Hydrophone

The hydrophone is located at the forward port corner of the radio room. The hydrophone has a large dial at the center with a fixed black outer scale, and a white movable inner scale. The outer scale represents relative bearing, and the inner scale moves with the gyrocompass, representing the true bearing. The dial also has a long thin indicator needle, which shows the direction in which the hydrophone is pointed. This direction can be controlled by clicking and dragging a large handwheel bellow the dial. In addition the station has four control knobs. On the Port side is the high pass and low pass filters. The high pass filter will allow signals above the indicated frequency pass, while the low pass filter will allow frequencies bellow the indicated frequency to pass. These can be helpful in isolating certain sounds. On the Starboard Side is the gain and volume controls. These each control it's own amplifier. Adjusting the gain knob will adjust the pre amp gain, while the volume knob will adjust the headset volume.

A hydrophone is a special type of microphone specialized for use under water. This is useful for locating other seagoing vessels when a visual contact is not available, as sound can travel very long distances under water. In order to use the hydrophone the boat must first be submerged. By turning the handwheel you can change the direction in which the hydrophone is listening, and thus be able to deduce the bearing to other vessels. The hydrophone on the type VII is however not very directionally sensitive, that is it picks up sound from a fairly broad range of bearings. Where the sound is the strongest is where the hydrophone is pointed directly towards the target. due to the broad range of the hydrophone however it can be difficult to ascertain a precise bearing. A popular technique for getting more accurate bearings is to adjust the gain until the sound is just barely distorted when the hydrophone is directed towards the target. Identifying the bearings where the sound transitions from distorted to undistorted is much easier to accurately gauge than simply the loudest bearing. By fine adjusting the gain the distorted zone can be narrowed down to a very narrow band of only a few degrees, at which point it is fairly safe to assume your target is in the center of that zone. However with larger convoys there are usually several ships overlapping, and so picking out a single ship can be very difficult, if not impossible. Therefore It is not recommended to use the hydrophone for precision measurements such as acquiring target solutions.

The gain knob can also serve as a rough estimation as to the range to a target. By adjusting the gain until it barely starts clipping, as described above, one can read the gain level as an estimate for range. Note however that it is not a proportional relationship, and that a hydrophone contact which starts distorting at a gain level of 2 is at most a few hundred meters away, and the signal will fade away completely as a vessel passes overhead.