsulated piezoelectric ceramic tube.  In air the transducer displays the same resonance modes as a tube. In water an additional resonance mode occurs.  The volume of water inside the tube interacts with the surrounding fluid as a Helmholtz resonator.  This Helmholtz resonance is also referred to as a cavity resonance or squirting mode and is well described in the DoD paper “Design Parameters of Free-Flooding Cylindrical Transducers“.

The squirting mode frequency is generally below the hoop mode resonance, providing an additional, lower frequency resonance point.  Also the coupling of the Helmholtz and hoop modes produces a broadband response in the transducer.  Both are valuable characteristics for a number of applications including long range communications.

Sensor Technology Ltd. can provide free flooded rings in frequencies ranging from below 5 kHz to more than 100 kHz. Contact us to discuss your requirements.