Apparatus is described which can be installed on board a vessel and used for the investigation and comparison of the strength of fog-signals at sea. Details of the construction and use of the sound-receiving instrument (a hot-wire microphone) and the recording apparatus are given, and a method is described whereby the ohmic changes in the resistance of the hot-wire grid can be readily interpreted in terms of a scale of sound amplitudes. An example of an acoustical survey carried out with the apparatus described is given.