Apparatus provides an output signal characteristic of the geometry of a confined volume. There is a conduit with an open first end for exchanging acoustical energy with the confined volume. An electroacoustical transducer is coupled to the conduit for launching acoustical energy into the conduit towards the opening in the confined volume producing an incident wave and a reflected wave to form a transient wave field representative of the geometry of the confined volume. At least first and second pressure-wave-sensing transducers are mounted along the length of the conduit in spaced relationship for providing first and second transduced signals representative of the transient wave field at spaced locations in the conduit. A processor processes the first and second transduced signals to provide an output signal characteristic of the geometry of the confined volume.