instrument to determine direction and distance of a fog-horn
• (n.) A double ear trumpet for estimating the direction from which sounds proceed, esp. for the use of navigators.
Top'o·phone noun [ Greek ... place + Greek ... sound.] A double ear trumpet for estimating the direction from which sounds proceed, esp. for the use of navigators. 
topophone A double ear trumpet for estimating the direction from which sounds proceed; especially, for the use of navigators.