The lower sound room is located in a bottom compartment well forward in the ship in order to minimize noise generated by the ship’s own propeller. The cross-sectional and cut-away views of a typical ship show the dome and projector lowered into working position, extending about 45 inches below the bottom of the keel. The dome is about 3 inches below the keel when retracted into the housed position.
The other unit of the sonar equipment that is not located in the sound hut console is the retracting gear, which is in the hull of the ship near the keel. This retracting gear is similar to that of the QGB equipment. However, it is slightly smaller because the QJB transducer is smaller. The Bell System retracting gear had large pillars of malleable iron and weighed about two tons.
The transducer is made up of ADP crystals (ammonium dihydrogen phosphate) mounted on a steel plate. ADP crystals can be used both for emitting sound and receiving sound and have very good sensitivity. These crystals project perpendicularly from it at a distance equal to one-quarter wavelength of sound in the crystal medium. Opposite the crystals on the reverse side of the mounting plate are steel rods extending a distance equal to one-quarter wavelength of sound in the steel medium. These steel rods are longer than the crystal units because the velocity of sound is greater in steel than in crystal. This system of mounting the crystals with the resonating rods results in greater power output than if the crystals were used alone. It has the disadvantage of making the transducer frequency sensitive. The crystal array is housed inside a rubber sound window. The space inside the sound window that is not occupied by crystals is filled with castor oil that has been treated to remove all air and moisture. This castor oil protects the crystals from damage by moisture because it excludes water.
In the retracted position, the dome protruded three inches below the keel and was 45 inches (3 ¾ feet) below the keel when extended. A two horsepower motor was required to raise and lower the dome with a provision to do it manually. A set of indicator lamps in the Sonar Office indicated the position of the dome -- red was for the working position, green was for housed and red/green for any intermediate state. In the housed position, the dome, the raft, and the oscillator were drawn up to the top of the trunk. The top of the raft butted up against a seating ring under the top of the trunk, thus making a watertight joint. This allowed the portable cover to be unbolted in the event that either of the oscillators had to be changed. The procedure would only work if the dome was undamaged.
The Staybrite window of the dome was aligned with the face of the oscillator so sound waves could be transmitted and received in a 360 degree arc in 5 degree steps. Staybrite was secured over a ribbed section of heavy cast metal and this ribbing is what produced the window effect.
Maximum design speed for the dome was 25 knots. When retracting or extending the dome, it was necessary to check the voltage of the mains supply. In a 220 volt mains' system, the voltage could not drop below 180 volts. If it did, the contactor in the control board could fail to operate thus causing the dome to bump against the end stops and cause damage.
The dome was normally housed for any of the following conditions: when working cables; when working bottom lines; when entering or leaving harbor; when steaming into a heavy sea; if cessation of sonar operations could be tolerated; and finally, when navigating in shoal water and sonar is not required for navigation.
Sound Dome with attached Retracting Gear