Apparatus is provided for adapting an analog depth sounder for the calling out of the depth at regular time intervals. In one embodiment, a self-contained adapter module is mounted to the face of the depth sounder so as to cover the depth sounder display, with the module including a transparent plate carrying a detector array and with electronics and speaker carried at the center of the array. In this embodiment, the detectors are arranged in thin radially extending lines to permit accommodation to displays of different diameter. Alternatively, small detectors are arranged along a circle having a diameter which overlies at least a portion of the orbiting radiating element which radiates along a considerable length. This alternative array configuration also accommodates a range of display diameters. As a further alternative, to accommodate different diameter displays, the thin line detectors may be replaced with radially extending light pipes with detectors at their ends. The transparent plate with almost invisible detection apparatus permits viewing of the depth sounder display through the plate so that visual as well as audible readout may be achieved. Electronics is provided to decode the angular position of the orbiting element of the depth sounder at the time the element is actuated to provide a digital number representing depth. This number is then used in generating its speech equivalent through a standard speech synthesizer and loudspeaker.